I - .~ ~EARCH FOR THE 

I UNIVERSAL ANCESTORS 

I\U\SI\ 

-----
Front Cover: 

Filaments of oxygenic cyanobacterium, which are involved in the 

deposition of modern laminated sediments. The laminated sediments 

are believed to be analogous to those found preserved in rocks from the 

prePhanerozoic. Filaments are 4 micrometers in diameter. 

(Nomarski Optics, Courtesy of 1. F. Stolz) 

l 

) 
NASA SP-477 

SEARCH FOR THE 

UNIVERSAL ANCESTORS 

Editors: 

H. Hartman 

Massachusetts Institute of Technology 

Cambridge, Mass. 

J. G. Lawless 

NASA Ames Research Center 

Moffett Field, Calif. 

P. Morrison 

Massachusetts Institute of Technology 

Cambridge, Mass. 

Prepared at Ames Research Center 

NI/\SI\ Scientific and Technical Information Branch 1985 

National Aeronautics and Space Admin istration 

Washington , DC 
Library of Congress Cataloging in Publication Data 

Main entry under title: 

Search for the universal ancestors. 

(NASA SP ; 477) 

1. Life-Origin-History. 2. Life-Origin. 

I. Lawless, James. II. Billingham, John. III. Series. 

QH325.S42 1985 577 85-5124 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 
1ll 

TABLE OF CONTENTS 

Page 

LIST OF CONTRIBUTORS. . . . . . . . . . . . . . . . . . . . . . . . . . . v 

PROLOGUE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. vii 

I. TOWARD THE ORIGINS OF LIFE. . . . . . . . . . . . . . . . . 1 

A Definition of Life? ............................ 1 

Life on a Tape ................................. 3 

The Localization of Order ... . .. . ..... . .... . ...... 5 

The Chemistry of Life: Why Carbon? What Else? . . . . . . . 7 

Water. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 

Suggestions for Further Reading ................... 11 

II. THE ORIGINS OF LIFE: A BRIEF HISTORY 

OF THE SEARCH ... .... . ......... . ............. 13 

The Early Questioners ...... ... .. ........ . ........ 13 

The Evolutionary Synthesis .... . . . . . . . . . . . . . . . . . .. 16 

The Experimental Era: Origins Enter the Laboratory. . .. 17 

The Space Age .... . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 18 

Suggestions for Further Reading ................... 21 

III. THE TWO RECORDS: IN THE ROCKS AND 

IN THE CELLS ......... . ....................... 23 

The Age of the Earth . . . . . . . . . . . . . . . . . . . . . . . . . . .. 25 

Visible Traces in the Rocks ... . . ............. ..... 26 

The Carbon Chemistry of Ancient Rocks: 

An Opportunity . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 30 

The Record in Living Forms . . . . . . . . . . . . . . . . . . . . . .. 33 

The Kingdoms of Life . .......................... 34 

Suggestions for Further Reading .... . .............. 40 

IV. THE NATURAL EVIDENCE...... . . ... . .... . ...... 43 

The Birth of the Solar System ... . . ..... ........... 43 

Comets and Meteorites . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 

Comparative Planetology . . . . . . . . . . . . . . . . . . . . . . . .. 55 
iv 

Page 

Ancient Atmospheres. . . . . . . . . . . . . . . . . . . . . . . . . . .. 59 

The Ancient Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . .. 67 

Suggestions for Further Reading . ... ... .... . . .. ... . 73 

V. THE LABORATORY EXPERIENCE . ...... . .. .... . . . 75 

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 

Extraterrestrial Evidence . ........ ... .. ... . .. . . ... 78 

The Experimental Era ...... . .... .. ... ... .. ... ... 80 

Inorganic Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 

Polypeptides, Polynucleotides, and the Beginnings 

of Natural Selection . . . . . . . . . . . . . . . . . . . . . . . . . .. 87 

Membranes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 98 

Suggestions for Further Reading . ..... . .. . . . . ... . .. 102 

VI. RECOMMENDATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . .. 105 

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 105 

Solar System . . . .... ....... .... .... ... . . . ... . .. 1 06 

The Earth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 09 

The Cell .... ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 

Chemical Evolution .... . . . ...... .. ... . .... .. .... 114 

Models for Early Life Forms .. .... . .. . .. . ... . ... . . 118 

The Replication of Surface Patterns - An Alternative 

Entry for Life. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 120 

Summary . . ... . .. .. .. . . . .. ... .... .. . .... ...... 123 

Next Organizational Steps ...... . . . .. . . . .... .. . . .. 123 

APPENDIX: MOLECULAR STRUCTURES OF 

IMPORTANT BIOLOGICAL COMPOUNDS ....... . . 127 
LIST OF CONTRIBUTORS 

M. Calvin, University of California, Berkeley 

S. Chang, NASA Ames Research Center 

D. Deamer, University of California, Davis 

A. Delsemme, University of Toledo 

J. Ferris, Rensselaer Polytechnic Institute, New York 

C. Folsome, University of Hawaii 

H. Hartman, Massachusetts Institute of Technology 

J. Hayes, Indiana University 

H. Holland, Harvard University 

N. Horowitz, California Institute of Technology 

J. Lawless, NASA Ames Research Center 

L. Margulis, Boston University 

S. Miller, University of California, San Diego 

P. Morrison, Massachusetts Institute of Technology 

B. Nagy, University of Arizona 

P. O'Hara, NASA Ames Research Center 

L. Orgel, Salk Institute 

J. Oro, University of Houston 

T. Owen, State University of New York at Stonybrook 

C. Ponnamperuma, University of Maryland 

A. Rich, Massachusetts Institute of Technology 

J. W. Schopf, University of California, Los Angeles 

W. Stoeckinius, University of California, 

San Francisco Medical Center 

D. Usher, Cornell University 

G. Wetherill, Carnegie Institution of Washington 

D. White, University of Santa Clara 

C. Woese, University of Illinois, Urbana 

v 

vii 

PROLOGUE 

The day was calm. The waves that lapped the shore were small, 

almost lazy; they were the waves of shallow waters, not of the open 

ocean. A ruddy sun shone in the hazy sky. The slow stream that 

came down to the foreshore rippled a little in the light wind, and the 

pebbles tumbled here and there without much energy. Drama was 

not wholly absent, for along the skyline all but lost in the distance 

were two or three volcanic cones. They were quiet just now, but a 

walk along the beach would soon bring a traveler to a stretch of all 

but impassable lava, where once, not so long back, the molten rock 

had oozed and hissed into the waters from an inland fissure. It would 

happen again, but no one could foresee just where and when the 

encounter would take place. 

The day was calm and the scene was lonely. The beach was 

devoid of shells. No flies buzzed; nothing at all hopped or crawled 

along the water's edge. No birds flew; no fish swam in the sea; 

no clawed creatures scuttled below the tidal waters. The rocky 

lands inward from the sea were utterly barren of life. Neither lizards 

nor mice could be found, and neither a tree nor a blade of grass 

spread green blades to the sunshine. Yet life was present, even 

abundant, in the scene. It grew everywhere that the shallow waters 

brimmed out to dry land: dense knobs and sheets of algae and bacteria covered all the shallows, out into the bay and up the stream 

toward the higher lands. That life was never out of touch with 

water; it never survives higher than a matter of inches from moisture. 

Inland, here and there, a few dry old knobs could be found, quite 

whitened, rocklike - a growing mat of the only life in this quiet 

land, stranded forever by some shift in the watercourse. 

Modern structures formed by microbial mats found in 

Australia are analogous to structures formed by ancient 

microbial mats billions o[ years ago in a similar environment. 

Photo courtesy of J. W. Schopf 
viii 

Just such a scene - we can infer the details rather well from the 

complex fabric of the rock samples - would present itself at the 

spot, now the western coast of Australia, where the oldest trace of 

life in all the Earth is found. The time is long ago indeed, a time we 

can estimate to within a few percent from a secure, mutually confirming set of radioactive decay measurements. The signs of copious 

algal life, which bears a remarkable resemblance to the same forms 

found throughout the record of the rocks up to the present day, 

occur almost as early as the first dated rocks. One must emphasize 

that this teeming life, single-celled , though colonial in nature, was 

about all that lived on Earth, not only for the first pages of the 

record but for four-fifths of our whole past. Not until a time only 

0.7 billion years (b.y.) back can we surely see any relic of life more 

mobile than the algal and bacterial mats. Indeed, they themselves 

become more complex in microstructure and more powerful in their 

chemistry over the 3 b.y. of their evolution and change. No life is 

mobile (beyond the drift of plankton) until about that time, 0.6 or 

0.8 b .y. ago. And it requires another couple of hundred million years 

before anything alive, either plant or animal, can break close contact 

with the waters, to stand well above the coast, the marsh, or the 

damp soil. All the forms of life we see in the familiar fossil record, 

everything so graphically drawn by the paleontological artists, all 

those feathery sea lilies, bulge-eyed trilobites, all sharks and dinosaurs, all ancient birds and beasts, all dawn palms or big kelp, all that 

crawls and flies and swims and stalks, all that branches or flutters in 

the wind, belong to the last 10% or 15% of life's long history on 

Earth. 

The direct rock record supports one major plausible inference: 

All life we know has evolved from single, small-celled beginnings, 

forms like those still to be found as copious and vigorous participants 

in the great geochemical cycles, the blue-green algae and their kin, 

the bacteria. These had filled the shallow-water world in full vigor 

even by the time of our earliest evidence. Theirs, of course, is the 

triumph of lowly biochemistry - not motion, not sensory response, 

not even structure on the scale of naked-eye visibility. It is rather the 

microstructure, the complexities at the level of molecular helixes, 

sheets, tubes and rods, and the complex biochemical pathways they 

enable, which evolved over the entire first half or so of life's biography. Thus, it is biochemistry we must search out back to the time 
IX 

of the most ancient rocks known, and before, if we can. The winds 

and the waters, the volcanic fires and the slumping sands, the lava 

pours and the rolling stones - those were not much different from 

today. But in fact we do not even know whether human beings -

time travelers - could have breathed that otherwise commonplace 

breeze. Was it oxygenated? Could the old algal mats already use 

oxygen and sunlight to build their substance in the open air? Or had 

they not yet made that invention, so that they subsisted in a very 

different atmosphere from ours, perhaps not yet even making good 

use of solar photons? We cannot be sure. We know that many bacteria today are fit for vigorous life oxygen-free. We know, moreover, 

that all the oxygen of today's atmosphere is turned over very rapidly 

by the green-plant world. But just when that ability first arose has 

not yet been fixed. It is in fact the molecular facts we still seek, with 

much difficulty, there in the most ancient rocks. For it is on that 

level that the mechanisms of life must have begun, about 4 b.y. back. 

But the rocks are steadily reworked, buried, heated, and reheated in 

the Earth's fiery mantle. We have to look hard indeed to find rocks 

older than Isua in Greenland, which may - or well may not - bear 

the crucial evidence we seek. Certainly, that search will go on, and 

with sharper analyses we will find more clues in the ancient, disturbed, and enigmatic samples. 

THE DEPTH OF SPACE 

Perhaps, we can work our way forward in time to life, forward 

from the date when the Sun and its planets were somehow condensed jointly out of the interstellar gases. We know that date rather 

well. The surface of the Moon, a large collection of meteorites that 

have been trapped from orbit by their chance encounters with Earth, 

and the strongly supported inferences from the dating of long series 

of Earth rocks, all lead to the conclusion that our planet was 

assembled by a complex set of processes just about 4.5 (±O.l) b.y. 

ago. In the time between 4.5 and 3.8 b.y., we know Earth became 

the round sphere it is, the rocks of the crust grew solid and were well 

sorted, and the processes of ordinary geology approached the familiar. This stormy period, with an epidemic of heavy impacts on planetary surfaces, is under detailed study which includes the hints given 
x 

by the other planets and satellites whose compositions we come 

more and more to know. The meteors, too, offer samples stored in 

the refrigerator of orbit by which we try to conclude what material 

made up the Earth. We are pretty sure that there was plenty of time, 

for the orbital processes are speedy compared to most geological 

ones. Free fall is faster than the drift of continents and the weathering of mountains to fill the seas with silt. Even such grand geological 

dramas take a fifth of a billion years at most. So there is ample time; 

the physical processes were more or less over and became familiar 

within a couple of hundred million years (m.y.) or so. But what that 

earliest stable Earth was like is a question still beyond the grasp of 

our models; the biologists look to the planetary sciences for the 

initial conditions of sea and air, and the physical studies still seek a 

constraining hint from what the biochemists will tell them of the 

initial atmosphere! 

There has been one unexpected finding in space within the 

last decade especially. It is the universal prevalence of carboncontaining molecules. The linked atoms of carbon were once thought 

to be solely the work of life. Now we find them overall, in plenty. 

A substance like ethyl alcohol exists in vast quantities, though 

gaseous and dilute to the point of vacuum, spread in the huge, thin 

clouds of interstellar gas. Substances of even more complex and lifelike kind are found in the meteorites that fall now and again to 

Earth, some of which resemble a blackened cheese, soft and 

carbon-rich in substance. These are certainly products of the evolution of the meteorites, perhaps somewhat associated with the debris 

of minor planets or at least with the conditions of the solar nebula 

where the planets grew. There is a chance that these carbon compounds may have contributed such useful organic resources directly 

to the early Earth, which was certainly heavily bombarded with gifts 

from orbit. Most workers believe that the surface of the Earth itself 

was fully suited for the production of organic carbon compounds 

from the commonplace atoms, like carbon and hydrogen, which were 

already in place. It is the density which must be of importance, for 

complex molecules arise out of the close proximity of several atoms 

leading to their eventual union, an event none too common within 

the dilution of space. Exactly that condition (the temperature, too, 

must be permissive) gives our Earth its liquid water, intimately, indispensably part of life, ancient and modem. Perhaps no other place we 
xi 

have yet found in the cosmos allows the presence of liquid water. 

Mars long ago had great rivers, we think. Water and carboncontaining molecules fit for living forms go tightly together. The conclusion which everyone accepts is that small carbon-containing 

molecules form spontaneously, under the right ambient conditions, 

whether or not life is directly involved. If not the true precursors of 

life, these molecules are at least patterns for the loom of life. They 

can be made by a variety of synthetic processes; they differ indeed 

only in detail from nonorganic molecules, copious but biologically 

insignificant in the atmospheres of the cooler stars. The molecule is 

plainly implicit in the atomic world. 

We will press the rock record back to its first pages. We will 

improve the planetary and meteorite studies forward to the Earth in 

birth. But can we look squarely into that half-billion year gap 

between the birth of the planet and the oldest relics of life? 

THE ESSENTIAL MEMORIES 

The extension of 35 years of molecular biology involving the 

idea of the genetic code and its work, has shown us much of the 

inner nature of life, especially of microbial life. The effort to frame a 

general definition of life, which might seem at the core of the matter, 

is in fact not so salient in the search for life's origin. For what we 

seek must be the long chain of events which gave rise to that specific 

complex web of present-day life on Earth of which we are a part. 

Even if quite other forms are possible, they would seem less relevant 

to the quest for what happened here. It is no surprise, then, that the 

key questions have been sought on theoretical grounds. For 20 years 

people have looked for conditions which, under the little-known 

natural circumstances of 4 b.y. ago, could plausibly lead to the rise 

of proteins - the working molecules of life which marshal the linked 

chemical reactions to build living systems and lend them function. 

Parallel effort has been spent with the nucleic acids, which on Earth 

alone form those subtle, long molecules (DNA and RNA), as well as 

the various copying peripherals that embody and reproduce the 

instructions for the proteins which implement the order, even the 
xii 

act of code reproduction itself. These complex molecules are universal in life forms today ; they have long seemed a wise starting point 

for the search for origins. 

Neither a coded message, whose slow elaboration is the only 

key to the evolutionary path, nor these protein jigs and fixtures, 

which alone allow the expression of the inert code within the world 

of living change, can be the whole story. No code, no way to elaborate the chain of life forever; but no jigs, no action on the world. A 

biological contract between these molecules, or more strictly, 

between these two functions in some molecular form , seems the 

center of the issue. We do not see it clearly yet. The clues from present life are useful, but today the mechanism has become so precise, 

so well-functioning, so long-elaborated that the steps that led to it 

from a simpler, nonliving world are hard to guess. It is possible that 

some simplified intermediate structures will be found; it is also possible that some key contrivance, now entirely superseded, must be 

found. Some think that the inorganic substructure of some mineral 

might have offered a crystalline framework for the first systems of 

molecular self-reproduction. Other essentials besides the message and 

the action are postulated ; or perhaps some early form of the 

message may have been at least weakly self-acting. The whole topic is 

complex, central, challenging. 

One conclusion seems stronger than ever, touching on the eons 

of evolutionary elaboration. It seems likely that the first cells of the 

microbial mat were the unusually small and structurally more or 

less simple ones which still distinguish the bacteria and the bluegreen algae as a group from all other higher forms of life. Sometime 

in the first 2 b.y., new cell types arose. More or less algal, they were 

big, hundreds of times the volume of their forebears, like most cells 

of the animals and plants of today. Moreover, they held the specialized organelles of cells, those which today carry out efficient photosynthesis, hold and take out the message molecules safely and precisely, arrange for sequential reactions 0 f energy storage and 

liberation, etc., in all higher forms. The smaller cells perform the 

same functions (indeed, as biochemists they are even more versatile), 

but they lack many advantages of rate and of diversity in reproduction. There is good reason to believe that here we see another later 

social contract - the organelles of the larger cells are perhaps the 

offspring of once-independent organisms, which long ago contracted 
,--

 

xiii 

to dwell within some host cell and share its world_ A few such symbiotic arrangements may have made the complex and protean cells 

of today, the eukaryotes. Their cooperation to form organelles, then 

whole creatures, has given rise to the multicellular forms of life that 

are now large enough to make an imprint in the world one by one, 

and enterprising enough to add swift mobility and varied macrostructure to the chemical virtuosity of that ancient algal mat. 

Probably, these several contractual unions are the heart of our 

problem; so far only the rise of the organelles seems to find some 

support in the world of life as we study it today. 

THE TENSIONS OF RESEARCH 

There is a certain tension in scientific research. No doubt all 

researchers would like to solve important problems, problems with 

impact on the mind, or problems whose solutions bear on human 

needs in a material way. But those problems are hard, often refractory. At any state of knowledge, the researcher is therefore led to 

seek out, not the most important problems, but the soluble ones. 

Galileo watched the lamp swing with uniform beat; he could get 

somewhere with that. But his early effort to explain the tides was 

not the source of much further work. In the study of the origin of 

life some balance must be struck. The problem is clearly important; 

perhaps no other problem speaks more to the common concern of all 

reflective human beings to learn our place in the world. But it is evidently no simple problem, more so because it cannot be sought 

within a single discipline. Here, molecular biology meets astronomy, 

both encounter geology, and each of these major disciplines draws 

upon the chief results of chemistry and physics over a very wide 

range. Such a specialty is institutionally fragile; its devotees generally 

must find their professional niches in quite different, better-focused 

enterprises - those with degrees, standard texts, students, clear 

applications. We believe that the maturity and importance of the 

problem begin to demand explicit support and recognition. The 

question lies squarely across the major currents of microbiological 

and of planetary research today. Those sciences could reach no more 

important outcome than to illuminate the origins of life on Earth _ 

_ J 
I~ 

xiv 

In this search, we therefore offer an account of the state of our 

knowledge, our hopes, and our puzzles as they are today. We hope 

the material will convince the reader how fascinating and how timely 

is this topic in the present state of science. We might remark also that 

a focus on the widespread but rather quiet, cryptic life of the 

microbial mats of the great watery flats is strangely relevant to 

today's world in which the geochemical and atmospheric cycles 

themselves begin to show the effects of human intervention for good 

and bad. The great role of that lowly life in the deep past is to a 

degree continued today; we know all too little of its complex and 

rich operations within the changing balance of contemporary nature. 

As we look back into the past, we will both employ and illuminate 

the present. But above all, what we seek is not only practical advantage, though that will come. Our chief goal is a kind of selfknowledge, as deep as our oldest myth: how it came about on this 

Earth that the quick were first parted from the dead. 

Philip Morrison 

Massachusetts Institute of Technology 

Cambridge, Massachusetts 

I 

,------ - - ~ ~--- .. 

I. TOWARD THE ORIGINS OF LIFE 

A DEFINITION OF LIFE? 

The search for a clear defmition of life is in itself a scientific 

problem of real depth. In the present context, that search appears a 

little peripheral, however deep and urgent the issue may be to a full 

understanding of biology. For our task is not at all to examine every 

path that the lifelike might walk. The classical metaphors of the 

flame, the whirlpool, the growth of a crystal, even so speculative a 

diversion as the notion of life based on silicon, whether within some 

computer mainframe or in the creaking motion on some far-off 

planet, are somewhat beside the point. The task is set for us sharply 

by the historical event: The life whose beginnings we seek is that of 

our own planet, carbon-based, molecularly elaborated, and capable 

of evolving into the web of life shown in the geological record up to 

our own time. 

The attributes by which we recognize living things as alive -

their capacity to grow, replicate, and repair themselves, to produce 

elaborate and seemingly purposeful structures and behaviors, to 

adapt to the most varied conditions - are derived ultimately from 

the genetic properties of living matter. By genetic properties we 

mean these two: self-duplication and discrete change. That is to 

say, living systems are systems that reproduce themselves closely, 

but that mutate as well, and then can reproduce their mutations. 

These properties define the living state. Such systems, mutating -

albeit blindly - in many directions, will evolve through the process 

of natural selection. In time , they can yield the living world in its 

endless variety and complexity. 

Within the notion of self-duplication, so intrinsic to life, there 

once seemed to rest a logical disaster. A mechanism which must 

reproduce itself needs to be complicated. But the more complicated 

Electron micrograph of DNA by Lom e MacHattie, in collaboration with Ken Berns and Charles A. Thomas, Jr. 
2 

it is, the harder it is to reproduce, and thus the more complicated it 

must still be - that particular kind of trouble was once and for all 

eliminated by the arguments of the mathematician John 

von Neumann 30 years ago. His argument was well-summarized by 

Freeman Dyson: 

"Von Neumann did not live long enough to bring his 

theory of automata into existence. He did live long 

enough to see his insight into the functioning of living 

organisms brilliantly confIrmed by the biologists. The 

main theme of his 1948 lecture is an abstract analysis 

of the structure of an automaton which is of sufflcient complexity to have the power of reproducing 

itself. He shows that a self-reproducing automaton 

must have four separate components with the following functions. Component A is an automatic factory, 

an automaton which collects raw materials and processes them into an output specifIed by a written 

instruction which must be supplied from the outside. 

Component B is a duplicator, an automaton which 

takes a written instruction and copies it. Component C is a controller, an automaton which is hooked 

up to both A and B. When C is given an instruction, it 

fIrst passes the instruction to B for duplication, then 

passes it to A for action, and fmally supplies the 

copied instruction to the output of A while keeping 

the original for itself. Component D is a written 

instruction containing the complete specifIcations 

which cause A to manufacture the combined system A plus B plus C. Von Neumann's analysis showed 

that a structure of this kind was logically necessary 

and that it must also exist in living cells. Five years 

later Crick and Watson discovered the structure of 

DNA, and now every child learns in high school the 

biological identifIcation of Von Neumann's four components. D is the genetic material, DNA; A is the 

ribosomes; B is the enzymes RNA and DNA polymerase; and C is the repressor and derepressor control molecules and other items whose functioning is 
still imperfectly understood. So far as we know, the 

basic design of every micro-organism larger than a 

virus is precisely as Von Neumann said it should be." 

3 

This qualitative triumph was not the last of the mathematical 

approaches. Perhaps the most sophisticated work which followed was 

that of the seventies by the distinguished physical chemist, Manfred 

Eigen. He and his colleagues have elaborated a carefully controlled 

model of linked chemical reactions, which shows the possibility of 

the steady increase of chemical complexity by various feedback 

mechanisms entirely within their model domain of interdependent 

reaction rates and yields: the explicit working out of the vague old 

notion of autocatalysis. But it seems rather far from our concrete 

problems, for it begins with a mechanism of reproduction with 

interaction. Yet it is the origin of that mechanism which is a central 

problem. 

Once again, the impact of this abstract work is, so far, stronger 

at the level of general understanding than upon the actual search for 

terrestrial beginnings; most workers prefer to take strong clues from 

the specificity of the life we know than to follow logical consequences of plausible general models. There are hints of experimental 

systems: Certain bacterial cells, products of some error in cell division, metabolically complete but without genetic apparatus, might 

allow a kind of direct experiment in the domain of self-reproducing 

automata. But most effort still seems directed to a more chemically 

characterized leve1.! For most work so far, that has been the pursuit 

of a bridge across the present wide gulf between the stable nucleotide 

memory string - the DNA - and the reactive peptides of enzymes, 

the structural proteins, and perhaps some other membrane constituents. The emergence of a self-reproducing system from single molecules is yet elusive. 

LIFE ON A TAPE 

Perhaps the most important achievement of modem biology has 

been the discovery of the chemical structures and mechanisms 

1 Important representative chemicals are illustrated in the appendix. 
4 

responsible for the genetic properties of living things, the components A through D of von Neumann's automaton. These properties 

derive from just two classes of large, information-rich molecules: proteins and nucleic acids. The nucleic acids are the ultimate selfreplicating, stable, yet mutable structures of all living matter today. 

They form the genes, the bearers of the genetic heritage, in every 

known species. This heritage consists of durable information, largely 

concerned, directly or indirectly, with the production of specific 

protein molecules. The latter form most of the cellular structures and 

the chemically active enzymes - the versatile class of highly efficient, interactive catalysts that control the chemical activities of 

cells, including the eventual self-synthesis of more enzymes, other 

proteins, nucleic acids, and other key molecules. The nucleic acids 

and proteins thus constitute an interlocking and interdependent 

association, the genetic system. Whatever is unique about living 

matter is inherent in this system. There is an instructing tape, the 

nucleic acid, which directs the assembly of the universal chemical 

tools, the enzymes. Note, though, that all the important structures 

are not single molecules, but complexes like lipid membranes. 

The duality of the genetic system arises from the circumstance 

that survival in the struggle for existence depends on the ability of 

organisms to synthesize a large variety of specific proteins; those 

proteins are highly ordered, hence, thermally improbable structures 

that must be built up by a long sequence of individual amino acids. 

If every generation had to discover for itself how to assemble amino 

acids in the right order to produce the proteins it needs, survival 

would be impossible. This information must be transmitted from 

parent to offspring, and a mechanism for storing and copying it is 

required. Amino acid sequences cannot be copied by any known 

chemical scheme from a preexisting protein, but nucleotide 

sequences can be copied from a nucleic acid. Consequently, the 

instructions for assembling protein molecules are encoded in nucleic 

acids. Only the latter are copied for inheritance; only the former 

are made anew to do the work. 

As indicated above, the lengthy information sequence contained 

in the genes was generated by random mutations in DNA, screened 

by natural selection. The genetic specifications are thus the evolutionary product, a record of discovered solutions to the problems of 

survival encountered by the species in the long course of its history. 
5 

Basic to this evolution is the mechanism - itself slowly perfected by 

mutation and natural selection - that brings about the nearly flawless replication of DNA and its mutants, and through them, of proteins and the entire organism. 

The spontaneous origin of so complex a system as that 

described above poses great conceptual difficulties. It must be recognized that the simplest modem genetic system we see is highly 

evolved; the original system was probably much simpler. Two logically possible predecessors are worth exploring: (I) polynucleotides 

which still retain some catalytic capability, and (2) polypeptides 

which yet have some replicative capability. Molecular systems such 

as these would be extremely inefficient by standards of life today, 

but they might have been sufficiently accurate under the lesscompetitive conditions of the primordial Earth to survive and to 

evolve. Either one might have been capable of developing into the 

modern dual mechanism. 

THE LOCALIZATION OF ORDER 

The search for interactions among molecules which might lead 

to self-replicating systems of either or both kinds of molecules, 

nucleic acids and proteins, has progressed to some depth. Biochemical preparations which are essentially homogeneous solutions can 

yield remarkable results whenever the polymer prototypes required 

are present in rather high concentration. The classical experiments on 

DNA replication in vitro are only one famous example. 

But this sort of system is not without its limitations. It is after 

all the cell, a large set of interactive molecules, which is the unit of 

present-day life. The notion that simple mutations - if one thinks of 

many successive single-point modifications within a long polynucleotide sequence - could in the fullness of time lead to the marvelous 

outcomes of natural selection is inherent in the point of view. Surely, 

that is too simple a view of the actual course of evolution. The time 

required for a given selective result can be much reduced if the process proceeded among quite distinct strings of genes, to join several 

of them later in an action which may confer complex, new capabilities all at once. This is of course seen in living forms at many genetic 
6 

levels, from that of the bacterial plasmids to sexual recombination in 

the classical way ; even the typical microbial cell may itself largely be 

a symbiotic union of once-independent organisms. Some such hierarchical architecture is probably essential, even a rare process which 

increases the probability that successful changes can come to dominate. The slow growtl}. of molecular tapes in solution - whether one 

imagines an organic ocean or a more plausible multitude of enriched 

tide pools - is bound to be speeded up by any process which tends 

to sequester reactants in the right way. 

The logical simplicity of the replicating single molecule is evident. On the other hand , the complex system we now see requires 

the participation, even in vitro, of an energy source and a number of 

specific auxiliary enzymes. In the cell of course much more is invariably present, from the multimolecular mechanism of the transcription apparatus to the enclosing membranes which maintain the 

concentrations and the integrity of every cell. The molecular coupling described above is a minimal early step. 

It is plausible to extend the idea of the single molecule. The 

simplest extension is to a system of a few molecules, which seems 

closer to the working system of life today. The difficulty is still in 

the coupling within any simple molecular system. 

The main logical point seems to be to extend the participation 

of the environment, in order to restrict the replicating event to a 

single protogene itself. To this end the free energy is thought to be 

provided by a medium rich in needed building blocks, stored there 

by nonbiological processes that produced the molecules required. It 

is not difficult either to imagine the presence of some nonspecific 

catalyst, say, a metal ion, which might assist the coupling sought 

between the major polymers. 

The next step beyond the single molecule self-copying in an 

organically rich and slightly metallized medium is the idea of a nonspecific substrate, some solid mineral surfaces where adsorption 

might locally increase concentration, provide nonspecific catalysis, 

and allow the use of rare components. At this point of hierarchical steps of unification, perhaps on the model of the simplest one, 

if A can make not itself but can make B, while B in turn makes 

A, the single union is self-duplicating. Once the possible role of 

substrate is admitted, the scheme can go even further. We shall 

discuss in more specific terms in chapter VI the conjectures that 
7 

involve substrate to the extent that a replication of pattern is itself 

seen as beginning first on a mineral surface. 

None of these schemes is yet much supported by experiment or 

quantitative theory, but given that life arose in an abiotic environment, some stable and yet not immutable spatial ordering was a key 

part of the process. Was that ordering all spatial, structural, in the 

sense of the fonnation of discrete phases beyond the molecular 

scale? Or was it in part temporal, kinetic in the sense of variable 

reaction rates among molecules? Those rates could be self-controlled 

by catalytic feedback loops, as well perhaps as by temporal, even 

cyclic, changes in the external chemical environment, e.g. , dry-wet, 

or light-dark. That both possibilities might have been of importance 

is not to be overlooked. In the present state of our knowledge we 

would hope for small steps along any of these paths. 

THE CHEMISTRY OF LIFE: WHY CARBON? WHAT ELSE? 

The capability for generating, storing, replicating, and finally 

utilizing large amounts of infonnation implies an underlying molecular complexity that is known only among the compounds of carbon. 

The special properties of the carbon atom that make it suitable for 

the construction of large, complex, three-dimensional molecules 

which are, in fact, thennodynamically unstable but kinetically 

metastable, are discussed in textbooks of organic chemistry. While 

living forms contain significant amounts of hydrogen and oxygen (in 

the fonn of H2 0), no other element enters as many and complex 

compounds as carbon. As every student knows, there are more compounds of carbon known than there are of all the other elements 

put together. 

Table I-I lists the relative number of atoms of the chemical elements which comprise the particular sample of life described. The 

data give in round numbers the number of atoms for each important 

element (not the weight) among 100 atoms of the sample. Note small 

round-off errors. 

Life on our planet is carbon-based despite the fact that carbon 

is a minor element in the Earth's crust (about 0.5% by weight). This 
8 

TABLE 1-1.- SOME ELEMENTARY RECIPES FOR LIFE FORMS 

Element 

Life form 

H C 0 N P S Total Other 

Key biological polymers 

D A 38 29 19 II 3 --- lOa 

Typical protein 50 3 1 10 8.5 --- 0.5 100 

Wh ole living forms 

Green plant (alfalfa) 57 6 33 4 0.2 0.03 100.3 Ca , 0.1 ; all others 

less than S 

Animal with bony 62 9.5 26 1.3 .3 .08 100.3 Ca, 1.3; K, 0.09; 

skeleton (human) Na , 0.07; all others 

less than S 

is comprehensible once the special properties of the carbon atom 

referred to above are considered. It becomes almost expected in the 

light of recent discoveries showing that carbon compounds complex 

enough to have biological importance are reasonably abundant in 

carbon-containing meteorites (carbonaceous chondrites), even in 

vast, cold, and dilute interstellar clouds. Analysis of carbonaceous 

chondrites has revealed the presence of numerous amino acids, 

including at least eight of the amino acids of living proteins. It has 

been generally concluded that these amino acids are of extraterrestrial and non biological origin. Equally remarkable is the demonstration, by microwave spectroscopy, of a variety of organic compounds in interstellar space, in association with dust clouds rich in 

molecular hydrogen. Among the substances identified in these 

clouds are intermediates familiar in the synthesis of amino acids and 

of purines, pyrimidines, and sugars - in short, precursors of the 

genetic system, albeit very dilute within those enormous astronomical volumes. 

It is clear from these discoveries that non biological reactions 

leading to the formation of biologically interesting molecules have 

occurred and are still occurring in the universe on a grand scale. 

This suggests that wherever life may be found it will be carbon-based, 

not greatly different in chemistry from our own. 

J 
9 

The cosmic abundance of atoms tends to fall steadily with 

atomic number. None of the key atoms in the biopolymers of early 

life is heavier than sulfur, atomic number 16. Living forms do make 

some use of a good many elements in addition to the major constituents of their biopolymers. Every shelly or bony creature uses calcium, while some forms have skeletal frameworks of silica; these are 

specialized structures, but they all use elements rather common in 

the surface minerals of Earth. Among the most abundant of minor 

atoms are the volatile elements of the mineral world which are easily 

outgassed, dissolved by water, and weathered out of the rocks to salt 

the sea and make up the fundamental electrolyte solutions within all 

living cells. These are magnesium, sodium, potassium, calcium, and 

chlorine (chlorine is in fact rarer than the others listed). Next most 

important is iron, a rather heavy atom (atomic number 26), which, 

because of its intrinsic nuclear stability, is unusually abundant in the 

cosmos for its weight. The iron atom plays a central role in life today 

within a number of indispensable metal-containing organic structures, the blood-red pigments of course, but others as well. Similar 

roles can be played by less common metal atoms, e.g., copper, 

cobalt, zinc, manganese, even molybdenum, and vanadium. These are 

by no means common elements, but their exploitation by life seems 

to be an opportunism of natural selection, making use even of a trace 

of some rare atom to take advantage of its special properties. The 

secondary elements in life, in addition to hydrogen (H), carbon (C), 

oxygen (0), nitrogen (N), phosphorus (P), and sulfur (S), amount at 

most to one or two atoms in a hundred; no living species is known to 

require any element heavier than iodine (atomic number 53). The 

vital iron atom is the most abundant heavier atom in the human 

body, yet there is in the body only about one iron atom for every 

15,000 of carbon! In all of biology only humans make any use of the 

rare heavy metals, like lead, gold, and uranium; that use is not 

biological, but cultural. 

WATER 

The two most abundant compound-forming atoms in the cosmos are hydrogen and oxygen. Their most familiar compound, water, 

is wide spread. But it is grains of solid ice and very dilute water vapor 
10 

that we find copiously in space. The familiar liquid water is not 

known to us with certainty anywhere in the universe save on Earth. 

We suspect it was in the bodies where the carbonaceous meteorites 

formed, probably some class of asteroids. We might have seen signs 

of it in the margins of the Martian polar cap; maybe it is present 

underground on Mars. The point, of course, is that liquid water is a 

fleeting substance; it can persist only within a limited range of temperature at reasonably high pressure. Such a regime is from a cosmic 

viewpoint intermediate in temperature; 10 or 20 times above the 

temperature of the cold gases of space, but 10 or 20 times below the 

temperature of a star surface. To realize liquid water, the pressure 

must stay rather high, compatible almost certainly only with the 

surface gravity of a modest, cool, planet-sized body. Thus, liquid 

water and life as well seem to be phenomena of high density. The 

near--vacuum of space cannot keep liquid water, and the atomic 

collisions which allow sequential reactions there are haltingly infrequent, even though quite numerous within the clouds in space. So 

familiar an "organic" compound as ethyl alcohol is well detected in 

such interstellar clouds. The total amount present there is huge; a 

single cloud contains more alcohol than all life on Earth has made 

over all its history, but it remains more dilute than a laboratory 

vacuum. It is so dilute there that any buildup to truly complex 

molecules is painfully slow. 

Water, the medium of life, dominates life today. Ninety percent 

of evolutionary time had passed before life could emerge from water 

(or perhaps take water along) to populate the land. Until that epoch, 

life was to be found only below the water's surface, or near that 

boundary, or, at the driest surface locked within damp enclosing soil. 

Land life now has elaborate and specialized devices to avoid dryness. 

It remains true that the biopolymers themselves depend on water-like 

chemical bonds for their very existence. The hydrogen bonds in 

which a proton forms a positive electrostatic link between two negatively charged electron clouds, is the chemical bond of intermediate 

stability that lends the big polymers their subtle mix of stable and 

labile properties. If it were not water within which life grew it must 

all the same have been some other hydrogen-rich medium. 

These logical inferences confirm our present fmdings: Life on 

Earth must have begun in or near water. That much seems sure. 
11 

SUGGESTION FOR FURTHER READING 

Judson, H. F.: The Eighth Day of Creation. Simon and Schuster, 

N.Y., 1979. 
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13 

II. THE ORIGINS OF LIFE: 

A BRIEF HISTORY OF THE SEARCH 

THE EARLY QUESTIONERS 

There is within modern science a curious anomaly, almost a 

paradox. Since the rise of modern science during the Renaissance, its 

fashioners have realized that the daily tasks of science could not be 

set by the great philosophical questions. The philosophers and the 

prophets over all history had raised the key questions; What is 

motion? What are being and becoming? What are the stars? Whence 

the Earth, life, man, and all the rest? But they could not supply 

growing insights, for all their keenness in setting great questions and 

opening logical conjecture. A more modest science could bring 

answers. But science by choice begins with small questions: How do 

bronze balls roll down inclines? What are the shapes of planetary 

orbits? What happens to the weight of charcoal as it burns? One had, 

above all, to supplement the inborn senses; the telescope, the microscope, the balance, the careful computations, these were the new 

tools of science. 

With such tools and maturing skills, with concepts and analyses 

beyond the reach of the common language of the general philosopher, the scientists broke new ground in every direction. But they 

left aside the great questions, often the questions of origins and of 

ends, for such questions were not ripe for answers. It takes a mature 

discipline of geology, for example, to ask where mountains come 

"But if (and oh, what a big if) we could conceive in some warm little 

pond, with all sorts of ammonia and phosphoric salts, light, heat, 

electricity, etc., present that a protein compound was chemically 

formed, ready to undergo still more complex changes, at the present 

day such matter would be instantly devoured or absorbed, which 

would not have been the case before living creatures were formed. " 

Charles Darwin 's letter to Hooker, February 1871 

(Copy courtesy of Prof Melvin Calvin, University 

of California, Berkeley) 
14 

from; until the whole world is mapped, even the sea floor, and many 

individual histories have been teased out of the data, the worldwide 

picture of plate tectonics can hardly be drawn. Even now, the 

specialized sciences, with their diverse techniques and conceptual 

structures, do not often aim directly at the larger questions. There is 

a kind of disparity between the great end which science, as a whole, 

seeks - the full understanding of our universe and the human place 

within it - and the everyday or every decade problems which it can 

and does solve. The great questions are often put aside for another 

generation. 

The problem of the origin of life is such a great question. Only 

in the last decades, since we have acquired a powerful molecular view 

of life's inner unity and a growing reach into space toward the old 

history of the planets and the Earth, can we begin to ask that question in so many words. The story of the growth of that topic within 

science, and the scale and scope of the community recently engaged 

with the issue are sketched here. 

The idea of life arising from nonlife, the idea of spontaneous 

generation, had been commonplace for millennia. One had only to 

accept the evidence of the senses, thought the ancients: worms from 

mud, maggots from decaying meat, and mice from old linen. 

Aristotle had propounded the doctrine, along with Virgil and 

Lucretius. This teaching was accepted by a long line of western 

thinkers. Eastern ideas were similar. In the ancient Hindu scriptures 

life is described as having originated from nonliving matter. The 

Rig Veda, for example, pointed to the beginnings of life from the 

primary elements while the Atharva Veda postulated the oceans as 

the cradle of all living things. 

The fIrst pointed experimental investigation of the concept of 

spontaneous generation was carried out by Francesco Redi of 

Florence in 1668. His experiment was as simple as it was decisive. 

Once the jar of meat was covered with a veil of muslin, no flies could 

lay eggs on the decaying meat, and therefore it bred no maggots. All 

life is from the egg! 

With the use of the microscope by Robert Hooke and Anthony 

Leeuwenhoek (ca. 1660-1700) a new false trail appeared. Many who 

used the new instrument saw many moving microorganisms grow 

amidst decaying vegetable matter but they were unable to explain 
15 

their ongm. The theory of spontaneous generation was thus kept 

alive for a century. 

In 1860, the French Academy of Sciences offered a prize to 

anyone who would provide a decisive experimental result to halt the 

old controversy. Louis Pasteur's experiments of the 1860s with the 

swan-neck glass flasks are part of our scientific heritage (fig. 11-1). 

Pasteur announced his results to the French in the following words: 

"Life is a germ, and a germ is life. Never will the doctrine of spontaneous generation recover from this mortal blow." 

About the same time there came a clear insight from the field of 

organic chemistry. Perhaps it is premature to use that term, for in the 

mid-nineteenth century the chemistry of carbon compounds had not 

yet come of age. The great Berzelius in 1815 had argued that organic 

compounds were produced from the elements by laws differing from 

those governing the formation of inorganic molecules. According to 

him, organic compounds were produced under the influence of an 

essential vital force and therefore could not be produced artificially. 

But Wohler's classic experiment of 1828, in which a product of animal metabolism, urea, was produced by the heating of ammonium 

cyanate, weakened the sharp distinction between the organic and 

A B 

Figure 11-1.- In the 1860s, Louis Pasteur showed that life did not arise spontaneously. The intact swan-neck flask (a) remained sterile, while the one with 

the broken neck (b) did not. 
16 

inorganic. Similarly, in 1845 Kolbe by a series of stepwise reactions 

produced the familiar acetic acid, surely a "genuine" organic compound, from carbon disulfide, which in turn had been prepared by 

reacting carbon with sulfur. The Chemical Abstracts today lists over 

5 million organic compounds, elequent testimony to our unified 

understanding of organic chemistry, one single chemistry of carbon 

compounds. 

THE EVOLUTIONARY SYNTHESIS 

It appears that it was Charles Darwin who flrst formulated the 

modern approach to the origins of life, with a view of the circumstances, not of today, but of the distant past when the fust life was 

somehow formed. He wrote in a private letter in 1871: "If we could 

conceive in some warm little pond, with all sorts of ammonia and 

phosphoric salts, light, heat, electricity, etc., present, that a protein 

compound was chemically formed ready to undergo still more complex changes, at the present day such matter would be instantly 

devoured or absorbed which would not have been the case before 

living creatures were formed ." In short, the logical needs for the 

origin of life include the absence of life: a sterile environment was 

exactly what was present then and what is utterly unknown in the 

biosphere today. But for 50 years such large ideas lay dormant. They 

were ahead of the state of biology and geology. The question was too 

grand. Pasteur's wonderful declaration is true for our geological 

epoch; the ancient epoch when life originated, which is not at all the 

present natural life-filled environment, was not brought under study. 

In 1924, a young Russian biochemist published a preliminary 

account of his ideas on the chemical origins of life. In a booklet' 

entitled Proiskhozhdenie Zhizny, he pointed out that the complex 

combination of manifestations and properties so characteristic of 

life must have arisen in the process of the evolution of matter. 

A. Oparin had learned Mendeleev's ideas on the possible origin of 

hydrocarbons from the carbides in the crust of the Earth, and 

injected into his own thinking a new notion concerning the reducing 

nature of the early atmosphere. To Oparin's great credit, this observation was made before the astrophysicists had realized that the stars 

were 90% hydrogen. 
17 

In 1928, 1. B. S. Haldane, the British biologist, independently 

of Oparin, wrote a classic paper, "The Origin of Life." Haldane 

speculated on the early conditions suitable for the emergence of life. 

According to him, when UV light acted upon a mixture of water, 

carbon dioxide, and ammonia, a variety of organic substances were 

made, including sugars and apparently some of the materials from 

which proteins are built up. Before the origin of life they must have 

accumulated until the primitive oceans reached the constituency of a 

hot, dilute soup. Haldane gave us the concept of the "primordial 

soup. " 

Almost 20 years after Haldane's publication, 1. D. Bernal of the 

University of London conjectured before the British Physical Society 

in a famous lecture entitled The Physical Basis of Life that clay surfaces were involved in the origin of life. He was looking for ways and 

means by which the primordial molecules in the hot, dilute soup 

could be brought together to give rise to polymers capable of replication. A physicist and crystallographer by training, Bernal was particularly attracted to the role of surface phenomena in the origin of life. 

He argued that favorable conditions for concentration, which may 

have taken place on a very large scale, were provided by the adsorption of organic molecules on the fine clay deposits. The role of clay 

in primordial organic synthesis is today a lively area of investigation. 

THE EXPERIMENTAL ERA: ORIGINS ENTER THE 

LABORATORY 

In the early fifties, the Oparin-Haldane hypothesis, enriched by 

Bernal's ideas, was being widely discussed. Many experimenters were 

interested in putting it to the test. We must, however, recognize 'that 

over a long period of time innumerable experiments had been performed to generate organic molecules of biological interest. For 

example, Haber, in 1917, exposed a mixture of gases to electric discharges and postulated that any compound which could plausibly be 

synthesized would come out of such a system. (Rabinowitz, 1945, 

narrates many other examples, although they were not undertaken 

with the set purpose of studying the origin of life.) 

The first reported experiment in the series of investigations to 

test the hypothesis of chemical evolution was done at Berkeley in 
18 

1950 by Calvin and his associates. The availability of radiocarbon 14 

and the Berkeley cyclotron provided the ideal tools for such an 

experiment. A mixture of carbon dioxide, water, and hydrogen was 

exposed to 40 Me V helium ions from the 60-in. cyclotron at 

Berkeley. (Unfortunately no nitrogen was added to the mixture of 

gases irradiated.) A curious fact about this experiment was that 

although they referred to Oparin's idea of a reducing atmosphere, 

they used an oxidized source of carbon. Among the products identified were formaldehyde and formic acid. 

Soon after the results of this experiment were reported in 

Science in 1951 , there appeared a detailed paper by Harold Urey on 

the early chemical history of the Earth's atmosphere. In this paper 

Urey clearly defmed the conditions under which a primitive atmosphere may have arisen and argued from the abundance of hydrogen 

in the universe, the rate of escape of the lighter elements, and equilibrium constants that the early atmosphere must indeed have been 

reducing. This paper was soon followed by the now-celebrated 

experiment by Urey's graduate student, Stanley Miller, in which 

methane, ammonia, and water were exposed to electric discharges 

(fig. 11-2). Among the compounds formed and identified were four of 

the amino acids commonly found in protein: glycine, alanine, 

aspartic and glutamic acid. 

Over the years, although new evidence on atmospheric outgassing and the geological conditions of the early Earth has led us to 

reconsider our thinking on the true nature of the Earth's primitive 

atmosphere, Urey and Miller had set the pattern for most of the 

synthetic experimental work in chemical evolution. 

THE SPACE AGE 

The scientific inquiry into the ongm of life soon began to 

receive worldwide attention. In 1957, under the sponsorship of the 

International Union of Biochemistry and the leadership of Oparin, 

who was at that time its vice president, an international conference 

on the subject was convened at Moscow. At this meeting a number of 

biochemists were gathered who reported on their theoretical or 
19 

Figure 11-2.- An apparatus used to produc,! amino acids from methane, 

ammonia, and water by electric discharge. 

experimental investigations on the origin of life. The mammoth volume of 691 pages bears testimony to the extent of the work and the 

depth of interest of the international scientific community. 
20 

With the establishment of the U.S. space program in 1958, a 

dedicated effort in space biology began to emerge. In an authoritative document, the Space Science Board of the National Academy of 

Sciences declared that the search for extraterrestrial life was a prime 

goal of space biology. "It is not since Darwin and, before him, 

Copernicus, that science has had the opportunity for so great an 

impact on the understanding of man. The scientific question at stake 

in exobiology is the most exciting, challenging, and profound issue 

not only of the century but of the whole naturalistic movement that 

has characterized the history of Western thought for over 300 years. 

If there is life on Mars, and if we can demonstrate its independent 

origin, then we shall have a heartening answer to the question of 

improbability and uniqueness in the origin of life. Arising twice in a 

single planetary system, it must surely occur abundantly elsewhere 

in the staggering number of comparable planetary systems." 

In 1963, the second conference on the Origin of Life was held 

at Wakulla Springs, Florida, under the sponsorship of the National 

Aeronautics and Space Administration. Much progress had been 

made since 1957. Many of the molecules of biological significance 

had been synthesized. The pathways for their chemical origin had 

been outlined, the conditions of reaction well defined, and the analytical techniques developed to a high degree of refinement. 

By now, primarily with well-planned support from the Space 

Sciences Division of the National Aeronautics and Space Administration, several laboratories across the world were engaged in sophisticated experimental programs related to the origin of life and to life 

beyond the Earth. Papers on the subject appeared in journals as 

diverse in discipline as microbiology and astrophysics. The landing 

of a man on the Moon and the availability of lunar samples for analysis intensified the geochemical aspects of the program. 

The year 1970 appeared a most appropriate time to organize 

the Third International Conference on the Origin of Life. This 

meeting was held at Pont-a-Mousson, France, about 250 km east of 

Paris. Over 150 researchers from several countries around the world 

were present. At this meeting was also born the International Society 

for the Study of the Origin of Life, with Alexander Oparin as its first 

president. 
21 

An accidental event of considerable importance at about this 

time was the fall of the Murchison meteorite in Australia. The techniques developed for the analysis of the lunar samples were soon 

applied to this fresh carbonaceous chondrite. Although meteorites 

had been analyzed before this fall, the finding of equal amounts of 

D- and L-amino acids and nonprotein amino acids in a-meteorite 

provided unambiguous evidence of nonbiological and indigenous 

organic compounds for the first time. Pre biotic organic matter had 

been discovered in the solar system. 

In 1973, the Fourth International Conference of the Origin of 

Life was held at Barcelona, Spain. The subject of chemical evolution 

had come of age. A quarterly journal, Origins of Life, devoted to 

the interdisciplinary approach to the subject, has been published 

since 1974. The Journal of Molecular Evolution and Biosystems 

often publish papers on the biochemical aspect of the question. 

Precambrian Research highlights those aspects related to the Archaen 

sediments. 

The pace of research has quickened. The Viking Mission to Mars 

gave man the first opportunity to sample the soil of another planet 

and search for clues for life. The results of the Martian program were 

carefully analyzed at the Fifth International Conference on the 

Origin of Life which was held in Kyoto, Japan, in 1977, and, at the 

sixth meeting in Jerusalem in 1980. 

Since that time, the growing interest in the subject has been 

demonstrated by the number of scientists from different disciplines 

and from different countries who have joined the Society for the 

Study of the Origin of Life. Apart from the international meetings 

held every 3 years, regional meetings in the United States, Europe, 

Japan, and India keep alive the expanding interest in the subject, of 

fundamental importance to all science. 

SUGGESTIONS FOR FURTHER READING 

Bernal, J. D.: The Origin of Life. World Publishing Co., Cleveland, 

Ohio, 1967. 

Haldane, J. B. S.: The Rationalist Annual, 1929. 

Oparin, A. I.: The Origin of Life on Earth, Third edition. Academic 

Press, New York, 1957. 

23 

III. THE TWO RECORDS : 

IN THE ROCKS AND IN THE CELLS 

From the perspective of the long sweep of human history, over 

the past two centuries or so, less than a dozen human generations, 

scientists have begun to study, and to comprehend, the history oflife 

on this planet. Progress has been impressive - nearly 200,000 species of fossil organisms have been discovered and described ; the 

evolutionary continuum that links life of the modem world to that 

of earlier biotas has been extended far into the geologic past; and 

great strides have been taken toward deciphering the timing and 

nature of the major events in the development of life on Earth. 

Early investigators interested in the history of life concentrated 

on those problems most readily amenable to investigation, and 

already by the mid-1800s when Darwin's On the Origin of Species 

first appeared, the broad outlines of the history of animals and 

plants were rapidly coming into focus. Indeed, it is this fossil 

record - that of the "Phanerozoic Eon" of Earth history, the ages of 

trilobites, coal swamp flora, dinosaurs, and the like - that normally 

comes to mind when one thinks of the history of life on Earth. Yet 

the Phanerozoic extends only a scant 600 m.y. into the geologic past 

(fig. III-I). But this is only 15% of geologic time. What came before, 

during the first 85%? How long before the advent of visibly large life 

did the earliest organisms first appear? And what does the geologic 

record reveal about the origins of life itself? These questions and 

others like them have long been pondered - indeed, Darwin regarded 

their solution as a necessary prerequisite to ultimate acceptance of 

his theory of evolution - but it has only been within the past quarter 

Finely layered stromatolitic structures, of bacterial and/or 

blue-green algal origin, from Early Precambrian rocks near 

Bulawayo, Rhodesia, about 2600 million years in age. These 

structures are among the oldest stromatolites - and thus 

among the oldest fossil evidences of life - now known. 

Photo courtesy of J. W. Schopf 
24 

OLDEST 

SEDIMENTARY 

ROCKS (GREENLAND) 

FORMATION 

OF EARTH 

4000 

4500 

Figure III-l.- A summary chart showing the known geological distribution of 

Precambrian fossil microorganisms (microfossils) and stromatolites. Photo 

courtesy J. W. Schopf 
25 

century or so that they have also proved amenable to scientific 

inquiry. Here, too, progress has been impressive - in recent years the 

known fossil record has been extended further and further into the 

remote past. The oldest known fossils are about 3.5 b.y. old, an age 

approaching, but markedly less than, the oldest rocks (approximately 

3.8 b.y. old) on Earth, only one billion or so years younger than the 

age of the planet itself. 

THE AGE OF THE EARTH 

Knowledge of the age of the Earth is of fundamental importance to our understanding of the time when life originated. To fix 

the age of the Earth, one must determine the isotopic abundances of 

certain elements. 

The isotopic composition of lead (Pb) at the time of formation 

of the solar system can be accurately determined by measurements 

of Pb in certain undifferentiated meteorites rich in water, carbon 

dioxide, and other volatile materials. Assuming the mantle source of 

terrestrial lead has always been a well-mixed reservoir containing 

uranium (V) and thorium (Th), decay of 238V, 235V, and 23 2Th will 

enrich this primordial lead in the radiogenic daughter isotopes to the 

extent observed in modern samples of lead from Earth's mantle in a 

time of 4.43 b.y. Measurement of lead samples of various ages, 

extending back to 3.8 b.y. ago, shows that the assumption of a 

well-mixed reservoir is a good, but not perfect, approximation. 

Correction of the calculated age of the Earth for the observed deviation from a uniform source leads to an age about 100 m.y . older; i.e., 

4.53 b.y. 

Accurate measurements by V-Pb, and other isotopic clocks in 

meteorites, yield 4.55 ±0.02 b.y. for the time of formation (or differentiation) of their parent bodies. Measurement of the decay products 

of extinct radioactivities; e.g. aluminum (26 AI) and iodine e 29I), 

shows that those bodies in the solar system from which the meteorites were formed are within about 1-100 m.y. of the time at which 

the forming solar system itself was produced. 

Formation of the Sun and planets post-dates this time. Thus, 

the age of the Earth is firmly bracketed between 4.65 b.y. and 
26 

4.43 b.y. The complete accumulation of the Earth is thought to have 

taken somewhat longer than that of the meteorite parent bodies, and 

it is therefore unlikely that the Earth is older than 4.55 b.y. Most 

likely the "corrected" lead isotope age of the Earth (4.53 b.y.), as 

discussed above, is within 50 m.y. of the time at which the Earth was 

formed and differentiated into silicate mantle and iron core. The real 

uncertainty in this figure could easily be 40 m.y., but is unlikely to 

be as large as 100 m.y. A value and range of 4.50 ±O.l b.y. would 

seem conservative. This implies at least 0.5 b.y. - indeed, something 

like 0.6 or 0.7 b.y. - between (1) the formation of the Earth as a 

solid differentiated planet and (2) the oldest known rocks. 

VISIBLE TRACES IN THE ROCKS 

What then is known about the history of life on Earth? What 

were the trends that shaped the course of evolution, and over what 

time periods did they occur? 

Two generalizations are clear. First, the history of life on Earth 

is on the whole the history of microscopic, rather than of visible, 

organisms. Because of our naturally anthropocentric myopia, as well 

as the relative ease with which fossils of the larger plants and animals 

can be discovered and studied, the history of organisms large enough 

to be seen with the unaided eye has received a disproportionately 

large share of scientific attention. The real situation is vastly different. Indeed, it is now known that the Earth's biota was composed 

solely of microscopic forms of life and their colonies for nearly 85% 

of the total history of life on this planet. All larger organisms are by 

comparison recent additions, interesting and significant, but they 

have been preceded by a long and well-developed evolution of 

microscopic forms. 

Second, the type of life inhabiting the globe and the nature of 

evolutionary trends through time depend upon the peculiar geology 

of our planet. Earth, unlike all other known bodies of the solar system, is an aqueous planet, some 71 % of its surface being covered by 

a thin, watery veneer. It is thus not surprising that liquid water 

(H2 0) is the major component of every known form of life. Water is 

the "universal solvent," the fundamental medium without which life 
1-

27 

as we know it could not occur. As a necessity for life, then, water 

serves also to limit life, and one of the principal themes that characterized the evolution of both plants and animals during Phanerozoic 

time was the development of structures and biochemical processes 

that enabled these life forms to spread to the land surface where 

water was in short supply. Indeed, many of the major evolutionary 

innovations of the Phanerozoic concerned the relations between 

water and life, such matters as the developments of lungs and of 

hardshelled eggs in animals, and the origin of seeds and of specialized 

pollenation mechanisms in plants, both once novel biological solutions to the scarcity of water on land. 

Another important generalization concerning the fossil history 

of life is its unevenness. Some portions of the record are well documented and understood, but others are nearly unknown. It is a general, but not a perfect rule, that the older the material, the poorer 

the record. For markedly different phases of the fossil record , 

"eons" can be recognized: 

1. The classical fossil phase: The Phanerozoic Eon, extending 

from about 600 m.y. ago to the present, is far better understood 

than any of the earlier phases; literally thousands of richly fossiliferous deposits of this age are known, units that collectively provide a 

sound and rather detailed basis for understanding the major aspects 

of the history of life. 

2. Before the classical fossils: The Proterozoic Eon, extending 

from about 2500 m.y. ago to about 600 m.y. ago, when the Phanerozoic began, is understood only in outline; the total fossil record now 

known consists of three kinds of deposits: (a) There are about a 

dozen latest Proterozoic (690 to 600 m.y. old) fossiliferous deposits 

with large organisms primarily as sandstone impressions. (b) There 

are also about 150 microfossiliferous deposits (fig. 111-2), spread 

somewhat unevenly throughout the eon (very roughly, 10-25 deposits per 100 m.y. during the later Proterozoic and only 1-5 deposits 

per 100 m.y. within the earlier portion of the eon). And (c) there are 

hundreds of limestones and dolomites that contain the structures 

called stromatolites. These structures are layered, commonly moundshaped, sedimentary rocks. They were built over time through the 

growth and metabolic activities of whole communities of microscopic organisms; but, with rare exceptions, they do not contain 

fossil remnants of the bodies of the microorganisms responsible for 
L 

28 

Figure III-2.- Prokaryotic microfossils.. about 850 m.y. old (Late Precambrian) 

from carbonaceous black chert of the Bitter Springs Formation of central 

Australia. 
29 

their construction. They are traces, rather than fossil organisms 

themselves. Research in this phase is vigorous and ongoing; over the 

past two decades it has resulted in substantial increases in our knowledge about Proterozoic evolution. 

3. The ancient phase: The later portion of the Archean Eon, 

extending from 3.5 to 2.5 b.y. ago, is very poorly known. Here, the 

whole fossil record consists only of some seven or eight stromatolitic deposits, and of a few units known to contain microfossils or 

suggestive microfossil-like objects. Indeed, only one diverse, cellularly well-preserved micro biota has as yet been detected in rocks of 

this age. The oldest assured, bona-fide records of life now known are 

those contained in the sediments of the Warrawoona Group of 

Western Australia (fig. 111-3). Those rocks are approximately 3.5 b.y. 

Figure 111-3.- Filamentous, prokaryotic (i.e., bacterium-like) microfossils in 

petrographic thin sections of carbonaceous chert from the Early Precambrian 

(ca. 3.5 b.y. old) Wa"awoona Group in the North Pole Dome region of the 

Pilbara Block, northwestern Western Australia. Photo courtesy of f. W. Schopf. 
30 

old; evidence indicates that diverse types of microorganisms, probably including photosynthesizers, were already extant at that time. 

4. The most ancient phase: The Early Archean (Hadean), 

extending from the time of formation of the Earth, 4.S b.y. ago, to 

about 3.S b.y. ago, is all but unknown; only one rock unit has as yet 

been detected in this oldest portion of the geologic column, and that 

unit (the Isua Supracrustal deposits of western Greenland) has been 

severely altered by postdepositional motion, heat, and pressure. 

Indeed , the sequence has undergone at least five stages of alteration, 

including severe metamorphism (technically of amphibolite-grade, 

viz., to SOOO -6000 C, and 2-8 kilobars), a sequence of events so 

severe as to make meaningful interpretation of the Isua "fossil 

record" virtually impossible (if, in fact, organisms that could give rise 

to a fossil record actually existed in Isua time). Isua has been studied 

mainly over the last S years. 

Thus, the fossil record as now known - rich and varied as it is 

in younger rocks, and scant to nearly nonexistent in older rocks -

provides only the most limited insight into the timing and nature of 

those events that led to the origin of life. The evidence establishes 

that communities of complex, diverse microorganisms were extant at 

least as early as 3.S b.y. ago, and that these organisms resembled 

modern bacteria in morphology, in ecology, and perhaps in biochemistry. Certainly, the origin of living systems must have occurred some 

time earlier, perhaps long before 3.S b.y. ago. When it occurred, and 

how long the origin-of-life event took, is certainly not known. 

THE CARBON CHEMISTRY OF ANCIENT ROCKS: 

AN OPPORTUNITY 

As just discussed, rocks more than a few billion years old are 

scarce, not because only few of them were made but because their 

survival has been difficult. The examples that we have are, quite 

literally, battered veterans. Even the layered pattern of stromatolites 

rarely remains. If the rocks themselves have been mostly destroyed 

or significantly altered, it follows that their organic molecular constituents, which are much more fragile, must be in relatively poorer 

shape. Delicate information-rich biological macromolecules have not 

a chance of survival intact. On the other hand, not all organic matter 
31 

has been so drastically altered that the study of organic geochemistry 

of ancient rocks is reduced to the study of plain graphite. There is a 

continuum between the extremes of "DNA" and "graphite." Sedimentary organic materials must inevitably move along this continuum as time progresses. As we move back more than 2.5 b.y. before 

the present into the Archean, the question arises as to what we will 

fmd first: The origin of life? Total degradation of all organic material 

to uninformative graphite? Or the end of the record? For at least 

15 years, every generally accepted minimum date for the appearance 

of life on Earth has been based on morphological evidence alone 

(stromatolites, microfossils), i.e., visible or microscopic traces, not 

molecular ones. Chemical analyses have been viewed as suggestive, 

but not compelling. The approach suggested here acknowledges the 

preeminence of such morphological evidence when it comes to a yes 

or no question about the existence of life, but then works toward 

more insight. Specifically, if micropaleontologists can show whet:e 

life existed, then perhaps organic geochemists can further support 

that interpretation and can determine what kind of life it might have 

been. 

The most important advance in organic geochemistry during the 

past 10 years has been the emergence of a coherent view of the 

nature and role of the substance called kerogen. This material is a 

blackish and insoluble macromolecular complex dispersed in sedimentary rocks and comprising the great majority of organic matter in 

sediments. It has effectively resisted many efforts at detailed macromolecular structural analysis, presumably because it lacks the regular 

structure like coal or the asphaltene particles in petroleum. But it is 

now recognized as a product of geochemical reactions occurring over 

a very long period of time. "Protokerogen," an organic substance 

composed mainly of cellular degradation products, is formed today 

by living microbial communities in modem sediments. It is added to 

and modified by a series of reactions grading from the microbiological through the high-temperature geochemical. The present level of 

understanding is not complete and does not fully extend into the 

Precambrian, but it raises questions regarding chemical fossils and 

points the way to useful investigations of kerogens. 

The times of the origin of photosynthesis, of respiration, of 

other biochemical processes of great ecological significance are not 

now known. Morphological evidence alone is unlikely to be decisive 
32 

in fixing these dates, but it does seem possible that both chemical 

and isotopic investigations might be of substantial use. Kerogen 

analyses might follow an approach already highly successful in other 

fields of geochemistry and concentrate on determinations of stable 

isotopes of the elements present. It seems likely, for example, that 

any shift in the mode of primary production of polyatomic carbon 

molecular skeletons - chains and rings and the like - would have 

been accompanied by some shift in the ratio of light to heavy carbon 

isotopes within the reduced organic matter. 

Whatever else they might do, kerogen analyses must deal with 

the fact that differences observed between specimens are as likely to 

be due to postdepositional effects as to differences in the original 

communities. The mineralogy, as it pertains to the origin and evolution of the rocks, and the structural geology of a given rock unit 

must be considered in addition to its organic geochemistry if this is 

to be done adequately. Such investigations are becoming more frequent, with many investigators realizing that the methods they 

choose should be designed both to decode the chemical message 

which might describe the original community and to assess the state 

of preservation of that message. 

It seems appropriate in the study of microfossils with single 

structural morphology, such as most of those found in the Archean, 

to seek chemical-supporting data to establish their biological origin. 

Combined electron microprobe-scanning, electron microscope systems can now detect C, N, 0, and P in micron-sized objects within 

reasonable limits of error. Micropaleontologists, using, e.g., microprobe techniques capable of detecting elements with atomic numbers 

as low as carbon, may be able to resolve details of the original 

organisms that left their remains. 

Just the same, Precambrian paleobiology has now made notable 

advances. These include the certain great antiquity of now uncontested stromatolites, the wonderful Ediacaran fauna, the Bitter 

Springs, Gunflint, Transvaal, Belcher Island, and Fig Tree microfossils, and the oldest "North Pole" fmds from W. Australia. Some of 

these deposits contain an abundance of well-preserved forms, e.g., 

Gunflint, Bitter Springs; others contain only relatively few forms, 

most of which are broken "debris," e.g., Transvaal. What is noteworthy is that these micropaleontological and organic geochemical 

findings can be related in some degree to living analogues. 
33 

The pursuit of kerogen investigation, together with related biological and geological studies, seems very likely to fulfill its promise. 

Organic geochemists now appear to have a good chance to make significant contributions, less to the blunt question "when did life 

arise?" than to the much more detailed set of questions dealing with 

the biochemical natures of the ancient communities once they are 

disclosed to the searching paleontological eye. 

THE RECORD IN LIVING FORMS 

While the paleontologist studies the fossils that the old organisms left in the rocks, the biochemist would like to study the biological processes in those earliest organisms. The descendants of those 

ancient organisms are alive today, and they, with their molecular 

traditions from the past, can be the subject of the biochemist's 

investigations. Some of these traces exist in a form much changed 

from the original, while others appear to have been handed down to 

generation after generation, practically unchanged. One important 

simple tenet of evolutionary theory is this: If several organisms are 

found to share a certain trait, that trait was most likely inherited 

from a common ancestor rather than evolved independently on 

several separate occasions. But certain important features are now 

shared in identical form by every organism extant. For instance, all 

life contains one set of amino acids, the building blocks of proteins, 

and one set of nucleotides, the building blocks of nucleic acids (ribonucleic acid (RNA) and deoryribonucleic acid (DNA), the genetic 

material). The main features of the complex system which serves to 

pass genetic information from one generation to the next are shared 

by all organisms. That is a clean example of what biochemistry can 

tell us about early common ancestors of all life: They contained 

nucleic acids, spelling out a genetic code for proteins, and they passed 

these down to all life forms present today. The random reorderings 

of amino acids and nucleotides observed in functionally related 

polymers must have evolved over the eons through various shuffling 

and mutational events. Locked in the arrangement of the monomers 

in various polymers in the cells of organisms is a coded record of the 

evolution of these organisms. The key to reading this record lies in 
34 

modern methods for "sequencing" or determining the detailed 

arrangement, one after the other, of nucleotides in nucleic acids and 

the amino acids in proteins. Since the information for the sequence 

of amino acids in proteins is carried in the nucleic acid hereditary 

material, the sequences of both these polymer classes reflect evolutionary changes handed down from organism to organism. Sequencing the proteins or the nucleic acids allows the estimation of genealogical relationships among organisms presently alive. We can trace 

relationships among a group of organisms back to a common ancestor of that group that lived hundreds of millions or even billions of 

years ago. 

The way this is done is to compare the sequences of related 

proteins or nucleic acids from a number of organisms. The degree of 

difference or diversity between these sequences is determined. The 

information concerning the degree of diversity among sequences can 

be used to estimate the order and perhaps the relative times of 

divergence of species from their ancestral relatives (fig. 1I1-4). We can 

estimate how old a particular family of organisms is by determining 

how diverse the sequences within chosen molecules are among its 

member species. If a family exhibits a relatively high level of 

sequence diversity, it is held to have existed as a group for a relatively long time. 

THE KINGDOMS OF LIFE 

Genealogical relationships traditionally have been defined by 

such characteristics of organisms as shape, photosynthetic ability, 

and mode of cell reproduction. Now we use the biochemical record 

to define groups in terms of shared genetic information. For many 

years, all life was held to be divided into ' only two major kingdoms: 

the plant kingdom, studied by botanists, and the animal kingdom, 

studied by zoologists. More recently we recognized that all plant and 

animal cells exhibit fundamental properties not shared by bacterial 

cells. The present view groups all cellular life into two major divisions: the prokaryotes and the eukaryotes. Prokaryotic cells, all bacterial, are generally small, simple, relatively undifferentiated cells. 

Eukaryotic cells, which make up all the plants and animals, fungi, 
35 

PAST~'----------------------------------------~. PRESENT 

TIME OF DIVERGENCE 

Figure 111-4.- Genealogical trees can be deduced from molecular-sequence 

information. Each box in the figure represents an organism, and the numbers 

indicate how many of the building blocks of a 200-block-long molecule in the 

organism are in the same arrangement as this molecule in organism A. Thus, the 

molecule in question in organism B has many building blocks (190) in an 

arrangement identical to organism A's molecule. This indicates they diverged 

from one another relatively recently. Organism D's molecules, on the other 

hand, has fewer blocks ( 1 00) in the same order as organism A's, indicating that 

they diverged from one another a long time ago. Organism C exhibits an intermediate time of divergence. 

and algae, are generally larger, more complex, and internally differentiated (containing several types of internal organelles which themselves are related to the free-living bacteria). 

The Eukaryotes 

One evolutionary puzzle that has responded to the biochemists' scrutiny is that of the origin of the eukaryotic cells (the 

nucleated cells of all larger forms of life). The currently accepted 
36 

theory of the origin of these cells (the serial endosymbiotic theory) 

postulates that they arose by symbiotic association between some 

unknown pre-eukaryotic cells (urkaryote) and certain types of 

prokaryotes, then free-living (fig. III-5). These symbioses gave rise to 

structures recognizable today as intracellular organelles, introduced 

in one step, not incrementally evolved. The organelles of eukaryotes, 

thought to have originated from living bacteria, are mitochondria, 

which react with chemical substrates and atmospheric O2 to produce 

chemical energy within all types of eukaryotic cells, and the plastids, 

subcellular sites of photosynthesis which convert radiant energy from 

light to chemical energy in plants and algae. All these quite complex organelles have their own nucleic acid genetic material, and they 

produce their own distinct proteins. The sequence of some protein 

molecules in organelles has been compared to that of similar proteins 

in extant prokaryotes. The amino-acid sequences of certain proteins 

in the plastids which have been examined indicate a close relationship 

to certain prokaryotes, particularly to the photosynthetic oxygenproducing cyanobacteria and prochlorons. The relationship of the 

r-------~ PRESENT DAY 

CYANOBACTERIA f------l 

ANCESTOR 

I URKARYOTE I O~---}-. ~'I 

PURPLE BACTERIA 1-------1 

ANCESTOR 

CYANOBACTERIA 

CHLOROPLAST 

PRESENT OAY 

PLANT CELL 

MITOCHONDRIA 

'"------ .. 1 PRESENT DAY 

PURPLE BACTERIA 

Figure III-S.- A schematic illustration of the endosymbiotic theory of the origin 

of plant cells. Bacterial entities from the cyanobacteria (blue-green algae) group 

and the purple bacteria group were engulfed by an urkaryote and formed today's 

chloroplasts and mitochondria, respectively. Animal cell mitochondria probably 

have a similar origin, although their ancestry has not been clearly established. 

I 

':
37 

mitochondria to prokaryote cells is not as clear, though the mitochondria exhibit a relationship to the purple bacteria. As a whole, 

though, the information contained in the biochemical record to date 

strongly supports the endosymbiotic theory of the origin of eukaryotic cells. Eukaryotes can be seen as evolutionary mosaics, depending 

on contributions joined from several distinct lines of descent. 

The Prokaryotes 

Study of nucleotide sequences (l6S RNA) in a large number of 

bacteria has lately revealed their enormous biochemical diversity. 

One group is called the eubacteria (true bacteria) and another called 

the archeabacteria (old bacteria). Still a third group, called the 

urkaryote, is represented by that part of the eukaryotic cell which is 

external to the organelles. The eubacteria and archeabacteria are no 

more closely related to each other than each is related to the third 

component, the cytoplasm. There are a great number of extant 

eubacterial species, but relatively few known archeabacterial species. 

Despite the paucity of extant species, the archeabacteria appear to be 

an ancient group which exhibit as much diversity as the eubacteria. 

Its members are generally restricted to unusual niches in the environment, hinting that the group might have originated during a period of 

Earth's history when prevailing conditions were different than they 

are today. The two bacterial groups together reveal greater biochemical diversity than all the extant eukaryotes. 

The Antiquity of Biochemical Traits 

The biochemical record can also yield inferences about the 

traits or phenotypic features possessed by very early organisms. A 

trait is an unlikely candidate for an ancient phenotype if the groups 

of organisms which now share the trait have all diverged from ancestral lines which did not possess it. A trait is probably not very old 

also if all extant organisms which possess it are closely related in 

terms of their biochemical record, indicating a recent time of divergence from ancestral lines. Conversely, if a trait is shared by groups 

so diverse that their biochemical record indicates a very ancient 

relative time of divergence, that trait may well have been possessed 

by very early organisms. 
38 

The first trait about which we have evidence concerns oxygen 

utilization by early organisms. There is some geological evidence indicating that on the very early Earth (Archean Eon) oxygen was much 

less abundant in the atmosphere. It is generally believed that significant quantities of oxygen first accumulated as the result of biological 

photosynthesis, a process in which free oxygen is a by-product. It is 

therefore thought that the first organisms must have been nonoxygen 

users, or anaerobes. The biochemical record supports this belief. 

Based on sequencing of nucleotides, anaerobic eubacteria are ancient 

compared with their aerobic counterparts. The major groups of the 

eubacteria are basically anaerobic, and the aerobic phenotype 

appears to have arisen relatively recently several times from various 

groups of anaerobic organisms. 

Recent studies have shown that oxygen-using enzyme systems 

in eukaryotic organisms require orders of magnitude less oxygen to 

function than that in the present atmosphere. These findings suggest 

that aerobic biochemical processes may have arisen earlier than widespread aerobic life. 

Even more fundamental is the nature of the energy source of 

the oldest organisms for which we have some clues. The biochemical 

record of the eubacteria has been examined to seek an origin for 

photosynthesis as a cellular trait, and suggests that photosynthesis is 

indeed ancient. Major eubacterial groups are photosynthetic; e.g., the 

purple photosynthetic bacteria, the cyanobacteria (blue-green algae), 

and green sulfur photosynthetic bacteria. Furthermore, nonphotosynthetic phenotypes have arisen several times from lines already 

photosynthetic (fig. lII-6). For instance, the nonphotosynthetic, 

common, human intestinal bacteria, E. coli, most likely arose from 

the group of purple photosynthetic bacteria. 

The usual view has been that the oldest forms of life were nourished not by internal biological photosynthesis but as heterotrophs 

that were supplied with energy by a rich environment, where organic 

nutrients that were abundant, were produced by processes which preceded life. We can find no sign of that early phase in the biochemical 

record among living cells. Photosynthesis in living cells goes back as 

far as the earliest groups of eubacteria. The lineages of the bacterial 

cells we know do not indicate whether heterotrophic or autotrophic 

life was first on the scene. 
39 

CHROMATlUM, A GROUP OF PURPLE 

PHOTOSYNTHETIC BACTERIA 

WHICH USE SULFUR 

..-- E. COLI, A NONPHOTOSYNTHETIC 

COMMON HUMAN INTESTINAL 

BACTERIA 

RHODOPSEUDOMONAS, A GROUP OF 

PURPLE PHOTOSYNTHETIC 

BACTERIA WHICH DO NOT USE 

SULFUR 

- ANCIENT RHIZOBIUM, A GROUP OF NON· 

ANCESTOR PHOTOSYNTHETIC BACTERIA 

OF PURPLE WHICH ARE AN IMPORTANT SOURCE 

BACTERIA OF A FORM OF NITROGEN FOR 

SOME CROPS 

RHODOSPIRILLUM, A GROUP OF 

PURPLE PHOTOSYNTHETIC 

NON·SULFUR BACTERIA 

ALCALIGENES, A GROUP OF 

NON·PHOTOSYNTHETIC BACTERIA 

Figure 111-6,- Nonphotosynthetic organisms arose from photosynthetic organisms a number of times. This figure shows part of the genealogical tree of the 

purple photosynthetic bacteria drawn in the same way as the top of figure III-4. 

The bacteria known as E. coli, Rhizobium and Alcaligenes are all nonphotosynthe tic, yet they appear to have independently arisen from the purple photosynthetic bacteria line of descent. This figure also illustrates the finding that aerobic 

lines appear to have arisen from anaerobic ancestry. E. coli, Rizobium, and 

Alcaligenes are aerobic, while the rest of the group is anaerobic. 

Finally we ask about that eukaryote component which is outside of all the cytoplasmic organelles - the nucleocytoplasm. It is 

quite distinct in its sequencing from both bacterial lines. That allows 

the idea that this urkaryote is the most ancient ancestral form we 

see, though it does not require that. We expect to fmd in that form 

only the features which the eubacteria, archeabacteria, and urkaryote share in common, and not much more. Those features present 

include of course the amino acids and their polymers as a class, the 

proteins; and the nucleotides and their polymers, the nucleic acids. 
40 

Apart from those generalities, the details of all three groups differ. 

The proteins, especially enzymes and their consequent metabolic 

pathways, the cell walls, and most of the membranes differ. This 

suggests that the three great groups diverged at the earliest stage, 

with not much but the building blocks, the genetic code, and its ribosomal machinery in common. 

More than that we have not been able to read from the details 

of the similarities of living forms. Perhaps we should have expected 

no more. For all the forms we know now are cellular, compact; the 

earlier forms we are looking for have still to depend for some necessity upon features of the nonliving environment, whether it be for a 

source of chemical free energy, the means of replication and variation, or the simple preservation of high concentrations of key constituents. All the primitive cells, we infer, are microbial, enclosed, 

and can replicate using the full normal apparatus of bacteria; they 

derive free energy either from some part of incident sunlight or from 

an organic-rich world or both. There is still a big gap to explore. For 

that we must turn to the laboratory. 

SUGGESTIONS FOR FURTHER READING 

L. Margulis, Symbiosis in Cell Evolution, W. H. Freeman Co., 

San Francisco, 1981. 

J. W. Schopf, editor, The Earth's Earliest Biosphere, Princeton University Press, 1983. 

C. W. Woese, Sci. Amer., "Archaebacteria," vol. 244, no. 6, 1981, 

pp.98-121. 


43 

IV. THE NATURAL EVIDENCE 

In chapter III we have seen how the rock record provides information about life on the early Earth. And we have seen that this 

record leaves us a gap in time from about 3.8 b.y. ago to the time of 

Earth's origin at 4.5 to 4.6 b.y. ago. We have also discussed in chapter I the essential elements of life. But what were the conditions 

under which life originated? What was the early Earth like? To 

understand this we seek additional knowledge : we must know how 

the solar system formed and how the Earth formed from it. In addition, we must know the chemical behavior of carbon and its compounds in the prebiological environments. Only then will we have the 

knowledge necessary to bridge the gap and understand how life came 

to be. 

THE BIRTH OF THE SOLAR SYSTEM 

Before there were any stars there was only gas, and this gas was 

essentially just a mixture of hydrogen and helium. But by the time 

the Sun appeared, several previous generations of stars had added 

other heavier elements to this interstellar gas as a result of the synthesis of elements in stellar interiors, and in the catastrophic explosions of supernovae. We find evidence for this change in composition 

by examining the various types of stars in the galaxy. The oldest stars 

we find today (having formed at an earlier epoch) contain a smaller 

amount of heavy elements than does the Sun. The age of the galaxy 

is estimated to be close to 12 b.y., while the oldest stars with heavyelement concentrations similar to the Sun's have ages of only 6 to 

7 b.y. The Sun itself is about 4.6 b.y. old. 

Today we can examine environments where stars form and then 

try to reconstruct the conditions that preceded the appearance of the 

Sun and its retinue of planets. We find these environments among the 

The solar magnetic field (white spiral lines) as it might have appeared 4.5 b.y. ago when created by the high spin rate of the Sun. This would have produced 

melting of rocks in the Asteroid Belt and elsewhere in the Solar System. 
44 

clouds of gas and dust in the spaces between the stars: the interstellar medium. As we explore these clouds, we must make some allowances for the further changes that have occurred since the Sun was 

born; however, most of what we find today appears characteristic of 

conditions that must have existed 4.6 b.y. ago. 

This exploration, still in its infancy, is yielding a continuous 

stream of new information about the chemical composition, mass, 

and distribution of these clouds, and about the relationship of different types of interstellar clouds to the process of star formation. It is 

already clear that the sheer mass of the low-density interstellar 

material dictates that most of the chemistry that goes on in the 

Universe takes place in interstellar clouds. 

If the material in the clouds were spread uniformly over all 

space, the concentration of matter would amount to about three 

hydrogen atoms per cubic centimeter. We consider two basic types of 

clouds: (l) diffuse clouds, which contain little dust, and where concentrations of gas molecules are very low and single hydrogen atoms 

are the dominant species; and (2) dark, dense clouds, which contain 

abundant dust, and where molecular hydrogen gas (H2 ) is the dominant species. In the latter clouds the gas concentrations range from 

about one thousand to about ten million molecules per cubic centimeter. Presumably, the material in dense interstellar clouds is utilized 

in star formation. Relatively small stars, such as our Sun, are probably able to form almost anywhere in these massive interstellar 

clouds. The dust in interstellar clouds is not well characterized; there 

is evidence to suggest that it is composed of ice, silicates, graphite, 

and both simple and complex carbon-containing compounds. While 

there is relatively little information about the dust, there is a growing 

body of data about the molecules present in these clouds. 

How are interstellar molecules produced? First, it is necessary 

to consider the environment in which synthesis might occur. The 

temperatures are very low, from -236° C to -173° C. In addition, the 

extremely low concentrations of molecules in the gas phase means 

that collisions (and therefore chemical reactions) are generally 

restricted to those that involve only two species (that are binary). 

These and other constraints have led to a model for the synthesis of 

interstellar molecules in dense clouds in which reactions are initiated 

by collisions of ubiquitous, high-energy cosmic rays with hydrogen 

and helium. These encounters produce reactive, positively charged 
45 

species which initiate chains of reactions leading to formation of the 

observed molecules. In this manner complex molecules and, perhaps 

even interstellar grains, are constructed within the clouds. A list of 

molecules observed in these interstellar clouds is given in table IV-l. 

At this writing, more than 50 species have been identified. Glycine, 

since it is the simplest amino acid (the building blocks of protein), 

is of obvious interest for the origin of life. To date, the search for it 

in the interstellar clouds has been unsuccessful. Rinally, in addition 

to the species listed in table IV -1, many other as yet unidentified 

molecules appear to be present. 

Alternative schemes for synthesis exist. One of those is the suggestion that the molecules are not formed in interstellar clouds but 

rather are formed under relatively high temperatures and highdensity conditions, such as in primordial solar nebulae. Those 

nebulae are clouds of more concentrated dust and gas that form from 

interstellar clouds and directly spawn suns. Regardless of the model 

chosen, examination of the compounds in table IV-l leads to two 

important observations. First, the compounds are chemically diverse 

and structurally complex. Second, many of them are also known to 

be produced by abiotic synthesis experiments in the laboratory (see 

chap. V). 

Clearly, the interstellar environment, as exotic and as seemingly 

inimical to chemical reactions as it may appear at first consideration, 

exhibits a rich chemistry which manifests itself in the production of 

organic compounds that, for the most part, are familiar to the 

TABLE IV-l.- SPECIES IN THE INTERSTELLAR MEDIUM 

Numbe r of atoms in the species 

2 3 4 5 6 7 8 9 II 

H, HCN H,CO CH,NH CH,OH CH,CHO CHOOCH, CH,CH,OH HC, C,C, C,CN 

CH H,O HNCO HC,CN NH,CHO CH,CHCN (C H,hO 

CH+ H,S H,CH CHOOH CH,CH CH,NH, CH,CH,CN 

OH OCS NH, NH,CN HC,CH, HC,C,C,CN 

CN HCO C,N CH, HC,C,CN 

CO SO, C, H 

CS HCO+ 

SiO HN,+ 

SiS C,H 

NS HNC 

C, 
46 

earthly experience. Although many scientists believe there is little 

basis for the speculation that individual interstellar organic molecules 

found their way, intact and unchanged, to the prebiotic Earth's surface, there is growing interest in the possibility that interstellar 

molecules and dust might be preserved intact in comets and, in 

altered form, in the carbonaceous meteorites. 1 These possibilities 

stem from the probability that all matter in the solar system originated from interstellar dust and molecules. To the extent that 

comets and carbonaceous meteorites contributed mass to the early 

Earth, interstellar organic compounds could have survived to take 

part in subsequent chemical evolution. 

Just as biological evolution implies that all organisms on Earth 

have a common ancestry, so chemical evolution implies that all 

matter in the solar system had a common origin. Observations of 

various stages of star formation and evolution in dense interstellar 

clouds support the view that our Sun and solar system formed from 

interstellar dust and gas. Serious scientific consideration is being 

given to the following scenario: 

An interstellar cloud of dust and gas underwent gravitational collapse, perhaps triggered by a shock wave 

generated by a nearby supernova; thus began the 

chemical evolution of the nascent solar system. Cloud 

contraction led to the formation of the primordial 

solar nebula, an enormous spinning disc of dust and 

gas with the proto- (or newly forming) Sun at the 

center. Heating, associated with gravitational contraction, produced a thermal gradient in the nebula, possibly with temperatures in excess of 1300° C, close to 

the protosun. According to the model, the temperatures were about 300° C at the present distance of 

the Earth to the Sun, but remained low (-150° C) at 

Jupiter's distance and beyond. While uncertainties 

exist about the temperatures in the nebula, the qualitative trend of decreasing temperature with increasing 

1 Carbonaceous meteorites are so named because of their high concentration of carbon (up to 6%). The nature of this carbon will be discussed in detail 

shortly. 
distance from the protosun seems to be acceptable. 

Cooling of the hot gas in the inner nebula close to the 

protosun led to condensation of solid mineral grains. 

As interstellar gas and dust were drawn into the solar 

nebula they would have been heated to varying 

degrees, depending on their distance from the 

protosun. 

47 

Further condensation accompanied by aggregation of finegrained material yielded planetesimals ranging in size from kilometers 

to tens of kilometers. Continued growth of these objects by accretion led ultimately to the formation of the solid bodies of the solar 

system. Formation of organic matter from gases containing carbon, 

nitrogen, hydrogen, oxygen, and sulfur could have accompanied the 

condensation processes; mineral grains could have provided surfaces 

to catalyze the synthesis. As previously noted, in the outer regions of 

the nebula, temperatures would have remained low «-150° C). 

There, low-temperature accretion of organic and inorganic material 

into planetesimals could have taken place, allowing the preservation 

of the ices and other volatile compounds originally in the interstellar 

medium as well as the rare gases helium, argon, neon, krypton, and 

xenon. Thus, from the solar nebula came the Sun, the planets and 

their satellites, comets, meteorites, and asteroids. In general, it is 

thought that material accreted in the inner solar system originated at 

relatively high temperatures and was depleted in volatile substances, 

while the materials that accreted in the outer solar system were 

volatile-rich. The validity of this simple model has been and continues to be subjected to critical assessment; inevitably, as theory, 

experiments, and observations progress the model will undergo 

changes, perhaps so many that a new model will emerge. In the 

meantime, it provides a useful framework within which to discuss 

various aspects of inorganic and organic chemical evolution. 

COMETS AND METEORITES 

Comets occupy an especially interesting place within this 

framework. They may have been a source of part of the atmospheres 

of the terrestrial planets, and they are believed to have been the 
48 

planetesimal-like building blocks for some of the outer planets and 

their satellites. Present knowledge places the origin of comets in the 

outer regions of the primitive solar nebula, both in and beyond the 

space now traversed by the giant planets (Jupiter, Saturn, Uranus, 

and Neptune). Perturbation of their original orbits, by the formation 

of these giant planets, is believed to have sent some protocometary 

bodies into the inner solar system (to collide with the Sun and inner 

planets - Mercury, Venus, Earth, and Mars) and others into orbits 

extending great distances from the Sun (up to 50,000 astronomical 

units (AU); 1 AU = l50X 106 km, or the distance from the Sun to 

the Earth). 

Comets consist of a nucleus, a coma, and a tail (see fig. IV -1). 

According to a current model, comet nuclei contain simple and complex organic molecules, and meteorite-like dust and rock imbedded 

in a matrix of frozen water, possibly solid carbon dioxide and other 

ices. As comets approach the Sun, heating occurs and the ices vaporize, ejecting volatile "parent" compounds (possibly water, carbon 

dioxide, methane, acetylene, ammonia, hydrogen cyanide, etc.) and 

entraining nonvolatile dust and rock from the nucleus. In the coma 

that results, interactions of the gaseous parent compounds with solar 

radiation can lead to physical and chemical processes that cause the 

partial to complete breakdown of the so-called parent molecules to 

"daughter products." The uncharged daughter products are observed 

in the coma, whereas the positively charged ones are observed in the 

tail. According to an alternative view, all the observed daughter products already existed "frozen" in the nucleus of the comet, and were 

simply released directly into the coma by evaporation. In addition to 

the species indicated in table IV-2, metallic elements (iron, silicon, 

magnesium, calcium, nickel, sodium, chromium) have been detected 

by means of spectroscopic analysis of comets that pass very close to 

the Sun, and of meteor showers associated with comets. The relative 

abundances of these elements suggest similarities between the chemical compositions of cometary dust and carbonaceous meteorites. 

The nucleus of a comet is thought to be small, typically 1 to 

10 km diam, but no direct measurement of a nucleus has ever been 

made. Nuclei appear as small points of light imbedded within the 

bright and extensive coma of the comet. The mass of nuclei could 

range from I 015 to 1018 g. The light from the visible coma and tail 

is emitted by atoms and molecules that have interacted with solar 
49 

Figure VI-l.- A comet showing the major features and species observed in the 

coma and tail. 

TABLE IV-2.- SPECIES IN COMETS 

Coma Tail 

HCN CN NHz HzO+ Nz + 

CH3CN CH C3 CO/ CO+ 

H2 O OH CH+ 

COz

a CO OH+ 

NH3 NH 

Cz 

CS 

aSuggested parent molecules, not detected. 
50 

radiation. The size of the coma is remarkable as measured by the 

light emitted by the atomic hydrogen in it. The coma may be 

> 1 X 106 km in radius. The tail, composed of dust grains and charged 

molecules, is even larger, >1 OOX 106 km in length in some cases. 

When comets become visible in the inner solar system they can be, 

spatially, the largest objects in the sky, bigger than the Sun itself. 

As mentioned above, comets are believed to be material condensed and accreted in the outer regions of the primitive solar 

nebula. Thus, a relationship may exist between interstellar matter 

and the dust and molecules that make up comets. If one compares 

the molecules observed in interstellar clouds (table IV-I) and in the 

coma and tails of comets (table IV-2) there do seem to be similarities 

between the popUlations. For example, both contain cyanide, and 

derivatives with a "eN" group. It is also possible that comets are 

related to some of the carbonaceous meteorites in that the latter 

objects, less rich than comets in various forms of the volatile elements and organic matter, may be derived from remnants of volatiledepleted, moribund comets. It is appropriate to note that if comets 

do not contain relatively unaltered interstellar matter, and if they 

formed at the outer edge of the .solar nebula, where temperatures 

were sufficiently low to condense gases like carbon dioxide and 

water, then the presence of parent organic molecules in comets is 

difficult to understand. No widely accepted model exists for the 

chemical reactions that could have occurred in the solar nebula to 

yield the chemistry of comets. Indeed, in the absence of direct 

observations of the nucleus, our knowledge of comet chemistry is 

unfortunately sparse and model-dependent. Since comets may 

represent a chemical evolutionary link between the primitive solar 

nebula and the interstellar medium but are poorly understood, their 

direct study by space probes constitutes a high-priority objective for 

many scientists. 

Unlike comets which have only been observed from afar, meteorites are rock samples of extraterrestrial origin that have survived 

passage through the atmosphere to the Earth's surface and are available for direct examination. Preserved in these objects are chemical, 

mineralogical, and structural information about the nature of the 

environments and the processes involved in their formation. Indeed, 

recent discoveries of anomalies in the isotopic composition of some 

elements (e.g., oxygen, aluminum, magnesium, noble gases) in 
51 

meteorites even provide a connection with nuc1eosynthetic events 

that preceded the solar system, perhaps that triggered its formation. 

With few exceptions, the ages of meteorites fall within the range of 

4.6 ±O.l b.y. Since these objects constitute the oldest datable 

material now available, their study provides clues to the very early 

history of chemical evolution in the solar system. Although some 

uncertainty remains in identifying the source(s) of meteorites, there 

appears to be agreement that most were derived from asteroidal 

parent bodies, either in the main asteroid belt or those with Earthcrossing orbits or both. Some meteorites may be fragments of the 

inactive cores of ice-depleted short-period comets. According to a 

current scenario for solar-system origin, meteorites and the bodies 

from which they were derived (parent bodies) were formed as a 

result of the condensation and early evolution of planetesimals from 

the primordial solar nebula; these represent the building blocks from 

which solid planets and moons were assembled. 

Because of turbulence and thermal and pressure gradients in the 

nebula, solid material that condensed at widely different radial distances and, therefore, different physical and chemical environments, 

could have been brought together and assembled into a common 

body. In this context, proto asteroidal and proto cometary bodies 

may be viewed as components of a distribution of planetesimals 

that accumulated increasing proportions of ice and other volatile-rich 

phases. The accumulation of the diverse ingredients into parent 

bodies, possibly resembling asteroids, would have been accompanied 

by various processes which would have further influenced the chemistry, mineralogy, and structural features of the material and, to varying degrees, masked the features that would have been characteristic 

of primary solar nebula condensates and of originally interstellar 

material. Presumably, perturbation of a parent body, perhaps by 

collision with another object, yielded fragments of the bodies, some 

of which eventually fell under the influence of the Earth's gravitational field. 

Meteorites can be placed in two general categories: (I) partially 

to fully differentiated and (2) undifferentiated objects. Differentiated meteorites exhibit strong chemical fractionation relative to 

average solar-system composition as represented by the Sun; they 

show clear evidence of having been derived from parent bodies that 

have undergone processes analogous to planetary core formation 
52 

and volcanism. Evidently, partial melting of primitive undifferentiated material in asteroidal-sized bodies gave rise to the oldest 

basalts in the solar system at about 4.5 b.y. ago. Elucidation of the 

circumstances of early differentiation of some meteorite parent 

bodies and the nature of the heat sources involved may have much to 

tell us about the course of the Earth's differentiation to form the 

mantle and core, and its subsequent thermal history. 

Undifferentiated meteorites have elemental abundances similar 

to those found in the Sun. Among these meteorites, the carbonaceous chondrites are closest to the Sun in bulk elemental composition and are considered to be among the least fractionated and, 

therefore, most primitive solid objects available for study in the solar 

system (fig. IV-2). It is noteworthy, however, that relative to the 

Sun, even the carbonaceous meteorites are depleted in hydrogen, 

carbon, nitrogen, and noble gases. Observations indicating less 

depletion of these elements in comets signify that comets are even 

more primitive bodies than meteorites. 

Carbonaceous meteorites consist of complex assemblages of 

relatively fine-grained mineral and organic matter that reflect a broad 

range of elemental compositions and textures. This is indicative of 

wide variations both in the environments of origin for the various 

components and in the evolution of the respective parent bodies. 

For present purpo.ses, we consider the classification of carbonaceous 

meteorites into three types: CI, CII, and CIII. Major differences 

among these types lie in their content of volatile elements and minerals of high-temperature origin; these are inversely correlated. Accordingly, the amount of organic matter increases in the order CIII, CII, 

CI, with the CIII containing about 0.5% and the CI having about 5% 

by weight. Similarly, minerals exhibiting a high-temperature history 

occur most abundantly in CIII meteorites, along with metals (iron 

and nickel). These minerals exist only in low to trace amounts in CII 

meteorites and are virtually absent in the CI meteorites. 

Mixtures of clay-like minerals compri~e the predominent minerals in CI and CII meteorites (50% to 80%) and a minor proportion in 

some CIII meteorites. These minerals resemble terrestrial clays in 

crystallographic structures, and the mixtures exhibit bulk elemental 

compositions remarkably similar to the pattern of solar abundances. 

Recent research suggests that like terrestrial clays, the clay-like 

materials in carbonaceous meteorites were formed in an aqueous 
r - - - - - - -

53 

Figure 1V-2.- The Murchison carbonaceous chondrite. Most of our understanding of the organic matter in meteorites has been derived from studies of the 

Murchison meteorite. 

environment. Thus, the oldest known clays in the solar system were 

probably produced on parent bodies of carbonaceous meteorites. 

The organic matter in carbonaceous meteorites occurs in various 

forms. A high-molecular-weight complex material characterized by 

insolubility in solvents and acids makes up the major carboncontaining component in all three types of meteorites. (Terrestrial 

sediments contain a material called "kerogen," which has similar 

characteristics but is obviously of different origin.) The source(s) and 

production mechanism(s) for this insoluble material are unclear, but 

may involve interstellar environments as well as environments in the 

solar nebula and on the parent bodies themselves. 
S4 

Solvent-extractable organic matter in ell meteorites (Le., 

the Murchison meteorite) is distributed among a variety of compound classes: alcohols, aldehydes, amines, amino acids, carboxylic 

acids, hydrocarbons (aromatic and aliphatic), ketones, purines, 

pyrimidines, etc. Carbon species found in the Murchison meteorite 

include the following: 

1. A carbonaceous phase not affected by solvent 

2. Carbonate 

3. Hydrocarbons (aliphatic and aromatic) 

4. Carboxylic acids 

S. Amino acids 

6. Ketones and aldehydes 

7. Urea and amides 

8. Alcohols 

9. Amines 

10. Nitrogen-containing heterocycles 

More detailed information on these chemical compounds will be presented in the next chapter. At most, these compounds constitute 

30% of the total carbon, and less than 0.5% of the total mass of the 

meteorite samples in which they are found. Some of these compounds have been sought and found in other carbonaceous meteorites, but only the Murchison meteorite has been studied in great 

detail because of the availability of samples and its relative freedom 

from terrestrial contamination. 

The variety of types of compounds found and their molecular 

structures point to origins in non biological processes. However, the 

nature of the processes and where they occurred remain to be clearly 

established. Electric discharges and other gas-phase processes and gassolid reactions requiring catalytic grain surfaces could have taken 

place both in the nebula and on parent bodies. An interstellar origin 

for some of these compounds should also be considered. 

Recent isotopic studies of organic matter in carbonaceous 

meteorites have revealed that large differences exist in the 

deuterium/hydrogen, carbon-l 3/ carbon-I 2, and nitrogen-IS / 

nitrogen-I4 ratios associated with different organic components 

within the same meteorite. Although the full implications of these 

findings remain to be elucidated, the magnitudes of the isotopic 
55 

variations and their occurrences among different components 

strongly suggest that more than one source region and/or more than 

one production mechanism must have been involved. 

Evidence that the clays in carbonaceous meteorites were produced in secondary aqueous alteration processes raises the possibility 

that some of the organic matter might also have been produced at 

the same time by alteration of preexisting compounds by water. Possibly, simple species (such as cyanide compounds), which occur 

abundantly in the interstellar clouds (table IV-I) and have been 

observed in comets (table IV-2), could have been present and served 

as precursors for some of the more complex molecules found in these 

meteorites. 

From studies of interstellar dust and gas, comets, and meteorites th~ initial conditions in the solar nebula and its subsequent 

chemical evolution are being elucidated. Continuing investigations 

into the cosmochemical origins of organic matter are crucial because 

organic chemistry occurs throughout the cosmos, and the organic 

matter that results constitutes a molecular and isotopic record of the 

materials and processes involved in its formation. 

We have outlined a scenario by which the building blocks of 

solar-system bodies may have developed from dusty and gaseous 

starting materials. We might ask if there are other solar systems in 

the universe. We feel that this possibility exists because of the frequent occurrence of binary and multiple stars (which now seem to be 

well over 50% of the star population), and the fact that the separations between most of these binary stars are comparable to the 

dimensions of our solar system. The inference is that stellar condensation tends to form more than one object; when the residual matter 

is insufficient to form a second star, planets may occur instead. We 

see this same tendency for multiplicity within our solar system. Only 

Venus and Mercury are without satellites, and this lack may be 

attributed to gravitational perturbations caused by the close proximity of these bodies to the Sun. 

COMPARATIVE PLANETOLOGY 

It is instructive to examine the Jupiter satellite system in this 

context. The Voyager spacecrafts have confirmed and extended the 
56 

impressions gained from ground-based and Pioneer spacecraft observations. The four large satellites of Jupiter exhibit gradients in average density and other properties that are strikingly reminiscent of 

the gradient observed among the planets. The inner satellites, 10 and 

Europa, have densities of 3.5 and 3.0, respectively, indicating a predominantly rocky composition. In contrast, the more distant satellites Ganymede and Callisto have densities of 1.9 and 1.8, respectively, suggesting compositions that include a high percentage of 

carbon, nitrogen, and oxygen compounds and, presumably, are 

dominated by water (ice). 

The surface appearances of these objects substantiate this interpretation. The icy crusts of Ganymede and Callisto have evidently 

been unable to support the topography associated with large-impact 

craters, although crater densities in the smaller size range are close to 

saturation. The surface of Europa appears to be covered with a 

layer of water ice that has obliterated any trace of its history of early 

bombardment. The few craters that do appear are comparable in the 

number per unit area to that found on Earth. 10 is so wreaked by 

continuous volcanism that its entire surface must be reworked on a 

time scale that is very short compared with 4.6 b.y. As a result of its 

tidally induced volcanic activity, this satellite appears to be extensively degassed, with the volcanoes possibly relying on sulfur dioxide 

as a working fluid instead of water. 

While obviously much smaller than the Sun, Jupiter is apparently large enough to have caused the same gradient in the properties 

of its retinue of satellites as the Sun has caused in the planets 

(fig. IV-3). This fractionation of material can be attributed to the 

heat released during the formation of the central body. The resulting 

similarity between these two systems (Jupiter's and the Sun's) 

strengthens our intuitive feeling that the gross characteristics of the 

solar system are probably representative of those found elsewhere; 

with small dense inner planets possessing secondary degassed 

atomospheres. The exploration of the Saturn system by 

Voyager has introduced an important caveat: For this gradient 

in properties to exist, the central body must be massive enough 

to heat the surrounding space during the time the system forms, 

and the planets or satellites must be large enough to represent 

a homogeneous sample of the accreting material. Neither of these 

conditions was met in the case of Saturn (fig. IV-4), with the result 
57 

Figure IV-3.- Jupiter, the largest planet in our solar system, has an atmosphere 

dominated by hydrogen, and has a retinue of satellites analogous to the planets 

of the solar system. 

that its moons are very different from those of Jupiter. By analogy, 

one might expect the planets of a red dwarf star to differ considerably from the planets in our own system. But even given another star 

like the Sun, what are the chances of fmding another planet like 
58 

Figure IV -4.- The ring system of Saturn is composed of ice. 

Earth? We are apprehensive about this question since our planet is so 

different from its neighbors. 

The planets in our solar system that are most similar to Earth 

are Mars and Venus. Historically, this apparent similarity in gross 

properties led many scientists to anticipate that these planets might 

also be populated with some form of life. Yet, as we learned more 
L 

59 

about these two planets, this possibility became increasingly remote. 

Our considerations of the origin of life on Earth must therefore 

include some discussion of the unique properties of Earth itself. 

Why is our planet so different from its nearest neighbors? 

We can quickly identify two basic characteristics that will determine most of a planet's development - size and distance from the 

Sun. If a planet is too large, hydrogen will not be able to escape from 

its gravitational field, and the result will be an object like Jupiter or 

Saturn, with a huge, dense atmosphere and no solid surface. At the 

other extreme, a body that is too small will not be able to retain any 

kind of atmosphere over geological periods of time. Examples of this 

end of the spectrum are Mercury and the Moon. Distance from the 

Sun becomes a part of this constraint, however, since a small body at 

a large distance could be cold enough to have an atmosphere. What 

this means is that the thermal velocities of gas molecules will be 

smaller than the velocity needed to escape from the body's gravitational field (given that the molecular weight of the gas is high enough 

and that the gas does not condense at the low temperature corresponding to this distance from the Sun). In our solar system, the best 

example of this situation is Titan, a satellite of Saturn, that has a 

dense atmosphere of nitrogen with a small amount of methane and 

traces of other compounds. 

The inner planets - Mercury, Venus, Earth, Mars - have clearly 

been drastically affected by their proximity to the Sun. They are all 

grossly deficient in the volatile elements, having formed in an environment that was evidently at too high a temperature to permit the 

common compounds of these elements to condense. In contrast, the 

outer planets - Jupiter, Saturn, Uranus, Neptune - have retained 

large amounts of hydrogen and helium. Jupiter may even represent a 

cosmic mixture of the elements; i.e., the composition of this planet 

may be identical to the composition of the Sun and other young 

stars. 

ANCIENT ATMOSPHERES 

We can now distinguish between two types of atmospheres -

those that are primitive, representing material captured from the 

solar nebula with only minor fractionations, and those that are 
60 

secondary - produced by the degassing of the material that accreted 

to form the planet. The fIrst type of atmosphere is found in the 

outer solar system; the inner planets exhibit the second type. To give 

a quantitative illustration of the extent to which the light gases are 

defIcient on the inner planets, one can imagine adding hydrogen and 

helium to the Earth until the abundance ratios of these elements to 

silicon were equal to the ratios found in the Sun. The resulting planet 

would have a mass approximately equal to the mass of Saturn. 

Since we have good evidence that the composition of our 

planet's atmosphere has changed with time, it is natural to ask 

whether at some earlier stage it could have been strongly reducing, 

like the atmospheres we now see in the outer solar system. In other 

words, did the Earth and the other inner planets capture atmospheres 

from the solar nebula as they formed? And if they did, were the 

compositions of these atmospheres similar to those we now fmd in 

the outer solar system? With time, these planets would inevitably 

have moved from a highly reduced to an oxidized condition, since 

hydrogen will escape from their small gravitational fIelds and their 

warm surface temperatures ensure that water will be available to provide a source of oxygen. They must have begun with no free oxygen 

in their atmospheres, but were these atmospheres ever as reducing 

(or hydrogen-rich) as Jupiter's? 

For many years this seemed the most likely scenario, but the 

evidence that was cited in its favor has become les.s compelling in the 

wake of new discoveries. An early argument involved the distribution 

of abundances of the noble gases in our present atmosphere. Compared with the cosmic abundance pattern, the noble gases are clearly 

defIcient on Earth, and this defIciency seems to be mass-dependent, 

such that helium is the most depleted gas, then neon, then argon, 

etc. This pattern was taken as evidence that the Earth once had an 

atmosphere with its full complement of volatiles, but a "catastrophic 

event" - perhaps a strong solar wind during an early unstable and 

flaring stage of the Sun's history (T-Tauri phase) - swept away the 

gases, removing the lighter ones most efficiently. It seemed that the 

noble gases left us a record of this event, since they are chemically 

inert, and (except for helium) are too heavy to escape from the 

Earth's gravitational fIeld . 

But now we know that this same abundance pattern is found in 

meteorites and in the atmosphere of Mars, although the atmosphere 

_ J 
61 

of Venus is distinctly different. It thus seems that the fractionation 

processes were acting in the solar nebula prior to the formation of 

the planets, and affected all of the solid bodies in the inner solar system in different ways. We have also learned that the T-Tauri phase of 

solar history was probably not cataclysmic enough to have blown 

gases away from the surfaces of planets. 

To reconstruct the Earth's early atmosphere, we must therefore 

turn this argument around. This fractionation of the noble gases in 

the solar nebula was presumably accompanied by a fractionation of 

other gases as well. Therefore, the maxirrzum amount of hydrogen 

that the Earth could have captured can be calculated by using the 

neon in the present atmosphere as an index. In other words, if we 

assume that neon was captured, the cosmic hydrogen-to-neon ratio 

would give us the maximum value for early atmospheric hydrogen. 

This turns out to be about 10 millibars, or 1/ I 00 of our present 

atmospheric density. Such an atmosphere would be lost by escape 

in less than 10,000 years. 

Methane and ammonia would have had abundances 1,000 times 

smaller than that of hydrogen. Ammonia is particularly unlikely as a 

long-term atmospheric constituent, since this small amount would be 

destroyed in less than 40 years by solar ultraviolet light. Ammonia 

would also have been out of equilibrium with crustal rocks. 

Thus, the only hope for a highly reducing early atmosphere 

would seem to reside in the possibility of producing it by degassing 

from the early Earth, either by internal melting processes caused by 

radioactivity or by external processes - bombardment by meteorites 

and comets that incidentally may have contributed reduced volatiles 

themselves. The first of these possibilities requires the presence of 

some reducing agent in the upper mantle. Free iron has been suggested as a candidate for this role, assuming that this early degassing 

took place prior to formation of the Earth's core. Some investigators 

are unhappy with this picture, however, arguing that the core should 

have occurred as the planet itself formed, since the energy of accretion would have been sufficient to initiate the melting of iron once 

the embryo-Earth attained lO% to 25% of its present size. 

At this stage of our knowledge, there seems no way to rule out 

a transient, early atmosphere rich in hydrogen, methane, and carbon 

monoxide "and containing ammonia. Because of the difficulties 

referred to above, however, interest is shifting toward the possibility 
62 

that the early atmosphere of our planet was only weakly reducing, 

consisting of a mixture of nitrogen, carbon dioxide, carbon monoxide, water, and a few percent hydrogen. Such an atmosphere would 

provide a sufficient greenhouse effect to keep the Earth warm even if 

the Sun were 25% less luminous (a model which has some support) 

during that period than it is today. There is no need for ammonia or 

some other reduced gas to provide this effect as long as the partial 

pressure of CO2 is on the order of 200 millibars. This large amount 

of atmospheric CO2 would gradually diminish as a result of rockweathering and the consequent production of carbonates. 

Everything that we have described for the Earth should apply to 

Mars and Venus as well. Why then has our planet turned out so differently from our neighbors? Let us return to our basic criteria for 

planetary differences - size and distance from the Sun. 

We first consider distance from the Sun. Suppose we could 

move the Earth to the position occupied by Venus. What would 

happen? The increased intensity of sunlight would cause the mean 

temperature of our planet to rise. Model calculations show that this 

increase in temperature would cause increased evaporation of sea 

water and would lead to a larger amount of water vapor in the atmosphere that could increase the greenhouse effect and the mean surface temperature further, leading to more evaporation, etc. In 

other words, the Earth's climate would go into a positive feedback 

loop that would lead to a condition known as a runaway greenhouse. 

The end result would be that the oceans would boil, putting all of 

the surface water into the atmosphere. The atmosphere itself would 

be so hot that there would no longer be a cold trap to confine water 

vapor to lower levels. Photodissociation by ultraviolet light would 

then become very efficient, and the hydrogen atoms resulting from 

this process would escape into space. 

This is one explanation for the absence of water on Venus. The 

process we have just pescribed would have occurred on that planet 

shortly after it formed. The oxygen left over from H2 0 dissociation 

would have combined with the crust and available volatiles; we see 

one of the results in the dense carbon dioxide atmosphere that now 

blankets the planet (fig. IV-5). Pla!lsible as this scenario seems, it has 

not yet been rigorously proven. A demonstration that deuterium is 

enriched on Venus would provide a strong supporting argument. If 

indeed there were once oceans that boiled with the subsequent 
63 

Figure IV-S.- Venus, the planet closest to the Earth in size, has retained a significant atmosphere of carbon dioxide. Cloud features indicate a significant 

atmospheric dynamics. 
64 

escape of hydrogen, the heavier mass of deuterium should have led to 

a net enhancement of this isotope in residual hydrogen compounds 

over geologic time. The alternative to this picture is that Venus had 

no (or very little) water from the beginning, a result again of its 

proximity to the Sun; the temperature at which Venus formed was 

so hot that water could not have condensed. This alternative seems 

less satisfactory in view of recent models that explain the bombardment histories of the inner planets by impa-cts from meteorites and 

comets that had formed in colder parts of the solar system. These 

impacts would necessarily inject additional volatiles such as water 

into the atmosphere of an evolving planet. While the visible results of 

this bombardment seem only superficial, it is likely that even during 

the early periods of planet formation, there was opportunity for 

ample mixing of materials from various parts of the solar nebula. 

Thus, if the Earth were much closer to the Sun than its present 

position, it would be too hot for liquid water to be stable on its surface. And without water, life as we know it cannot survive, and 

probably cannot even originate. 

What if Earth were farther from the Sun? This situation is more 

complex. A change in the composition of our planet's atmosphere 

could lead to an enhanced greenhouse effect, resulting in temperatures above the freezing point of water, even at the distance of Mars. 

Indeed, model calculations show that if Mars itself had an atmosphere in which the partial pressure of CO2 was equal to the total 

pressure of our own atmosphere, the mean surface temperature on 

Mars would be above 0° C. This result has been used to explain the 

presence of sinuous channels and other landforms on Mars that seem 

to provide evidence for the action of running water on the planet's 

surface at some time in the distant past - perhaps 3.5 b.y. ago. 

If Mars had a sufficiently dense CO2 atmosphere during its early 

history, it would have been warm enough to allow liquid water to 

exist, and that water could have cut the channels we observe today. 

Thus, it seems possible that Mars may have had an early history 

much like that of the Earth. Yet we do not see evidence that there 

were ever lakes or oceans on Mars. It looks more as though the water 

came out in periodic floods, but never accumulated in large basins. 

Perhaps conditions were simply not that stable. Was the atmosphere 

never dense enough to do more than raise the temperature closer to 

the freezing point than we fmd it today? 
65 

In any case, the water present produced its own demise. Dissolving atmospheric CO2 caused weathering of the rocks that led to the 

formation of carbonates, thereby reducing the amount of CO2 in the 

atmosphere. This negative feedback would ultimately lower the surface pressure and the surface temperature. Abetted by other factors, 

such as the loss of nitrogen by escape, this process led to the point 

where liquid water could no longer exist. 

But we must now consider our other basic planetary characteristic: size. What if Mars were a larger planet? We can return to our 

hypothetical experiment of moving the Earth to the position of 

Mars. In that case, more carbon dioxide could have been released 

initially, and the greater amount of tectonic activity associated with 

the larger heat engine in the bigger planet would provide a much 

more efficient means of recycling that gas. Perhaps a planet the size 

of Earth or slightly larger would be able to maintain a reasonably 

warm climate at the Mars distance from the Sun, provided that it 

could maintain a sufficiently large amount of carbon dioxide in its 

atmosphere. This in turn might heat up the tropics of a planet still 

largely frozen. Weathering would proceed very slowly, since much of 

the crust would be protected by ice. Whether life could originate and 

persist on such a planet is a matter of speculation. But these considerations do show that an Earth-like planet could exist at a greater 

range of distances from its central star than we would have concluded had we required that the planet have an atmospheric history 

identical with our own. 

Still farther from the Sun, we must consider other liquids and, 

hence, other kinds of life. Ammonia is often suggested as an alternative to water as a medium for alien types of biology. We should 

expect to find such environments on the surfaces of satellites of the 

outer planets, since the planets themselves do not have solid surfaces 

that would allow ammonia (or any other liquid) to collect. 

The best example of an object that might meet these criteria is 

Titan, the largest satellite of Saturn. This is the only satellite in our 

solar system known to have a substantial atmosphere; this atmosphere contains both nitrogen and methane. The problem with Titan 

is that it is too cold to be very interesting. The surface temperature 

has been shown to be below -175° C by both spacecraft and Earthbased measurements. If there is a liquid on Titan's surface, it is liquid 

methane or liquid nitrogen, not ammonia; ammonia would be solid. 
66 

The development of alien life at these low temperatures seems 

unlikely in view of the slowness of chemical reactions under these 

conditions. Yet some very interesting chemistry is taking place On 

Titan, for we can detect traces of reaction products in the atmosphere. In addition to CH4 , C2 H2, C2 H4 , and C2 H6 that had been 

detected by Earth-based telescopes, the Voyager spacecraft succeeded in identifying N2, H2, HCN, C2N2, HC3 N, C4 H2, C3 H4 , and 

C2 Hg. Furthermore, the atmosphere is charged with a brownish 

photochemical aerosol that may include polymers of one or more of 

these substances. Since hydrogen can escape from Titan, fragments 

of hydrocarbons that are produced by UV irradiation or charged 

particle bombardment in the satellite's atmosphere are free to combine to form more complex substances. Here, we have a highly 

eVQlved atmosphere that has remained reducing, since oxygen is 

safely trapped as water ice in Titan's interior. 

This lack of liquid water makes the current chemistry on Titan 

fundamentally different from the chemistry on the primitive Earth. 

But the chemistry occurring in the Earth's atmosphere in its early 

history may well have been very similar to what we find on Titan 

today, making further investigation of this object particularly 

appealing. It depends, of course, on how reducing our early atmosphere was. At this stage of our ignorance, a mixture of CH4 , N2, 

and H2 can't be excluded. 

The photochemical reactions are taking place in the satellite's 

upper atmosphere, which is some 80° C warmer than the surface. 

The reaction products will gradually settle out and be preserved in 

this cold trap (or dissolve in liquid methane or liquid nitrogen!). The 

early history of Titan may have been even more interesting, however. 

The nitrogen we now find in the atmosphere is presumably the result 

of the photodissociation of ammonia. In order for the ammonia to 

get into the atmosphere, Titan must have been much warmer. This 

would have led to much more methane in the atmosphere as well, 

so we can imagine an early atmosphere even denser than the present 

one, which has a surface pressure of 1.6 bars. How warm was the 

satellite during this early period? How long did this time last? What 

kinds of chemistry occurred? Here in this cold, alien environment, 

we find ourselves confronting the same questions we have considered 

on Mars. 
67 

We can find in this example another important property of 

water that makes it well-suited as a liquid medium for life. When 

ammonia is photodissociated, the reaction products are nitrogen and 

hydrogen, neither of which protects the ammonia from further 

photodissociation. It thus seems problematical whether one can ever 

have an environment in which liquid ammonia is stable. In contrast, 

the oxygen produced by the breakup of water can act to shield the 

water while also providing a potential source of chemical energy (far 

more accessible than nitrogen) for evolving life. An alternative would 

be to provide a UV-protective smog layer, such as Titan in fact seems 

to possess. The trick is then to maintain a warm enough surface to 

have liquid ammonia but a cold trap high in the atmosphere that prevents the ammonia from diffusing up to altitudes where the smog 

cannot protect it. On Titan, no such ammonia sea is present, but perhaps it is present somewhere far away. 

The outer planets themselves are less promising. There is ample 

evidence for chemical reactions, particularly on Jupiter where we see 

a variety of colors among the clouds (fig. IV-6). The expected condensed ammonia, ammonium hydrosulfide, ammonium hydroxide, 

and water all preclude white clouds. Hence, the existence of colors 

indicates that more complex, nonequilibrium compounds are being 

fonned. With solar ultraviolet light, lightning stonns, bombardment 

by charged particles, and escaping internal heat all available as energy 

sources, we have a giant natural laboratory in which experiments 

bearing on the first stages of chemical evolution are continuously 

being performed. At the present time, these colored substances have 

not been identified. It is becoming increasingly evident that we shall 

have to probe Jupiter to solve this problem. 

THE ANCIENT SURF ACES 

The recent exploration of the planets has given us a model of 

the early Earth, but what terrestrial evidence do we have of what our 

Earth looked like in the past? The most ancient metamorphosed sedimentary rocks now known are those at Isua in western Greenland. 

They were deposited roughly 3.8 b.y. ago as sediments carried by 

water into a volcanic basin to which volcanos contributed solids and 
68 

Figure IV-6.- The giant red spot of Jupiter and the complex cloud patterns 

suggest a natural laboratory for chemical evolutionary processes. 

probably hydrothermal solutions as well. Some metamorphosed sedimentary units at Isua approach the composition of normal presentday sedimentary rocks, which bear conclusive evidence of an atmosphere that contained sizable quantities of carbon dioxide. Little can 

be said about the oxygen content of the atmosphere at the time of 

deposition of the Isua sediments. There is a fair amount of evidence 

that the oxygen content of the atmosphere more than 2 b.y. ago was 

substantially less than today. Therefore, there must have been less 

oxygen at the time of deposition of the sediments at Isua. The 
69 

atmosphere at that time was certainly not highly reducing; in fact, 

the bulk composition of the Isua rocks is surprisingly similar to that 

of younger rocks in equivalent settings. In this sense they give eloquent testimony to the notion that 3.8 b.y. ago the Earth had 

already settled down to a regime that is quite similar to that of the 

present day. 

Elemental carbon is present in these sediments. The Isua sediments have been heated to such high temperatures since deposition 

that virtually no extractable organic compounds remain. At present, 

a biological origin for carbon in the Isua rocks seems unproven. Life 

could have started before the time when Isua was formed, perhaps 

much closer to the birth of our planet. A search for sedimentary 

rocks older than 3.8 b.y. is obviously needed. 

Several continents are known to contain 3.S-b.y.-old sedimentary rocks, and these ancient areas may contain enclaves of even 

older rock units. Areas in Australia and southern Africa, and an area 

in central Greenland that is currently covered by ice may tum out to 

be particularly promising targets in the search for more ancient 

rocks. 

We believe that our Earth is about 4.6 b.y. old. At present we 

are forced to look to other bodies in the solar system for hints as to 

what the early history of the Earth was like. Studies of our Moon, 

Mercury, Mars, and the large satellites of Jupiter and Saturn have 

provided ample evidence that all of these objects were bombarded by 

bodies with a wide variety of sizes shortly after they had formed. 

This same bombardment must have affected the Earth as well. The 

lunar record indicates that the rate of impacts decreased to its present low level about 4 b.y. ago. On the Earth, subsequent erosion and 

crustal motions have obliterated the craters that must have formed 

during this epoch. Since it is generally believed that life on Earth 

began during this period, the bombardment must have been part of 

the environment within which this event occurred. 

Perhaps the most significant aspect for our consideration is the 

realization that some of the impacting objects were large enough to 

punch through the crust of Earth and formed large basins that could 

be flooded by lava. These would be the terrestrial analogues of the 

large circular maria on the Moon, the basins on Mars, and the ringed 

structures on Jupiter's moons - Callisto and Ganymede. This is a 
70 

reminder that the early history of the Earth was more turbulent than 

a simple volcanism model would suggest. 

We have seen how interesting and important it is to find rocks 

from the earliest possible times in the Earth's history. Yet, it has 

been much easier to find ancient rocks on the Moon, where many 

samples have been dated at 4 b.y. or more. Unfortunately, these 

ancient lunar rocks do not tell us about the Earth's primitive atmospheric conditions, since the Moon evidently possessed no longenduring, substantial atmosphere, even in those early times. The 

lunar rocks are grossly deficient in volatile elements and compounds 

compared with the Earth, suggesting that they were thoroughly 

degassed but that the gases escaped rapidly into space. 

There are large regions on the surface of Mars that exhibit crater 

densities similar to those found on the lunar highlands, the oldest 

region of the lunar surface. Although there is still some dispute about 

absolute chronologies, this similarity in the distribution of impactcraters has led several investigators to suggest that these regions of 

the Martian surface are probably as old as the comparable areas on 

the Moon. Thus, it seems reasonable to expect that on Mars as on the 

Moon, rocks with ages greater than 4 b.y. should be reasonably abundant, provided one were to go to the right region of the planet to 

look for them. 

The great difference between Mars and the Moon is that Mars 

has an atmosphere. This atmosphere has apparently developed from 

an inventory of volatiles very similar in composition to the one that 

formed the atmosphere of Earth. Indeed, there is every reason to 

expect that the first steps in the development of the atmospheres on 

these two bodies were essentially identical. Although we would 

expect hydrogen to escape more rapidly from the low gravitational 

field of Mars, this may have been partially compensated for by the 

lower temperature of the exosphere on this more distant planet. 

Thus, we can suggest that if Earth ever had a strongly reducing atmosphere, Mars probably did too. 

This probability lends a special piquancy to the search for 

ancient rocks on Mars. If we could fmd such rocks in a suitably protected setting, we would have an opportunity to test the possibility 

that the early Martian atmosphere was strongly reducing by examining the mineral assemblages that the rocks contain. Our conclusions 
1- --- -- 71 

would then be very useful in helping us to evaluate the composition 

of the atmosphere on the primitive Earth. 

But we can go further than this. The surface of Mars is also 

marked by many different examples of fluvial erosion. Once again 

the timing of the events that led to these landforms is controversial. 

But it is probably conservative to say that the youngest of the 

numerous floods took place at least 3.5 b.y. ago. In other words, 

liquid water was evidently available on some kind of intermittent 

basis for the fIrst billion years of Martian history. The controversy 

that still exists centers on the issue of how much later than this there 

might have been epochs when water flowed on Mars. For our immediate purpose, it doesn't matter; I b.y. is long enough! 

As already stated, our current view of the development of 

inner-planet atmospheres suggests that Mars, Earth, and Venus all 

began with a very similar volatile inventory. Models for the early 

Martian climate indicate that a dense CO2 atmosphere could have 

melted ice by a greenhouse effect. The morphological evidence that 

liquid water once flowed on Mars seems to substantiate both of these 

points. The special signifIcance of this picture of primitive Martian 

history is apparent as soon as we ask what was happening during the 

first billion years of our own planet's history. There is almost no 

direct evidence available to answer this question. But the recent 

discovery of stromatolites dated by three different methods to be 

3.5 b.y. old indicated that life had originated, evolved, and become 

fIrmly established on our planet within the fIrst billion years. If this 

happened on Earth, why not on Mars? 

There seems no way to exclude this possibility. We might be 

more comfortable if there were a record of ancient seas and lakes on 

Mars - proof that the presence of water was more than a series of 

very transient events. But the evidence for such smooth landforms is 

much more likely to disappear under the shifting sands of the windblown Martian terrain than is the high relief associated with individual stream beds. In fact, there are many examples of craters with 

diameters greater than 30 km that once contained standing water. 

These "lakes" existed at the same time as the large fluid channels. 

Periodic wetting and desiccation and/or freezing could help to concentrate prebiotic material, as has often been stressed in considerations of the origin of life on Earth. We are thus confronted with the 

arresting possibility that since life originated on Earth within the fIrst 
72 

billion years of its history, and since conditions on Mars were probably similar to those on Earth during this period, there is an excellent 

chance that life originated on that planet too. We can easily see why 

and how such life could have died out in the ensuing millenia, leading 

to the negative results obtained by the Viking investigations 

(fig. IV-7). But even if there is no life anywhere on Mars today, there 

seems to be good reasons for returning to Mars to look for evidence 

of early life forms. 

This will require a careful search in "the right places," as has 

been required on Earth. A Viking-style lander mission has virtually 

no chance of success in such an endeavor; a manned mission or a 

sophisticated system of rovers with capabilities for sample return will 

be required. It will be an expensive and difficult task, but the 

rewards would be so great that a considerable effort is justified. To 

find one more example of the origin of life, to know that this 

Figure IV -7,- NASA's commitment to the exploration of the planets is most 

dramatically seen in Viking Martian lander. 

- 1; Ln 

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75 

v. THE LABORATORY EXPERIENCE 

INTRODUCTION 

In chapter III we traced the history of the Earth back through 

time by means of the rock record, yet it stops at 3.8 b.y., and 

we know the Earth to be 4.5 b.y. old. We sought clues to the early 

stages of the Earth's formation in chapter IV in our reconstruction of the evolution of the solar system, especially of the Earth, 

from the interstellar gas and dust, yet we still found great gaps. One 

theme which runs like a thread throughout our previous chapters, 

however, is the role of organic molecules in the universe. 

A brief glimpse into the nature of organic matter in our biosphere thus seems in order. It is necessary to spend some effort at a 

rather more technical level than in most of this book to discuss what 

are chiefly chemical questions. 

In the 19th century, as the microscope was perfected, the cell 

was discovered. All organisms including ourselves are composed of 

cells. The main function of a cell is to grow and divide. By the use of 

dyes, various parts of the cell were recognized: nucleus, mitochondria, chloroplasts, and centrioles. It was a great age of observation; 

A drawing made by an unknown artist to illustrate an early edition of 

Gulliver's Travels. Dr. Lemuel Gulliver (in "Gulliver's Travels"} reported 

on this device, which he saw in the National Academy of Laputa. 

"It was Twenty Foot square . .. The Superficies was composed of 

several Bits of Wood . .. all linked together by slender Wires . .. " 

On them "were written all the Words of their Language in their 

several . .. Declensions, but without any order . ... The Pupils at 

his Command took each of them hold of an Iron Handle. .. and 

giving them a sudden Turn, the whole Disposition was entirely 

changed . . . the Professor shewed me several Volumes . .. already 

collected, of broken Sentences, which he intended to piece together 

. .. to give the World a compleat Body of all Arts and Sciences . .. " 

The same logiC applies to the origin of life, although we are looking for a 

less random hierarchy of processes, and a less elaborated system of 

selection. 

Provided by Phil Morrison, Cambridge, Mass. 1983 

--------
76 

the biology of microscopic life received a powerful stimulus from the 

rise of medical bacteriology. 

It was left to the 20th century to analyze the cell into its chief 

chemical components: proteins, nucleic acids, fats, and carbohydrates. Of these chemical species, proteins were soon discovered to 

be giant molecules made up of thousands of atoms. If proteins are 

gently broken down, they fall apart into amino acids; there are only 

20 different amino acids in all life. Thus, a protein can be described 

as a word string in an alphabet of 20 letters (the amino acids). It was 

found as well that the nucleic acids (DNA and RNA) were strings 

over an alphabet of four letters (nucleotides) (fig. V-I). In the case of 

DNA, the letters (or molecules) are adenylic acid (A), guanylic acid 

(G), cytidylic acid (C), and thymidylic acid (T). In the case of RNA 

it is A, G, C, as above, but uridylic acid (U) instead of thymidylic 

acid (T). The number of combinatorial possibilities are more than 

astronomical. Assuming we have formed a string of amino acids 

100 letters long, how many different ones could be present? There 

are 20 different possibilities for the first member of the string and 

20 for the second and so on. Therefore , there are (20)100 different 

proteins, a number large past imagining. 

The DNA in E. coli is found in a single molecule which is about 

I mm long. There are about 3 million nucleotide base pairs in one 

molecule of DNA. The different protein strings realized are encoded 

in the sequence of bases in the double-stranded molecule of DNA. 

A protein molecule called RNA polymerase transcribes the 

sequences within the DNA molecule into RNA molecules, called 

messenger RNA molecules. An RNA molecule then enters a proteinsynthesizing machine, which is best compared to a molecular tape 

recorder, in which the RNA molecule is read and the output is a 

sequence of amino acids. 

Since there are four bases (A, G, C, U) in the RNA alphabet 

that can be used to code for amino acids, it can be seen that three 

bases (43 = 64) are required to accomplish this code: neither one 

base (41 = 4) nor two bases (42 = 16) would provide a unique code 

for the 20 amino acids. The representation of the sequence of amino 

acids of proteins by a sequence of nucleotides in RNA is called the 

genetic code (table V-I). In table V-I the fust base in the triplet is 

listed in the first column, the second base is listed along the row, and 

the third base is listed in the last column. The 64 triplets are thus 
- - --- ---

77 

AMINO ACID 

CH3-CH-COOH 

I 

NH2 

NUCLEOTIDE 

NH2 

I 

N /C~ 0- /j---C/ ~N 

I H-C" I 

\ C C 

O==P-O-CH? N-- ~ -;:::? ~ I - 0"" / N H 

C/ C-H 

\ H H / / '\ 

C-C 

/ \ 

0-

OH OH 

Figure V-I.- Shown are representative examples of an amino acid, a nucleotide. 

Amino acids are the building blocks of the proteins, while the nucleotides are 

the building blocks of the nucleic acids. A complete set of these molecules used 

in biology can be found in the appendix. 

related to the 20 amino acids. For example, the triplet GGG stands 

for the amino acid glycine. In summary, the "central dogma," which 

can be stated DNA ~ RNA ~ Protein, dictates that the synthesis of 

proteins is controlled by the nucleic acid and that the genetic code 

describes the relationship of the simple molecules that make up the 

strand of nucleic acids and proteins to each other. But where did 

these simple molecules come from and how were they organized? 
78 

TABLE V-l.- GENETIC CODE 

Bases 

First Second Third 

G C A U 

gly ala glu val G 

G gly ala glu val A 

gly ala asp val C 

gly ala asp val U 

arg pro gIn leu G 

C arg pro gIn leu A 

arg pro his leu C 

arg pro his leu U 

arg thr Iys met G 

A arg thr Iys ile A 

ser thr asn ile C 

ser thr asn ile U 

trp ser tenn leu G 

U tenn ser tenn leu A 

cys ser tyr phe C 

cys ser tyr phe U 

EXTRATERRESTRIAL EVIDENCE 

The study of meteorites - stimulated by the fall of one big 

unusual carbonaceous meteorite on Murchison, Australia, in 1969 -

has allowed close examination of extraterrestrial material. Numerous 

laboratories have shown that the Murchison meteorite contains 

many organic compounds that appear to have been synthesized 

extra terrestrially by non biological processes. One of the first classes 

of compounds studied was the amino acids. As compared to the 

20 amino acids found in proteins, a much larger number (over 100) 

have been estimated to be present in carbonaceous chondrites. 
r 

I 

I 79 

(The amino acids in the Murchison meteorite are optically inactive, 

unlike those in proteins, showing that both handed forms of a given 

amino acid were present in roughly equal proportions; see fig. V-2.) 

In addition to these fundamental building blocks of life, other 

important classes of organic molecules have been identified. These 

include heterocyclic bases, hydrocarbons, fatty acids, hydroxyacids, 

etc. These solvent-soluble materials represent at most 30% of the 

carbon found in these meteorites (or about 0.5% of the total meteorite weight). The remaining carbon is predominantly present as a 

solvent-insoluble phase. 

At this point it is important to remark that while the types of 

organic compounds found in meteorites are consistent with those 

expected to have served as precursors for the biochemicals of terrestrial life, we do not know the details of their synthesis on the 

meteorites. We cannot be sure that either the environments in which 

MIRROR 

o - ALANINE L - ALANINE 

Figure V-2.- An important property of carbon is one that allows the formation 

of two forms with identical composition - one form is the mirror image of the 

other, just as the left hand is the mirror image of the right hand. Thus there are 

two forms of alanine, L (levo-) the left handed and D (dextro-) the right handed 

versions. The L-amino acids predominate in terrestrial biology. 
80 

the meteorites originated or the organic matter synthesized were the 

same as those on the primitive Earth. However, the complex suite of 

organic molecules found in carbonaceous meteorites provides a new 

basis for our understanding the phenomenon of chemical evolution. 

As mentioned in chapter IV, the widespread occurrence of 

organic compounds - not produced by any life form - in the 

cosmos and within our solar system confirms the expectation, based 

on elemental abundances in the stars, that organic chemical evolution 

is one natural consequence of the evolution of matter in the universe. 

But organic chemical evolution is inextricably intertwined with the 

evolution of environments, be they interstellar clouds, meteorite 

parent bodies or planets; its progress toward the origin of life may be 

terminated at different stages depending on the physical and chemical constraints imposed by the environment. 

As noted in previous chapters, the Earth's atmosphere, oceans, 

and crust during the first 500 million years are difficult to define in 

any detail. Too little is known to fix the actual time of accretion, the 

heterogeneity of the accreting materials, the state of the Sun, and the 

quantity of short-lived radioisotopes that could have influenced the 

thermal structure of the early Earth. 

In many respects our knowledge of the early Earth is much like 

our knowledge of the early solar system. It is model-dependent and 

relies on the reconstruction of an environment by extrapolation from 

a record preserved in (but deciphered only in fragmentary fashion 

from) lunar rocks, meteorites, and remotely discernible features of 

Venus and Mars and of very ancient rocks and sediments of the 

Earth. As more of the record is unveiled , new evidence is revealed, 

new interpretations arise, and models undergo revision. Thus, all that 

can be done at present is to define a rather wide range of possible 

compositions for the early atmosphere and oceans and to suggest the 

implications of atmospheres and oceans within this range for the 

problem of the origin of life. These models have provided the basis 

for experimental simulation studies. 

THE EXPERIMENTAL ERA 

Laboratory efforts have been extensive and have provided much 

of the insight we have gained into the origins of life on Earth. 
81 

Because of the uncertainty of the nature of the primitive atmosphere, experimentation has explored a range of plausible models 

from those whose gas composition is strongly reducing to those with 

a more oxidizing composition. 

Strongly Reducing Atmosphere 

We will now explore that model which takes as its premise that 

the initial atmosphere of the early Earth was rich in hydrogen compounds, the one which has received most attention in the last 

generation. 

According to the Oparin-Miller-Urey paradigm, a highly reducing atmosphere consisting of methane, ammonia, and water - all 

hydrogenous - prevailed on the primitive Earth. Passage of energy in 

various forms through this hypothetical atmosphere produced the 

reservoir of organic molecules from which life evolved. 

The first successful prebiotic amino acid syntheses were carried 

out using his reducing gas mixture of CH4 , NH3 , H2 0 (or CH4 , NH3 , 

H2 0, H2 ) and an electric discharge as an energy source. The result 

was a large yield of amino acids (the yield of glycine alone was 2.1 % 

based on the amount of carbon present), together with hydroxy 

acids, short aliphatic acids and urea. One of the surprising results of 

this experiment was that the products were not a random mixture of 

organic compounds, but rather a relatively small number of compounds were produced in substantial yield. In addition the compounds produced were, with a few exceptions, of biological 

importance. 

The special mechanism of synthesis of the amino and hydroxy 

acids was further investigated. It was shown that the amino acids 

were not formed directly in the electric discharge but were the 

result of solution reactions of smaller molecules produced in the 

discharge, in particular, hydrogen cyanide and aldehydes. These 

reactions were studied subsequently in detail and the equilibrium and 

rate constants of these reactions were measured. These results show 

that amino and hydroxy acids could have been synthesized at high 

dilutions of HCN and aldehydes in a primitive ocean. 

Ultraviolet light acting on this mixture of gases is not effective 

in producing amino acids except at very short wavelengths 
82 

«1500 A) and even then the yields are very low. The low yields are 

probably a result of the low yields of HCN produced by UV light. If 

the gas mixture is modified by adding gases such as H2 S or formaldehyde, then reasonable yields of amino acids can be obtained at relatively long wavelengths «2500 A) where considerable energies from 

the Sun are available. The H2 S absorbs at these longer wavelenths 

and is photodissociated to Hand HS. The H atoms have a high velocity ("hot atoms") and react with the CH4 and NH3. It is possible, 

but not demonstrated, that HeN and other molecules are produced, 

which then form amino acids in the aqueous part of the system. In 

experiments simulating thermal environments, pyrolysis of CH4 and 

NH3 gives low yields of amino acids. 

A second model of a reducing atmosphere with less hydrogen 

would consist of CH4 , N2, and traces of NH3 and Hz 0. This atmosphere is more consistent with current models of the primitive 

Earth, though still consisting of hydrogenous compounds. Large 

amounts of NH3 would not have accumulated in the atmosphere 

because of photodestruction and also because the NH3 would dissolve in the ocean. 

This mixture of gases is quite effective with an electric discharge 

in producing amino acids. The yields are somewhat lower than with 

CH4 , NH3, and Hz ° but the products are more diverse. Hydroxy 

acids, short aliphatic acids, and dicarboxylic acids are produced 

along with the amino acids. Ten of the 20 amino acids that occur in 

proteins are produced directly in this experiment. Methionine is 

obtained if H2 S is added to the mixture, while cysteine, another 

sulfur containing amino acid, was found in the photolysis of CH4 , 

NH3, H20 , and H2 S. Phenylalanine, tyrosine, and tryptophan can 

also be synthesized under putative prebiotic conditions. Thus, only 

the basic amino acids - lysine, arginine, and histidine - have not 

been produced in prebiotic synthesis of amino acids. There is no 

fundamental reason that the basic amino acids cannot be synthesized, and this problem may be solved before too long. 

Mildly Reducing Atmospheres 

The geochemists, especially W. Rubey, were not happy with the 

proposal by Urey that the early atmosphere was composed of meth-
83 

ane, ammonia, and hydrogen. They favored a model for the atmosphere provided from volcanic outgassing. It was dominated by 

carbon dioxide, nitrogen, and water vapor; Abelson argued that 

"volatiles from outgassing interacted with the alkaline crust to form 

an ocean having a pH 8-9 and to produce an atmosphere consisting 

of CO, CO2, N2, and H2." A series of experiments were initiated in a 

mildly reducing atmosphere. 

CO, N2, H2 - Electric discharges acting on this mixture of gases 

are not particularly effective in amino acid synthesis, but HCN is 

produced in 'significant amounts. Glycine is produced in fair yield, 

but only small amounts of higher amino acids are formed. However, 

formaldehyde, which is important in the prebiotic synthesis of 

sugars, is obtained in large amounts. 

CO2, N2, H2 - The CO2 is more oxidized than the CO, but the 

excess H makes it a reduced mixture. As with CO + N2 + H2, the 

amino acid synthesis is quite low with electric discharges unless 

H2 /C02 ratio is ;(:2. In this case glycine is produced in fair yield, 

but again very few of the higher amino acids are formed. 

CO, H2 - This mixture is used commercially in the FischerTropsch reaction to make hydrocarbons in high yields. The reaction 

requires a catalyst, usually Fe or Ni supported on silica, a temperature of 200°-400°C, and a short contact time. Depending on the 

conditions, aliphatic hydrocarbons, aromatic hydrocarbons, alcohols, 

and acids can be produced. If NH3 is added to the CO + H2, then 

amino acids, purines, and pyrimidines can be formed. The intermediates in these reactions are not known, but it is likely that HCN is 

involved together with others. 

CO, H2 ° - Electric discharges are not effective with this mixture, but UV light that is absorbed by the water (:$1849 A) results in 

the production of formaldehyde and other aldehydes, alcohols, and 

acids. The yields are fair. The mechanism seems to involve splitting 

the H2 ° to H + OH with the OH converting the CO to CO2 and the 

H reducing another molecule of CO. 

The amount of hydrogen needed in the synthesis outlined 

above, except in the CO, H2 ° experiment, is still very high and it 

does not fare well with present ideas about the atmosphere of the 

early Earth. 
84 

Nonreducing Atmosphere 

If the early Earth's atmosphere were dominated by the gases, 

carbon dioxide, nitrogen, and water, then the Miller-Urey experiment 

would not be relevant to the origin of terrestrial life. The reduction 

of carbon dioxide and nitrogen would have to have taken place by 

means other than molecular hydrogen. Recall that reduction means 

the acceptance of electrons by a molecule or ion. Thus, an organic 

molecule in water, upon accepting an electron, gains a net negative 

charge which is neutralized by a proton donated by the water 

molecule itself. 

If the early atmosphere were not hydrogen-rich, the reduction 

of carbon dioxide could only be carried out if another supply of electron donors were available. A conceivable source of electron donors 

on the early Earth would be iron - the ferrous ions. 

In 1960, Getoff irradiated an aqueous solution of ferrous sulfate and carbon dioxide with light (~2600 A) and got a yield of 

formaldehyde of approximately 1 %. These observations should be 

considered in the light of recent proposals that the reducing conditions on a primitive Earth were to be sought in the abundance of 

ferrous iron in the crust rather than in the amount of hydrogen in 

the atmosphere. The problem of photochemically reducing N2 and 

NH3 is currently under active investigation. Low yields of NH3 form 

from N2 in the presence of the metals Mo, Fe, and Ti when irradiated by long-wavelength UV light. The reduction of carbon dioxide 

and nitrogen may have been possible on a primitive Earth if the right 

electron donors were present. This opens up new experimental vistas 

in the studies of the origin of life, especially in the synthesis of the 

amino acids. While the above discussion has dealt predominantly 

with the synthesis of amino acids, we would be remiss if we 

neglected the laboratory synthesis of the other organic species we 

have encountered in this text. 

As pointed out many times in the text, the nucleic acids playa 

fundamental role in terrestrial biology. In addition to phosphorous 

in the form of phosphate, the nucleic acids require both sugars and 

certain nitrogenous molecules, the purines and pyrimidines. There is 

ample laboratory evidence to show that these molecules are produced from the reactions of the simple molecule HeN in water. Of 

the purines, adenine (see Appendix) is the major one produced from 
r 

85 

HCN oligomers. Of the pyrimidines, cytosine is formed directly by 

the reaction of cyanoacetylene and cyanate, while uracil is the fmal 

product using either HCN or cyanoacetylene as the starting material. 

These are only examples from an extensive scientific literature. 

Sugars and Nucleosides 

The self-condensation of formaldehyde was a likely route to 

sugars on the primitive Earth, the formaldehyde being formed by the 

action of electric discharges or UV light on a mildly reducing atmosphere. This condensation is inhibited by HCN which reacts rapidly 

with formaldehyde. Therefore, the synthesis of sugars probably was 

delayed until the bulk of the HCN had condensed or was hydrolyzed. 

It is not clear how the relatively few sugars which have the central 

role in contemporary living systems (ribose and glucose) were 

selected from the very complex mixture of compounds which is 

formed from formaldehyde. 

Lipids 

The prebiotic formation of lipids has not been extensively 

investigated, but the limited experiments which have been performed suggest that lipid-like materials might have also formed 

spontaneously on the primitive Earth. 

Biosynthetic Pathways as a Guide to Prebiotic Chemistry 

Since the primitive metabolic pathways probably evolved from 

prebiotic syntheses, some steps in the current metabolic pathways 

may be "chemical fossils" of reactions which occurred on the 

primitive Earth. Consequently, a proposed prebiotic pathway gains 

validity if it can be correlated with a contemporary biosynthetic 

process. 

There is a good correlation between the contemporary biosyntheses of purines and pyrimidines and some of the steps in their proposed prebiotic syntheses. 
86 

As an example, the decarboxylation of the nucleotide of orotic 

acid is one of the steps in the biosynthesis of the nucleotides of 

uracil and cytosine. This same decarboxylation is a key reaction in 

the prebiotic synthesis of uracil from HCN. In addition, aspartic acid, 

the starting material for the biosynthesis of orotic acid , is produced 

in a variety of prebiotic experiments. It would not have been a major 

change for early life to evolve a system for the biosynthesis of orotic 

acid from the readily available aspartic acid once the low supply of 

preformed orotic acid limited the growth of primitive life forms. 

INORGANIC ASPECTS 

While much of the work in the study of chemical evolution and 

the origins of life have dealt with the formation, polymerization, 

and interaction of important organic molecules, there is an awareness 

on the part of most students of the field that inorganic chemistry 

was undoubtedly of fundamental importance in the processes responsible for the origins of life on Earth. Since it is known that all life on 

Earth now requires the metal ions for its chemical function, a number of scientists have questioned at what stage such fundamental 

processes became important. Early in the history of this field of 

study, Granick suggested that the first organization of preprotoplasm 

could be a primitive energy-conversion unit that could perform the 

elementary processes of photosynthesis and respiration ; that this 

unit originated within the domain of some common minerals; that 

the minerals that contain metal ions served both as coordinating templates and catalysts for various reactions; and that around this unit 

were formed organic molecules that gradually became organized into 

units of ever-increasing complexity. Thus, biosynthetic chains developed in a stepwise fashion. The metal catalysts of minerals became 

modified into the metalloenzymes; in these new complexes the same 

metals would become more efficient. 

In a similar vein, Bernal had earlier suggested in his book "The 

Physical Basis of Life" that clays were sites upon which organic 

molecules could be concentrated and react with each other. 

In recent years, studies of the role of metal ions and minerals in 

prebiological chemistry have shown promising results. As noted 
i87 

earlier, the reduction of CO2 to formaldehyde and methane has been 

accomplished by the interaction of UV light with an aqueous solution of CO2 and ferrous ions. In preparation for the Viking mission 

to Mars, the reduction of CO2 was also observed to occur on silicate 

surfaces. The reduction of nitrogen to ammonia by titanium dioxide 

has been accomplished, again by using UV light. 

In laboratory experiments, biomonomers have been synthesized 

in the presence of clays. Investigations have shown that clays affect 

the formation of amino acids and nitrogen heterocycles from CO and 

NH3 at temperatures of about 300°C. In addition, biomonomers can 

be adsorbed onto clays. This adsorption provides an excellent 

mechanism of concentration to facilitate subsequent chemical reactions. The clay- and/or metal ion-mediated oligomerization of 

biomonomers has also been demonstrated. Through this mechanism 

polypeptides and oligonucleotides have been formed in higher yield 

or with longer strands than in the absence of these inorganic components. Thus, inorganic chemistry may have played an important role 

in the emergence of life on Earth. 

POLYPEPTIDES, POL YNUCLEOTIDES, AND THE 

BEGINNINGS OF NATURAL SELECTION 

The transition from a mixture of organic molecules to an 

organized system that is capable of reproducing itself, represents the 

most puzzling problem in the study of the origin of life. We know 

that contemporary cells rely on proteins, very complex molecules, to 

catalyze specifically almost all biological reactions, including the 

replication and translation of nucleic acids. The proteins are themselves the products of the translation process, and their synthesis is in 

turn dependent on the presence of preformed nucleic acids. The 

origin of the genetic process thus appears to be a chicken-and-egg 

problem; which came first - proteins or the coded nucleic acids? 

Not everyone agrees that studies of the origin of self-replicating 

systems should concentrate on nucleic acids and proteins. Some 

researchers suggest a variety of simple, alternative self-replicating systems. Cairns-Smith, for example, proposes an entirely inorganic 

genetic system based on cation substitutions in clays. The major idea 
88 

is that clays could not only adsorb and catalyze reactions between 

organic molecules but that they could, like DNA, replicate. If we 

now suppose that, as in the case of DNA, the possibility exists of an 

error of replication or mutation, the replicating clays would evolve! 

At present, however, the self-replicating systems we know best involve 

molecules that resemble proteins and nucleic acids. The remainder of 

this section, therefore, is concerned with studies of these molecules. 

The Contemporary System 

A genetic apparatus is an essential requirement for all living 

things on Earth. It is by means of the genetic material that living 

organisms are able to store, express, and upon reproduction, transmit 

to their progeny the information for all of the capabilities which 

they possess. Cellular life forms usually store genetic information in 

double-strand DNA polymers (fig. V-3), though some viruses make 

use of RNA instead. As mentioned earlier, the information is coded 

in the sequence of the nucleotides in such a way that each of the 

64 possible trinucleotides codes for one of the 20 acids to be incorporated into a protein (table V-I), or codes for a stop signal to 

terminate protein synthesis. The two strands of the nucleic acid are 

held together by relatively weak (hydrogen) bonds, made specific by 

a unique and essential feature of the conformation of the four 

nucleotides - adenylic acid hydrogen-bonds specifically with thymidylic acid, while guanylic acid pairs only with that of cytidylic acid 

(fig. V-4). These interactions are referred to as the Watson-Crick 

pairing rules. Partly because of these unique pairing specificities, 

either strand can serve as a template for the synthesis of the other, 

given free energy, in the enzyme-catalyzed process known as replication. In this fashion, one double strand can yield two new doublestranded molecules, one for each of the progeny after cell division. 

The expression of genetic information requires that the information in the DNA be converted into protein. This is accomplished 

in two steps. First, in a process called transcription, one strand is 

copied by an enzyme, yielding a complementary strand of RNA 

(messenger RNA). This new strand then serves as the template for 

the synthesis of protein. The process of protein synthesis is termed 

translation because the nucleic acid "language" is now translated into 
SIDE VIEW 

I 

I 

89 

Figure Y-3.- DNA (deoxyribonucleic acid) consists of two polymers linked 

together by pairs of purine and pyrimidine bases. Of the four types of bases, 

adenine can pair with thymine and guanine can pair with cytosine (Watson-Crick 

pairing rules,' A -T; G -C). 
90 

Figure V-4.- The two strands of nucleic acids are held together by hydrogen 

bonds which are specific for the four nucleotides. A "double helix" is thus 

created and these interactions are referred to as the Watson-Crick pairing rules. 

protein "language." The "dictionary" which establishes the rules by 

which this translation is accomplished is called the genetic code. The 

genetic code, shown in table V-I, was thought to be entirely universal until quite recent experiments showed that in mitochondria, one 

of the stop signals actually codes for one of the amino acids, tryptophan; another amino acid, methionine, has two codons rather than 

only one. While these exceptions are rather minor variations on a 

major theme, they do emphasize the fact that isolated systems can 

evolve slightly different codes. 

Which Came First: Proteins or Nucleic Acids? 

Several people have proposed that the first genetic system was 

composed of proteins alone. They point out that it is much easier to 

synthesize and polymerize the amino acids than the nucleotides. 

Furthermore, the resulting polymers have some catalytic ability; in 

the contemporary cell, proteins play the major role as catalysts. 

Nucleic acids, on the other hand, are not usually thought of as having 

catalytic activity, although they do act as templates for replication 

and transcription (and a "bringing together" of reactants is certainly 
91 

one important function of many catalysts). In what follows, we will 

examine first the systems that contain only proteins, then those that 

contain only nucleic acids, and finally the combined systems. 

Proteins first- The nonordered polymerization of amino 

acids to fonn analogs of proteins (called peptides if the chains are 

short) has been demonstrated in the laboratory under a wide variety 

of experimental conditions. The energy needed for this reaction can 

be provided in a number of ways, one of which is heat. Polyamino 

acids (sometimes called proteinoids) have been produced by heating 

mixtures of amino acids to about 180°C. These temperatures are 

greater than the boiling point of water; however, in a few experiments similar products were fo und at temperatures below the boiling 

point of water, after very long periods. Clays also catalyze the reacwhen alternately wetted and dried. The fonnation of peptides on 

clay is further catalyzed by a simple peptide, i.e., histidyl-histidine. 

This suggests that the first enzyme-like molecules may have been 

very simple peptides. 

Another approach is to provide the energy needed for synthesis 

of peptide bonds through the use- of other energy-rich chemical 

species called condensing agents. Still another approach is to use 

amino acids which have been "activated" prior to reaction. These 

experiments are convincing evidence that amino acids readily enter 

into combinations with one another, but these experiments do not 

address the problem of reproducible organization into specific 

sequences which would participate in self-replication. 

The main difficulty with the "protein first" hypothesis is that 

it does not seem possible to fonnulate a plausible scheme for protein 

self-replication based on known properties of amino acids and peptides. One type of proposal postulates a complementary pairing of 

amino acids on two chains, analogous to base-pairing in nucleic acids. 

Complementariness might depend on size, charge, hydrogen 

bonding, or some combination of these properties. Such schemes 

are certainly possible in principle, but there is no experimental evidence for them. 

Another group of proposals suggests that a family of peptides 

forms a cycle in which each member of the cycle catalyzes the 

synthesis of one or more other members of the cycle. It has been 
92 

shown that the synthesis of certain peptide antibiotics of welldefmed sequence is brought about by a group of specific proteins 

(enzymes) without the help of nucleic acids. However, the peptide 

antibiotics that are synthesized are relatively simple and molecules 

of this size would not be capable of catalyzing the synthesis of complex specific proteins. Certainly, the enzymes involved are very complex and are themselves synthesized with the help of nucleic acids. 

We do not think, therefore, that this system is a good model for 

protein self-replication. It is dangerous to be dogmatic about general 

schemes of self-replicating peptide cycles, but we suspect that they 

all suffer from the same problem: simple peptides lack sufficient 

specificity, while large peptides are too hard to make so that it would 

be impossible to close the cycle. Furthennore, even if one selfreplicating cycle existed, it is hard to see how it could evolve to 

greater complexity. 

Nucleic acids first- There are a number of theories in which 

polynucleotide replication is proposed to have preceded the synthesis 

of ordered polypeptides. Proponents of such theories emphasize that 

nonenzymatic complementary replication of polynucleotides seems 

plausible in light of the known interactions between nucleotide 

bases. We shall see that there is already a substantial body of experiments supporting the idea that a preformed polynucleotide can 

direct the synthesis of a complementary oligonucleotide according to 

the previously described Watson-Crick pairing rules. The required 

"preformed" nucleic acid may itself have arisen originally in a nonordered joining reaction of mononucleotides; such reactions are 

already known. The polymerization of nucleotides, just as in the 

related reaction of amino acids to form a polypeptide, effectively 

requires the removal of one molecule of water for each addition of a 

nucleotide to the growing chain. Therefore, it is not surprising to 

find that successful polymerizations have usually employed drying 

conditions, the addition of "condensing" reagents, or the removal of 

water in a prior step to form an activated monomeric nucleotide. The 

addition of a catalyst, such as a metal ion, can increase the yield of 

polynucleotides quite appreciably. It has been shown that divalent 

metal ions enhance the formation of oligoadenylates from an 

activated derivative of adenylic acid. In the case of lead ion a 56% 

yield of oligomer was formed, while in the absence of this metal ion 

I 

J 
93 

the yield of oligomer was about 4%. It has recently been shown that 

simple catalysts can be important in more sophisticated sequencecopying reactions, and certainly cont.emporary replication enzymes r 

have all been found to contain a metal ion at the active site. 

Template-directed polynucleotide synthesis-The central reaction responsible for the stability of inherited characteristics is nucleic 

acid replication, while a major source of genetic variation is the 

inaccuracy of this process. This had led many researchers to postulate that nucleic acid replication, in which a preformed polynucleotide template directs the synthesis of a new complementary strand, 

was the first "genetic" process of the primitive Earth. This theory 

is appealing, but it should not be accepted as dogma. 

Under certain conditions, organized if unusual helical structures can form between polynucleotides and the complementary 

monomeric nucleotides or nucleosides. It has been shown that if 

certain activated condensing agents are added to energize the system, the monomers can join to form short oligonucleotides. These 

reactions have established that the Watson-Crick pairing rules apply 

to these systems too in a non biological setting. A detailed analysis 

of the products of early experiments revealed a startling structural 

difference between this chemical condensation and the normal 

enzymatic reaction. (The predominant internucleotide linkages in the 

chemical product are 2'-5' rather than 3'-5' (fig. V-5), but in contemporary cells, it is the later linkage which is found almost 

exclusively.) 

Several years ago, it was shown that the Zn2 + and Pb2 + ions are 

effective catalysts for energized template-directed synthesis yielding 

oligomers of guanylic acid with chain lengths in excess of 30. (The 

Pb2 + ion gives predominantly 2'-5'-liriked products, while the Zn2 + 

ion gives mainly 3'-5'-linked products.) The Pb2 + reaction has an 

error rate of about 0.1 in the presence of a "wrong" base, while the 

Zn2 + reaction has a much lower error rate of 0.005. Thus, the Zn2 + 

catalyzed reaction produces products with the "usual" linking and 

with an accuracy comparable to the accuracy required by a nonenzymatic replicating system. 

A more recent modification with a slightly different activating 

group produces a comparable preference for the "usual" 3'-5' 

linkage and comparable accuracy, without requiring Zn2 +. True 
94 

0-

I 

O=p--O ___ 

I CH2 BASE 

0-

o OH 

I 3'-5' BOND 

0-

I 

O=p--O I ----CH2 0 BASE 

0- K 

OH 0 

I 

2'-5' BOND 

0=1--0---cH2 0 BASE 

0- K 

OH OH 

Figure V-S.- There are two possible ways to form phosphodiester bonds in 

the process of building a polyribonucleotide from the monomers. Biology utilizes only one of these linkages in RNA. That linkage is the 3'-5' one illustrated 

above. 
r I 

95 

nonenzymatic replication of a nucleic acid with a mixed base 

sequence through several generations of "offspring" has not yet been 

demonstrated experimentally. 

Thus, the question of whether nucleic acid sequences could 

evolve by natural selection cannot yet be answered in a prebiotic system, but experiments using biological enzymes were done to address 

this question. Replicating systems consisting of the enzyme QB-RNA 

polymerase and certain small RNA substrates have been used to 

demonstrate molecular evolution in the test tube. One set of experiments started with an RNA substrate that absorbs a dye, ethidium 

bromide, and is then unable to replicate efficiently. By allowing the 

RNA to replicate repeatedly in the presence of gradually increasing 

concentrations of the dye, a new RNA substrate was generated which 

was no longer inhibited, because it no longer bound the dye-stuff so 

tightly. This system does not show self-replication, because each 

round of synthesis required the addition of new enzymes. However, 

experiments like these do show that RNA molecules can adapt by 

natural selection to bind or reject specific organic molecules, and 

this is relevant to discussions of the origins of the genetic code. 

Recent experiments demonstrate for the first time that nucleic 

acids themselves might cause interesting reactions to occur. An RNA 

strand was observed to "snip out" a portion of its own sequence, 

apparently without help from enzymes. This cannot really be called 

catalysis, since the molecule acts on itself, and does so only once for 

each molecule, but it does suggest that nucleic acids may be capable 

of at least a few specific catalytic-type reactions. However, in the 

absence of additional evidence that polynucleotides are able to function as catalysts, one cannot feel confident that nucleic acids could 

do enough interesting chemistry to "go it alone." 

As with amino acids (fig. V-2), nucleotides possess optically 

active configuration. Observations that D-nuc1eosides react more 

efficiently than L-nucleosides on a nucleic acid template made of 

D-nucleotides, suggests that nucleotide chains that are made up of 

monomer with the same enantiomeric configuration can undergo 

template-directed replication, while "mixed" oligomers cannot. Of 

course, either all D- or all L-oligomers would replicate equally well, 

so this result shows only that the components of the primitive 

nucleic acids must either all have been L-, or all have been D-isomers. 
96 

It does not explain why only D- instead of L-nucleic acids are important in biology. 

Proteins and nucleic acids together- Since neither proteins 

alone nor nucleic acids alone seem likely to be able to account for all 

of the genetic properties needed for self-replication, the alternative is 

to consider the development of a combined system of both proteins 

and nucleic acids. This requires a coupling between the two kinds of 

molecules in the form of at least a primitive genetic code. 

While speculations as to how genetic coding might operate even 

predated elucidation of the structure of DNA in 1953 by Watson and 

Crick, an understanding of the essential nature of the genetic material greatly stimulated the desire to understand how it is expressed. 

As information accumulated in the 1950s and 1960s about the 

molecular mechanisms of transcription, translation, and the coding 

process, another question began to emerge. Why, for example, is 

UUU a code for phenylalanine? There has perhaps been more speculation about the basis for the origin of the genetic code than any 

other aspect of molecular biology, and to enumerate and discuss 

them all is impossible in a short review. The theories fall into two 

groups: (a) Were the genetic code assignments based on some relationship (perhaps affmities) between amino acids and nucleotides, or 

(b) were they the result of random processes? The idea that the code 

is based on chance evolutionary processes, implies that we are not 

likely ever to understand the basis for the origin, so that experimentalists have necessarily been concerned with the first theory. Evidence for a physicochemical basis for the code, however, has not 

been abundant. Experimentalists have shown that mononucleotides 

have differential affinities for polybasic amino acids, but these affinities relate more to the self-associative properties of the mononucleotides than to code-related specificities. However, recent work has 

shown a preferential affmity of certain polyamino acids, polylysine 

for A-T rich DNA, and polyarginine for G-C rich DNA. In a similar 

fashion, it has been shown that the aromatic amino acids (tryptophan, phenylalanine, and histidine) have different affinities for polyadenylic acid. This information was important in showing that, at the 

monomer level, selectivities do exist. 

Investigations have demonstrated that there are variations in 

affinities of nucleotides for amino acids affixed to a column material, 
97 

but these variations were not clearly related to the code. Similarly, 

studies have shown a differential uptake of nucleotides and amino 

acids into detergent droplets called micelles, but code-related correlations were not evident from this work either. In 1976 the first 

clear-cut, code-related correlations between hydrophilic (water 

loving) and hydrophobic (water hating) properties of amino acids 

and nucleotides were reported. In addition, other researchers showed 

that a number of additional properties, including hydrophobicity, 

polarity, and bulkiness were also correlated between amino acids and 

nucleotides. Thus, very weak interactions can be detected between 

amino acids and nucleotides, but whether these interactions provide 

enough selectivity for a translation process is not yet clear. 

The demonstration of a weak, specific interaction between an 

amino acid and a nucleotide does not put us much farther forward 

unless we can couple it to an efficient peptide synthesis reaction. 

Many mechanisms have been proposed for simple systems that might 

be capable of translating nucleic acid sequences into peptides of protein sequences, but no experiment has ever shown actual translation 

in the absence of the complex components of the contemporary cell. 

A few experiments have shown a slight influence of nucleic acid on 

the yield of condensation of a single amino acid, with variation of 

yield as either the amino acid or nucleic acid is changed, but these 

experiments are not convincing evidence of a translation effect. As a 

result further experimental verification is in order. 

There is no paucity of suggestions for the origin of protein 

synthesis and of the genetic code, but there are few suggestions that 

are both chemically explicit and believable. Some papers are rather 

philosophical and do not lead immediately to a testable experiment. 

Others are explicit but the chemistry appears to be inconsistent with 

present knowledge. Actually the requirements of an efficient peptide 

synthesis system can be stated quite simply. The reacting amino 

acids, perhaps attached to specific oligonucleotide carriers, need to 

be lined up on the template, such that adjacent amino acids are held 

close enough together and in the proper orientation to allow reaction. The amino acids need to be in a sufficiently reactive form, so 

that the peptide bond formation is a spontaneous reaction, but not 

so reactive that hydrolysis competes with peptide bond formation. 

In addition, the mechanism has to be recursive, i.e., repetition of the 

process should result in a gradual elongation of the peptide chain. 
98 

Proposed translation models- Although most of the models 

that have been suggested for the first translation apparatus closely 

resemble the contemporary process of reading a linear message with 

triplet adapters, a number of other models have to be kept in mind 

as possibilities. Examples include systems involving adapters reading 

a single base rather than triplets, mechanisms with direct bonding of 

amino acids to polynucleotide templates, and various feedback 

linked systems. These tend to suffer from the problem of explaining 

how the contemporary genetic system could have evolved from such 

a different mechanism. On the other hand, some of these alternative 

models tend to be more easily evaluated experimentally. For 

instance, recent experiments have demonstrated that amino acids 

attached to single nucleotides give an enhanced yield of peptide 

formation when the nucleotides are lined up on a complementary 

strand of polymer. There clearly remain many other novel mechanisms to be discovered and suggested. 

It must be kept in mind that the problem may not yet be solva~ 

ble because some vital information may not have been discovered. 

Additional knowledge of the three-dimensional structure of ribosomes and transfer RNA is beginning to shed more light on how the 

complex translation apparatus operates in the contemporary cell. 

Structural studies of nucleic acids can still yield some surprises. It 

seems likely that increasingly detailed knowledge will suggest new 

possibilities for prebiotic studies of the translation mechanisms. 

In spite of the caveats that we must acknowledge, the correlations of properties and affinity data between amino acids and nucleotides, while certainly leaving us far from final answers, at least suggest that discernible patterns exist in the coding mechanism and give 

hope that primitive translation can be elucidated when sufficient 

data are available. 

MEMBRANES 

We can now ask what kinds of structures could coevolve with 

a replicating system that might enhance the ability of the system to 

incorporate the functions described above, and to evolve toward 

the structure we accept as a living cell. A key component of all 
99 

cells is the cell membrane, which allows cells to maintain an internal 

milieu different from the external environment in the composition 

and concentration of compounds. The main structural component of 

all membranes - with rare minor variations - is the lipid bilayer. It 

is as fundamental a structure in cells as the DNA double helix, in that 

it constitutes the basic permeability barrier which delimits the cells 

and controls their interaction with the environment. 

In prebiotic experiments, some alternatives to lipid bilayers as 

permeability barriers have been proposed, including coacervates, 

microspheres of proteinoid, or other heterogeneous polymers, 

micelles, and surfaces of minerals. All of these show some ability to 

selectively concentrate or retain some of the molecules characteristic 

of living cells. However, none of them is as efficient as lipid bilayers, 

and in addition it is difficult to account for how these could evolve 

into the lipid bilayer membranes of modern cells. 

In the absence of an understanding of exactly how membranes 

or other structures became coupled to genetic mechanisms, it is best 

to remain undogmatic about which structures were most important, 

or about whether genetic mechanisms developed before or after the 

structural components. 

It is useful at this point to list some properties of lipid bilayers 

that may be relevant to the development of replicating systems on 

the prebiotic Earth. These properties fall into subcategories that 

include physical, chemical, and supramolecular aspects of lipid 

organization in aqueous environments. Only one class of lipids, the 

phospholipids (fig. V-6), is generally involved in the formation of 

membranes. It is a striking property of most phospholipids that in an 

aqueous environment they form stable bilayer structures that typically close to form vesicular membranes. Since phospholipids have 

been synthesized under plausible prebiotic conditions and have been 

demonstrated to form vesicles, it is assumed that lipid bilayer vesicles 

were present on the prebiotic Earth. 

The next question concerns how such structures might contribute to prebiotic evolution. The chemical properties of lipid bilayers 

include a highly charged surface and a nonpolar interior of the 

bilayer. These two properties represent an almost totally unexplored 

area for research in prebiotic evolution, and suggest a number of new 
100 

LAR HEAD 

NONPOLAR 

TAILS 

.l=:::::::::::::: _:::: ..... =--: ........ 

• • 

:=~ • 

=: :::::::::: • 

• ~=::::.-:= :::;::: 

WATER. ::::::::: ::: 

.J:=:=::==: :::::::: 

• • • 

• ::::::: :::::::= ::;;:: • • -:: ::::: :::: :: . 

• :=:= ::: • • :::::::::: ::::::::~ . 

WATER 

Figure V -6.- The membranes of most living cells are made up of (a) phospholipids. They spontaneously form a lipid bilayer (b) in water as illustrated above. 

research directions. For instance, it is likely that the charged surfaces, like those of clays, have catalytic properties, and this possibility should certainly be investigated. 

The hydrophobic moiety also holds considerable interest. All 

contemporary light energy transducing systems depend on a nonpolar membrane phase to embed specialized pigments and enzymes, 
101 

and to provide a barrier for the electrochemical gradients produced 

by the pigment-enzyme systems. It is reasonable to assume that pigment molecules, formed under prebiotic conditions, would partition 

into the nonpolar phase of lipid bilayer membranes and offer primitive light energy-trapping functions. 

A significant physical property of the lipid bilayer is its relative 

permeability to various ionic and molecular species. In contemporary 

cells, the bilayer is understood to be the major barrier to free diffusion of water-soluble substances. The special permeability properties 

of cell membranes must be attributed to their other major constituent, the proteinaceous channels and enzymes that permit or drive 

ion and chemical transport. Recently, small peptides (e.g., valinomycin) have been shown to form specific ionic channels in a lipid 

bilayer. 

Energy Transduction by Early Membranes 

We can now go on to discuss possible contributions of membrane structure to evolution of energy transduction systems on the 

prebiotic Earth. The energy necessary to generate and maintajn the 

organization of early life forms has usually been assumed to have 

come from chemical reactions of compounds synthesized by prebiotic processes. There is, however, another source of energy, which 

has largely been overlooked. Given the existence of lipid bilayerenclosed vesicles, concentration gradients across the lipid bilayer 

would arise if the environment should change after the vesicles have 

formed. Chemical reactions favored by the milieu inside the vesicles 

would also set up concentration gradients. Concentration gradients 

of charged solutes could generate diffusion potentials and the flux of 

one charged species across the lipid bilayer could be coupled to the 

flux of any other charged species. If amino acids are polymerized 

inside a vesicle, this would give rise to an amino acid gradient, as 

would the breakdown of the polymer. In a similar fashion, many 

chemical reactions would change the acid/base nature of the vesicle 

interior. These could couple chemical reactions in the vesicle interior 

to transport processes. For example, many organic weak acids and 

bases permeate lipid bilayers easily in their undissociated form and 

would accumulate or be depleted inside vesicles if pH gradients 

existed. 
102 

Gradients of other ions such as sodium, potassium, and calcium 

can be modulated by carriers or channels. Relatively simple organic 

compounds are known which act as carriers for these ions, and it is 

possible that analogous compounds could have arisen abiogenically. 

It should be noted, however, that in contemporary cell membranes 

channels rather than carriers appear to be the main mediators of specific ion perrneabilities. 

Closed membrane compartments play, of course, a crucial role 

in the two most important contemporary energy conversion systems - photosynthesis and respiration. Their evolution has been 

extensively discussed and will not be further pursued here. However, 

cells presumably existed before either of these mechanisms evolved 

and our main purpose is to discuss the problems of evolution from 

abiogenic organic matter to protocells. The speculations advanced 

here are necessarily based on our knowledge of present-day living 

cells. Unfortunately, fossils tell us little about the chemistry of 

Precambrian life. 

Thus, we have been taken full circle back to the Precambrian. 

We have traced our knowledge from the fossils dated 3.5 b.y. 

through our speculations about the early Earth from 4.5 b.y. until 

3.5 b.y. We must note here that this gap of almost 1 billion years is 

slowly being filled by our experiments in the laboratory and by our 

expanding knowledge of our Earth and solar system. We have therefore deemed it necessary to outline in the concluding chapter the 

next steps in our quest to close the gap and elucidate the nature of 

the origins of life on Earth. 

SUGGESTIONS FOR FURTHER READING 

A. G. Cairns-Smith: Genetic Takeover. Cambridge University Press, 

London, 1982. 

W. Day: Genesis on Planet Earth. Talos Press, East Lansing, Mich., 

1979. 

R. E. Dickerson: Scientific American, vol. 239, no. 3, 1978, 

pp.70-87. 
103 

J. G. Lawless, C. E. Folsome, and K. A. Kvenvolden: Organic Matter 

in Meteorites. Scientific American, vol. 226, no. 6, 1972, 

pp.38-46. 

S. Miller and L. Orgel: The Origins of Life on Earth. Prentice Hall, 

Englewood Cliffs, N.J., 1974. 

105 

VI. RECOMMENDATIONS 

INTRODUCTION 

As a field of active scientific inquiry, the study of the origins of 

life is clearly in its infancy. By its very nature, it is multidisciplinary, 

requiring contributions from astronomers, biologists, chemists, 

geologists, physicists, and many others. 

We have seen that the gaps in our knowledge of the steps from 

the nonliving to the living are numerous. Among these gaps are 

(a) a solar system formation with its accumulation of raw materials, 

(b) the synthesis of the life-forming monomers, such as the amino 

acids, nucleotides, and lipids, (c) the condensation of these monomers into useful polymers such as proteins and nucleic acids, (d) the 

sequestering of these materials into droplets of proteinoid or 

membrane-like structures, and (e) the development of a chemical 

memory (the genetic code) to pass on to the progeny the information acquired. 

Throughout the text we have shown the partial answers to the 

many questions we have asked about organic chemical evolution and 

the origin of life, yet much remains to be learned. Thus, it behooves 

us to identify those unanswered questions. The following discussions, 

therefore, represent an attempt to identify these issues with the 

provisions that (a) the compilation is not intended to be complete, 

but rather, it represents important areas of future research identified 

by a specific group of people, and (b) progress in anyone of these 

areas might change the whole direction of research in the field. 

Professor Harold Urey was a remarkable American 

scientist who as much as any other man opened up 

the field of origins of life to modern study. 
106 

SOLAR SYSTEM 

The record of the early history of the solar system may be preserved in the organic and inorganic matter of the comets. Present 

evidence is consistent with two possible scenarios for the origin of 

the organic molecules sublimating from the cometary nucleus. Either 

they represent the "frost" of interstellar molecules that condensed 

onto those interstellar grains that have later accreted into comets, or 

they represent the "snows" that condensed onto silicate grains during the cooling phase of the presolar nebula. If they represent the 

frost of interstellar molecules, we must obtain a more detailed understanding of what this frost is. To accomplish this we propose to 

elucidate the major processes (stellar, circumstellar, interstellar, etc.) 

involved in the formation of gaseous organic molecules detected in 

interstellar space. For example, is the formation of much more 

complex solid polymeric organic molecules possible in interstellar 

dust-ice grain surfaces? How could they be detected? 

In addition, we need to acquire new and improved receivers at 

millimeter and submillimeter wavelengths, and new, accurate, and 

large telescopes to collect these wavelengths. Unavailable funds in 

this area would likely set the United States back in its worldwide 

lead in the study of the interstellar molecules during the 1980s. 

Therefore, a greater effort should be exerted to convince the Government of the significance of these research tools. 

However, if the comets represent the snows that condensed during the primitive solar nebula, then the organic and inorganic constituents would record for us the composition of the early solar 

system. 

We can't go on much longer without knowing more about 

comets. Even if these objects did not bring biomonomers to the surface of the primitive Earth, they may have been a major source of 

the volatile elements from which these compounds were later 

formed. We need to study comet nuclei with a rendezvous mission 

that brings a spacecraft close enough to permit detailed measurements of the nucleus and inner coma. The program should build 

toward a capability for sample return and interception of a large, 

active comet that is approaching the inner solar system for the first 

time. 
107 

Another possible record from which to study the composition 

of the early solar system is meteorites. Consideration of models for 

the origin of solid bodies in the solar system, and data obtained from 

the study of meteorites raise a number of important issues. 

Differentiation and hydrothermal processes occurred on small 

bodies very early in the chemical evolution of the solar system. Similar processes took place on the primitive Earth, but the chronology 

and the consequences of these processes on the thermal, geochemical, and atmospheric evolution of the Earth's prebiotic environment 

are major unknowns, and require elucidation. Insofar as processes 

operating on meteorite parent bodies can be generalized or related to 

similar processes on the Earth, then continued study of their manifestations in meteorites will contribute significantly to understanding 

the environment in which life originated. 

The hypothesis that an interstellar cloud of dust and gas provided the spawning ground for the entire solar system is generally 

accepted. Models based on this hypothesis hold promise of providing . 

a coherent framework for understanding the origin of the solar system and of, ultimately, life itself. In this context, relationships 

should exist between the organic matter in interstellar clouds, 

comets, and carbonaceous meteorites; thus, research efforts aimed at 

elucidating the nature of the relationship should be strongly encouraged. Contributions to the confirmation or denial of models for the 

origin of bodies in the solar system should become a major objective 

of organic chemical evolution research. 

Planetesimals resembling meteorites and their parent bodies are 

believed to have supplied the building blocks for formation of the 

planets. Carbonaceous meteoritic material would have contributed 

significantly to the crust, mantle, and inventory of volatiles of the 

primitive Earth. How these contributions influenced its physicalchemical evolution and, therefore, the setting in which life originated should be of major concern in the study of the origin of life. 

We still don't know where the carbonaceous meteorites came 

from; however, asteroids represent prime candidates. We do know 

that there are large numbers of dark asteroids whose reflectance 

spectra resemble those of carbonaceous chondrites. Is there a connection? Do these objects also contain organic compounds? 

From the above considerations it is obvious that detailed 

studies of meteorites should be continued. And, a mission to the 
108 

asteroid belt which could sample and return materials to Earth is 

important in our attempt to elucidate the history of the early 

solar system and, hence, to the origins of life. 

Evidence bearing on the problem of the chemical setting for 

prebiotic synthesis may yet be forthcoming from comparative 

planetology. The information derived from the knowledge of our 

sister planets will provide constraints for the origin and development of our Earth. 

The colors on Jupiter have long been thought to indicate the 

presence oJ organic chemical synthesis. Since the highly reducing 

atmosphere of Jupiter is consistent with some models of the Earth's 

early atmosphere, Jupiter may well represent a model for primitive 

Earth's chemistry. It is becoming increasingly evident that we are 

going to have to go there to find out. The Galileo Project will make 

a first step in this direction, but it is clear that more sophisticated 

explorations are required. 

In the same vein, Titan, despite its low temperature, remains 

important because of its methane/nitrogen atmosphere, its red 

color, and the variety of evidence suggesting the presence of a photochemical smog. Because chemistry is taking place in this primitive 

reducing atmosphere today, and 'the products of these reactions are 

believed to be accumulating on the satellite's surface, a wellpreserved record of carbon cosmochemistry undoubtedly exists. 

Thus, preliminary explorations of this environment are in order. 

Within the context of a totally different planetary environment, 

there is still much to be learned from Mars after Viking. For example, judging from the ages deduced for some parts of the surface 

from densities of impact craters, there should be very old (older than 

4 billion years) rocks on Mars. These rocks should presumably tell us 

whether or not Mars ever had a strongly reducing atmosphere. 

In addition, detailed surface analysis of Mars may yet provide 

information about organic matter in protected environments, and 

information on the ages of various strata would be fundamental in 

understanding the epochs of liquid water. Finally, the mineralogy 

would help to answer two important questions. First, are the "missing volatiles" tied up in the form of carbonates, nitrites, nitrates, and 

sulfates? And second, laboratory studies using iron-rich clays appear 

to satisfactorily explain the results of the Viking experiments. Are 

these clays present, and if present what is their nature? Thus, we are 
109 

presented with a comparative approach in assessing the early evolution of the solar system and the origin of life. Weare adjusting our 

sights to the understanding of the planetary stage on which life 

enters as a player. Was the ancient Earth the only stage on which life 

could play? This is the deepest question that future research must 

answer. From comets to planets, NASA is the prime agency gathering 

the data necessary for providing an answer to this question. 

THE EARTH 

Studies of the origin of life require an accurate reconstruction 

of conditions and events on the early Earth. Investigations of those 

matters, once rare, are becoming more common as new techniques 

become available and as other developments in Earth science allow 

problems to be more clearly defined and profitably attacked. As this 

work proceeds, it must constantly be borne in mind that the early 

Earth was, quite literally, "a different planet." The contrasts 

between surface conditions on the early Earth and those on the 

modem Earth are nearly as large as the contrasts between the surface 

conditions presently found on the Earth and on Mars. It is not just a 

play on words, therefore, to speak of a "mission to the early Earth" 

just as we might speak of a "mission to Mars." While the latter 

involves the use of spacecraft and a journey of millions of miles, the 

former presents equal challenges (and promises equal scientific 

returns) in a journey across billions of years. 

The space sciences thus have a special contribution to - and 

should have a special interest in - the studies of the earliest phases of 

Earth's history. The space scientist's avoidance of "geocentric bias," 

the unwarranted attribution of Earth-like characteristics to other 

planets, is perfectly appropriate to studies of the early Earth. 

Equally, the development of an accurate view of the earliest stages of 

Earth's history can provide crucial information regarding the origin 

and formation of planets generally, and can significantly constrain 

theories regarding the origin and development of the solar system. 

Contrasts between the ancient and the modem Earth are 

nowhere greater than in biology, which has embodied a progression 
110 

from no life, to primitive forms, to bacteria resembling those encountered today, to - after billions of years - single-celled plants and 

animals with biochemical systems like those found in higher life 

forms. This progression of microbes has caused events of planetological significance - the development of an oxygenic atmosphere and 

the deposition of vast mineral deposits being but two examples. 

Studies of the origin and early development of life, thus, simply 

cannot be separated from general investigations of the early history 

of the Earth as a planet. The geochemical record of the history of 

the volatile elements on Earth is the record of the history of life on 

Earth, and an understanding of that record is crucial to an accurate 

reconstruction of events in the early solar system. 

The record from the early Earth has no parallel at later stages of 

Earth's history; close analogs may eventually be found on other 

planets, but no similar environments occur on the modem Earth. In 

a situation so without precedent, it will be necessary to constantly 

avoid unjustified extensions of present geochemical models. As it is 

acknowledged that the early Earth was without multicellular organisms or land plants, it must be recalled that this places all the primary 

productivity in the hands of microorganisms and creates a global ecosystem very different from any which has been considered for the 

past 0.5 b.y. As microbial ecosystems are then recognized as especially important subjects for study by planetary biologists, it must be 

recalled that the global impact of modem microbial ecosystems is 

buffered, perhaps powerfully, by the great mass of the biosphere 

which lies outside them. 

The establishment of a research program at the interface 

between planetology, geology, and microbial ecology is, if anything, 

overdue. As it is welcomed and carried forward , the origins of its 

importance and the uniqueness of the problems it addresses must be 

kept constantly in mind. The best contributions will result from the 

arduous confrontation of all the evidence: biological, chemical, geochemical, geological, and astrophysical. 

Such a research program must answer a number of questions. 

For example, can there have been any survival of prebiotic organic 

matter? How (chemically) might this prebiotic organic material be 

recognized? When did life first appear? To answer this question, the 

analysis of rock systems (e.g. , Isua, Greenland ; Swaziland, S. Africa; 

and Pilbara, N.W. Australia) is of prime importance. Studies should 
L 

111 

include those on organic geochemistry and isotope analyses, mineralogical and elemental analyses, and micromorphological and analogue community analyses. 

Fossils are the records of historical events important in the 

origin and evolution of life. They exist in three forms: (1) embedded 

in rocks, (2) inherent in the complex, metabolic patterns of living 

organisms, and (3) recorded in the sequences of amino acids in 

proteins and in the sequences of nucleotides in RNA and DNA. 

The knowledge we have gained from the rock record, in recent 

years, has improved measurably. In particular, photosynthesis, as 

represented by the photosynthetic bacteria, appears very early in the 

rock record. These fmdings are consistent with the hypothesis that 

autotrophic (or photosynthetic) organisms are the original life forms. 

This would upset the current conjecture that the original organisms 

were heterotrophic and hence, would change our perspective on the 

problem of the origin of life. The search for older rocks should be 

actively pursued. 

To supplement the microfossil records, isotope fractionations of 

lighter from heavier isotopes of carbon and sulfur have been used. In 

the case of carbon the fractionation of isotopes has been interpreted 

as due to the fixation of carbon dioxide in photosynthesis by the 

Calvin cycle. The sulfur isotope fractionation is related to the reduction of sulfate to sulfide by various sulfur bacteria. 

In the realm of metabolism, the fixation of carbon dioxide by 

various organisms by metabolic pathways other than the Calvin cycle 

and the related fractionations of isotopes has not been extensively 

studied. Examples are the fixations by Chlorobium (photosynthetic 

green sulfur bacterium) of carbon dioxide by a reverse citric-acid 

cycle, and the fixation by the methanogens of carbon dioxide by 

metabolic pathways still unknown. Thus, the interpretation of the 

isotope fractionation in the geological record is woefully weak on the 

biological side. A much more systematic study of carbon dioxide 

fixation is in order. 

In the case of sulfur isotopes, the role of sulfide to sulfur 

metabolism in photosynthetic bacteria has been ignored in recent 

years in spite of the postulated early appearance of H2 S photosynthesis in the biosphere. Again this is evidence of a poor liaison 

between biologists and isotope geochemists. 
112 

Thus, the lack of a coherent attack on the problem is evident 

in carbon and sulfur isotopic fossil geochemistry and a close collaboration between the biochemist and the isotope geochemist is 

imperative to solve this important problem. 

THE CELL 

Studies in recent years have shown that the halophiles (e.g., 

Halobacterium halobium) create a proton gradient across their membranes after the absorption of light by bacteriorhodopsin. This is 

considered by many to be the most primitive photosynthetic model 

which we have available for study. As noted in chapter III the halophiles belong to the archeabacteria. These organisms which include 

the methanogens have very different cell membranes and 

coenzymes, and a protein-synthesizing machinery which has properties intermediate between procaryotic and eucaryotic cells. A current conjecture is that the archeabacteria branched off from other 

bacteria about 3.4 b.y. ago. Therefore, a detailed comparison of the 

two should push our knowledge of the biological record back. A 

coherent and systematic use of sequences of amino acids and nuc1eotides to clarify this split has just begun. The continual investigation 

of these sequences will be important in elucidating the nature of 

early life for us. 

Among the kinds of questions or problems that should be 

addressed is a general one - the description and reconstruction of 

the universal common ancestor. At what stage in evolution did the 

entity exist? Was it preprocaryotic? Why was there a common 

ancestor? Was it chance? Was it necessary? What are the salient 

differences among the three major lines of descent? 

In addition, the problem of evolution of metabolism needs to 

be addressed. What was the nature of pregenetic "metabolism"? 

Was it a basically dark reaction, solution biochemistry as is commonly believed? Or was it basically a "membrane" (surface) chemistry? Although pregenetic, was iJ nevertheless cellular? Did a primitive, pregenetic metabolic network develop any refining quality ; i.e., 

did it tend to organize itself, become restrictive, more specific? What 

were the primitive catalysts? Reaction centers? How do they relate 

J 
113 

to today's co-enzymes and prosthetic groups? What was the relationship, the transition, between pregenetic "metabolism" and what 

would later be the true cellular metabolism? It has been customary 

to think that the first cells were heterotrophs and so had very little 

metabolism of their own (they took all amino acids, nucleotides, 

etc., from their rich growth medium). Is this a correct view? Is the 

Horowitz hypothesis for the origin of metabolic pathways (Le., 

by backward evolution, one enzymatic step at a time) a correct view? 

Also, we must be concerned with the problem of primitive 

energy sources. What mechanism generated chemical energy for prebiotic, pregenetic systems? Was there a pregenetic photosynthesis 

(visible-infrared range)? To what extent were chemical reactions, 

such as CO2 + 2H2 -+ CH4 + 2H20, present and utilized? Did 

membrane-associated energy production occur, such as through systems that automatically generated transmembrane H+ (or other) 

gradients? 

We should, in addition, elucidate the nature of the development 

of a genetic (informational) system. What is the molecular mechanism of translation? How do various versions (eucaryotic, archeabacterial, eubacterial) of the ribosome differ from one another, 

and what does this tell us about the origin of that structure? What is 

the most primitive form of the translation apparatus? How did the 

genetic code evolve? What is the relationship between its evolution 

and the various stages in the evolution of the translation mechanism? 

What is the relationship between nucleic-acid replication (or its 

transcriptions) and translation? What is the significance of the fact 

that bacterial RNA viral replicase has four subunits, three of which 

are associated with the translation process in the host cell? What was 

the nature of the aboriginal genome? Was it RNA or DNA? How was 

it organized? What were the aboriginal genes (what functions were 

encoded)? What was the relationship between the aboriginal genes 

and their gene products? 

Finally, we must investigate the nature of the eucaryotic cell. 

How did it evolve? To what extent and in what ways are endosymbioses responsible for the uniqueness of the eucaryotic cell? Was the 

eucaryotic cell basically formed by the fusion of various pro caryotes? Or, did most of the important characteristics that are specifically eucaryotic stem from a pre-procaryotic stage in evolution? To 
L 

114 

what extent and in what ways (if any) is the eucaryotic cell polyphyletic in origin? More broadly, to what extent is it chimeric? How 

many (and what) major lines of eucaryotes are there? (In other 

words, how many kingdoms lie hidden in the general classification 

protista?) This question is underlain by the important general one of 

how easily various major states in evolution are arrived at. The wide 

range of questions raised points to a future growth of knowledge in 

this area. The origin of the cell may lay hidden in the biological 

record. 

To quote the biochemist, Szent-Gyorgyi: "Life has developed 

its processes gradually, never rejecting what it has built, but building 

over what has already taken place. As a result the cell resembles the 

site of an archeological excavation with the successive strata on top 

of one another, the oldest one the deepest. The older a process, the 

more basic a role it plays and the stronger it will be anchored, the 

newest processes being dispensed with most easily." 

CHEMICAL EVOLUTION 

As we have pointed out in chapter V, the field of chemical evolution has been guided by the premise that the primitive atmosphere 

was hydrogen-rich, a reducing atmosphere with the major f01ms of 

carbon and nitrogen being methane and ammonia. However, recent 

models of the Earth's early atmosphere have placed severe limitations 

on the amount of hydrogen originally present. Since such a primitive 

atmosphere would be dominated by carbon dioxide and nitrogen, the 

capability of this atmosphere to sustain synthesis of amino acids and 

other biomonomers might be limited. The possibility that there was 

no soup of any complexity must therefore be considered and 

explored experimentally. 

Thus, the field of the synthesis of biomonomers must be 

broadened to encompass not only experiments as they have been 

classically conceived (Le., in the reducing atmosphere of the MillerUrey model) but also, experiments utilizing nonreducing gases (i.e., 

CO, CO2 , N2 , etc.), minerals, and light. To the above must be added 

the constraints imposed by our expanding I.mowledge of the early 

Earth. 
115 

These research areas overlap the research interests of scientists 

in the solar-energy conversion field who are actively exploring metal 

surfaces and minerals which, in the presence of light, will photolyze 

water and reduce carbon dioxide and nitrogen. Therefore collaboration in this area should be encouraged between prebiotic chemists 

and scientists attempting to capture solar energy by chemical means. 

Recent studies suggest clays, mineral surfaces, and metal ions 

had important roles in activating organic molecules for reaction, and 

stabilizing polymers by binding them to surfaces and then catalyzing 

condensation reactions. The oxidation state of the metal ion used is 

critically important and it should be compatible with the expected 

oxidation level of the metal ion in the presence of the reducing 

environment of the primitive Earth. Further research on catalysis and 

absorption by inorganic substances may help explain why only a 

limited group of organic molecules were included in living systems. 

Many compounds with UV and visible chromophores are 

produced in experiments which simulate chemical transformations 

on the primitive Earth. In only a few instances the role of UV light in 

the further transformation of these compounds has been investigated. Since light in the UV-visible range was one of the most potent 

energy sources impinging on the primitive Earth, this area of research 

merits more extensive investigation. Of particular importance is the 

possibility of utilizing the light energy to drive reactions that would 

normally be energetically unfavorable, such as peptide or nucleotide 

bond formation. 

It was recently observed that the sulfate in sea water is reduced 

to sulfide when it comes in contact with molten portions of the 

Earth's crust at the Galapagos Rift. This sulfide is emitted as H2 S or 

metal sulfides from these thermal vents. The ease of reduction of 

sulfate suggests that most of the sulfur on the primitive Earth was in 

the sulfide oxidation state. The role of sulfide ion and insoluble 

metal sulfides in chemical evolution deserves more detailed investigation. The pronounced (nucleophilic) reactivity of the sulfide ion 

could result in a marked change in the reaction pathways observed in 

its absence. 

A case for a role of HeN in chemical evolution has been established by previous research. Purines, pyrimidines, and amino acids 

have been synthesized from HeN. Further analysis of the monomeric building blocks formed from HeN merits investigation. This 
116 

study should include the investigatioQ of catalysis by mineral surfaces and metal ions, since little has been done to study the complex 

interaction between organic and inorganic species on the putative 

primitive Earth. 

Apart from the fashionable areas of amino acids and bases, the 

synthesis of other biomonomers should be pursued. For example, is 

the "formose" reaction the correct answer to the prebiotic formation 

of the more important sugar compounds? Ribose is formed in low 

yield by the base-catalyzed condensation of formaldehyde. A complicated mixture' of C4 , Cs , C6 , and C7 branched and linear sugars are 

made in this reaction. Either ribose was formed under a more unique 

set of reaction conditions or else there was a mechanism for the 

selection of ribose from this complex mixture. In addition, we still 

do not have prebiotic syntheses for long chain unbranched fatty 

acids, alcohols, aldehydes, or isoprenes with relatively high yields. 

In the near future we can anticipate more work on membranes, 

and two specific questions need to be resolved. How and when did 

specific transport systems for solutes through lipid bilayers arise? 

Can a plausible protocell be built from amphipathic lipids, proteins, 

and (pre)nucleic acids? We anticipate a large input into prebiotic 

chemistry from the experimental study of lipid bilayers, and research 

in this area should be encouraged. 

An important adjunct to the question of the formation of biomonomers is the origin of optical asymmetry. For example, was the 

selection of L-amino acids as the constituents of proteins a matter of 

chance, or was it the result of some asymmetric process on the preprebiotic Earth? Further work in these areas is warranted. 

In the last few years an abiotic synthesis of glycine has been 

suggested as occurring in a spreading rift zone, specifically in the 

Red Sea. This suggests that the submarine hydrothermal systems 

should be studied as a possible model system for prebiotic chemistry. 

Environments on the Earth are subject to all kinds of fluctuations: diurnal, seasonal, and tidal. For example, in some experiments 

environmental fluctuations of temperature and moisture content 

have been successfully used to produce peptides from amino acids. 

Overall, how important were fluctuations for organic chemical evolution? Were they necessary? These are but two questions we can ask 

relative to this potentially important regime. 
r 

117 

Most experiments that model events on the primitive Earth 

focus on one or two steps in the process of chemical evolution. With 

the advent of sensitive analytical techniques it is now feasible to set 

up long term experiments where reactants are gradually added and 

removed from a flask over a long period of time in a situation which 

mimics the formation and reaction of biomolecules on the early 

Earth. The effect of changing the temperature, the exposure to light, 

and the reactants can be followed over periods of weeks and months 

by analysis. This experimental approach provides a more accurate 

model of the flux of chemicals through the primitive oceans and 

hence should provide useful information concerning the rates at 

which specific biomolecules were formed. This approach would also 

be ideal for the investigation of reactions taking place using mixtures 

of several reactants. For example, the reaction of carbon dioxide, 

hydrogen cyanide, formaldehyde, and UV light could be investigated 

in one such system. 

As has been pointed out, much clearly remains to be learned 

about monomers. If, however, we assume for the purpose of the following discussion that, in principle, the problem of monomer production is solved, the next step requires condensation of amino acids 

into polypeptides and of bases, sugars, and phosphate into monoand poly-nucleotides. This problem is still unsolved, despite the fact 

that many successful condensations have been carried out utilizing, 

as reagents, products of the electric-discharge reaction. The reason 

this problem remains unsolved is that, for ease of experimentation, 

only a limited number of organic molecules are used in most of these 

condensation studies. For example, amino acids (used in experiments 

to produce peptides) are only a fraction of the total organic compounds produced in the spark discharge, and amino acids would be 

expected to react with non-amino acids most of the time if the complex mixture were heated. The same criticism can be made of most 

of the "prebiotic" dehydration condensations that have been 

published. 

Unless some experiment using complex mixtures of monomers 

actually yields polymers of interest, it would appear that a process of 

fractionation must have intervened between the formation of the 

original prebiotic pool of organic compounds and their condensation 

into biologically useful polymers. Some related questions include 

whether amino acids and nucleic acids can polymerize in the same 
118 

system without interfering with each other. Also, can relevant 

reactions carried out with optically pure starting materials be done 

with racemic starting materials? The study of pure systems is certainly easier, but the study of mixed systems should be encouraged 

due to the important unanswered questions in this area. These 

questions are among the more important ones facing workers in this 

field at the present time, since they bear on the order of events in 

the origin of life ; e.g., whether polypeptides or polynucleotides 

appeared first, as well as whether laboratory experiments are relevant 

to more complex situations on the prebiotic Earth. 

MODELS FOR EARLY LIFE FORMS 

Since we define life in terms of its genetic properties, and since 

the only known system possessing these properties is the proteinnucleic acid system, the most easily defended position holds that the 

fIrst living things were based on this system. However, the spontaneous origin of such a complex mechanism poses great conceptual 

diffIculties. Therefore, other possibilities should be considered. An 

important constraint is that the original self-replicating system, 

whatever it may have been, must have had the capability of evolving 

into the protein-nucleic acid system. Possibilities that are worth 

exporing include: (1) polynucleotides with some catalytic capability 

and (2) polypeptides with some replicative capability. Obviously, 

these systems would be extremely ineffIcient in comparison with the 

highly evolved modem mechanism, but they might have been suffIciently accurate to survive and evolve under the benign conditions of 

the primordial Earth. Either one might have been capable of developing into the modem cell. 

The question of whether polynucleotides alone can constitute a 

self-replicating system can in part be answered by the development 

of experimental models for nonenzymatic replication. Some progress has been made in understanding how preformed polynucleotide 

chains can function as templates to direct the synthesis of their 

complements in nonenzymatic reactions. However, much remains to 

be learned about the catalytic effect of peptides, metal ions, etc. 
119 

Work on the incorporation of pyrimidines in template-directed reactions is also important. Further study of template-directed reactions 

should contribute to our understanding of the origins of nucleic 

acid replication. 

The minimal living system must be self-duplicating and mutable, 

and it must have, at least latently, the capacity for heterocatalysis for 

bringing about chemical changes in the environment that support the 

self-duplication function. In other words, a living thing must be 

capable of rearranging the universe to produce more of itself; and the 

"self' must be capable of continual change. All of this implies information storage, replication, retrieval, and utilization. Contemporary 

life displays these properties in a combined protein/nucleic acid system, so further work in this area is especially desirable. The major 

unresolved problem is in the area of translation and genetic coding 

which features the specific interactions of amino acids with nucleic 

acids or short oligonucleotides, and formation of peptide bonds by 

template mechanisms. Both are worth pursuing. The demonstration 

of translation in an experimental model that combines both of the 

above features is of the greatest importance. A wide variety of 

experimental approaches is justified in the absence of a consensus on 

precise mechanisms. 

The study of simple peptides or polypeptides as catalysts is 

important in understanding how a genetic system, once started, 

could have gained a selective advantage. The processes of replication 

and translation might be more efficient or more selective when catalyzed by protoenzymes rather than without them. Case models 

which leave out catalysts will not tell the whole story. Since the 

synthesis of molecules resembling proteins and nucleic acids requires 

removal of water, it is not surprising that molecules which have been 

studied as catalysts often lead to degradation rather than synthesis 

when the reaction occurs in water solution. The study of condensed 

phases, mineral surfaces, membranes, and other heterogeneous 

systems may be more conducive to synthetic catalysis than the study 

of the homogeneous aqueous phase. 
120 

THE REPLICATION OF SURFACE PATTERNS - AN 

ALTERNATIVE ENTRY FOR LIFE 

The identification of the large biopolymers - proteins and 

nucleic acids - as the significant agents in self-replication has been 

the motif of most of this volume. The implication taken is plain -

the earliest examples of such a well-developed system as we see at 

work universally in life today must have been in some way simpler 

analogies of the two classes of substances. The essentially homogeneous nature of the self-replicating preparations of enzyme, energy 

source, monomers, and template in water suspension has been seen as 

an advanced version of the original natural system. That beginning 

system would have been a simpler, less adapted set of much smaller 

molecules in the same sort of solution where the chemistry i basically similar to that of the modem examples. 

It is clear that the inference, suggestive and powerful though it 

is, is not unique. The gap in time allows the postulation of other distinct systems of entry that are discontinuous with the present informational but continuous in metabolic mechanisms, and which might 

step by step have come to develop the powerful self-replication 

that is universal today. For example, very recent experimental 

support has been found for a suggestion, 15 years old or more, 

that differs in most ways from the prototype of a linkage between 

the two classes of polymers. This hypothesis suggests replicating 

molecular patterns in two dimensions from a solid surface and not in 

one dimension from a long helical polymer. It links an inorganic 

substrate in an essential way with the organic product. It seems 

prudent to recognize that neither of these two incomplete models is 

apt to contain the whole story, but the whole domain of simple 

models spans between them as extremes. Consideration for both of 

the poles is surely the wisest strategy. In the next pages we present a 

summary of the model that rests on an inorganic notion of the first 

steps toward life. 

Monomer Supply and the Early Atmosphere 

The organic chemists view the decisive polymers as links of the 

very common atoms, C, H, N, 0, P, and S. The two essential heavier 
r 

i 

I 

\ 

121 

atoms, S in proteins and P in the nucleic acids, are full partners of 

the more usual atoms of organic molecules. The most abundant 

mineral compounds of the planetary surface contain as well, atoms 

still more abundant than are P and S, but not found at all within the 

great polymers. Among these are in particular magnesium, silicon, 

iron, and the sources of the ions of natural waters, especially Na and 

K. These inorganic elements are involved in life today in a more or 

less essential way, though they are not part either of protein or of 

nucleic acid. The inorganic geochemists take these atoms as central 

to their studies just as the biochemists look on the others as dominant. Their possible interaction certainly should not be overlooked 

simply because it does not quite lie within the most-studied disciplines of carbon chemistry. 

As pointed out in chapter V, the reduction both of carbon dioxide and nitrogen are possible on a primitive Earth, once the right 

electron donors (e.g., Fe ++) are present. The involvement of light to 

drive the reactions is at least reminiscent of photosynthesis. These 

photosynthetic models should be further explored. 

Replication 

As we have previously defined, a minimal living system capable 

of evolution must be self-duplicating and mutable; it must have, at 

least latently, the capacity for heterocatalysis. An example of such a 

minimal definition has recently been met by an experiment in which 

surface charge patterns on clay particles in water were shown to 

replicate and mutate. The preliminary results of these experiments 

indicate that clay minerals such as montmorillonite, which can swell 

to a large degree, may be capable of replicative self-multiplication. 

These minerals may, therefore, be looked upon as models for protolife, or possibly for most primitive life ; their catalytic capabilities and 

selectivities can be altered, and thus can accelerate or retard the rate 

of self-multiplication. 

Results like those were foreseen by Cairns-Smith in 1965. He 

suggested that the primitive genes were patterns of substitutions in 

colloidal clay crystallites. The theoretical information density in 

such crystallites is comparable to that in DNA. Evolution proceeded 

through selective elaboration of pattern mutations that had survival 

value for the clay crystallites that held them. 
122 

If we take it as a fact that such clay-surface charge patterns are 

replicable, and that changes in them can induce stable changes in 

their progeny, it is hard to deny the self-replicating quality. Notice 

one very substantial difference from the biopolymer model: replication in dimension is different. The clay particl~s replicate twodimensional patterns (the laboratory examples show that some 

106 ionic sites are replicated) while the nucleic .acids instead replicate 

a linear sequence that is one dimensional. 

Naturally there is a very long path from such curious " living" 

mineral particles to the cells we ascribe to or even find in the fossil 

stromatolites. The gap can be closed only by hard work in the laboratory and by new ideas. But it is interesting to put forward an optimistic, if vague, scenario about how the scheme might have gone 

forward in the gap between 4.5 and 3.5 or 3.8 b.y. ago. Of course, 

this is meant only as a pedagogical example. 

One such scenario would go as follows. Once the early Earth 

had well-differentiated into core, mantle, and crust, the atmosphere 

would be mainly carbon dioxide, nitrogen, and water, with some 

minor constituents like hydrogen sulfide. This atmosphere would not 

give rise to a soup of monomers, even locally. The interaction 

between atmosphere, sea water, and the silicious, iron-rich crust of 

the Earth, would lead rather to copious formation of clays. The ironrich clays replicated during many cycles of inundation and dryness, 

mutated, and began to fix carbon dioxide photochemically, using 

solar UV and ferrous ion. This could lead to sugars, to the citric-acid 

cycle, and even to fatty acids. In a later stage, the flXation of molecular nitrogen occurred · as well, and the surface formation of amino 

acids and nucleotides became possible. The evolving clays began to 

polymerize these surface monomers. In this system, the nucleic 

acids became coupled to the polypeptides through a genetic code. 

From this complex surface-borne system, a newly self-enclosed system based on nucleic acids and proteins began a new and independent evolution of its own, free of solid substrate - a protocell with 

its membrane consisting of some lipid-rich layers. 

It is evident that in such ideas we have the beginnings of a rich 

and promising experimental campaign which is complementary to 

the search for coupling between the simpler biopolymers held in 

solution. 
123 

SUMMARY 

What is the simplest chemical system that is capable of complete genetic self-replication and open-ended Darwinian evolution? 

This question has been at the heart of our discussion of the origin of 

life. Experiments designed to demonstrate true self-replication and 

natural selection in prebiotic situations, although perhaps a distant 

goal, are of obvious value. Three experimental systems which might 

conceivably lead to such a demonstration are nonenzymatic nucleicacid replication, clay replication, and combined peptide/nucleic acid 

systems. Although none of these systems has demonstrated all of the 

characteristics for self-replication, each has promise worth pursuing. 

Another possible route to demonstrate self-replication and 

natural selection, although without addressing the origin of such a 

system, is to try to construct a minimal self-replicating system from 

components of biological cells. This would indeed be worthwhile if it 

could be done, since it would to some extent plug the huge conceptual gap between simple self-replicating systems and the complex 

genetic system of even the simplest contemporary cell. 

In summary, the most critical need in the areas of nucleic-acid 

replication, translation, and other aspects of self-replicating systems 

is for experimental studies rather than more speculations and diffuse 

theories. 

NEXT ORGANIZATIONAL STEPS 

This whole report has sought to sum up the present state of our 

knowledge and the questions which remain, the many small questions whose answers will lead us to see into the great question, so 

important for most reflective people, scientists or not. 

A modest world community - a few scores of laboratories and 

a thousand or two scientific workers - will encounter great difficulty 

in its pursuit of this very important question. People in such a community work with a given discipline; they are geochemists, or microbiologists or nucleic acid chemists, or biochemists, or specialists in 

planetary dynamics. The classes they teach, the techniques they use, 

the meetings they attend, the journals they read and contribute to, 
124 

are typically diverse. The nonna1 structure of science, especially 

university science, is disciplinary. Unlike the mission-oriented interdisciplinary teams of NASA, scientific experiment tends to be 

discipline-bound. A journal on the Origin of Life is exceptional since 

it centers on a large question, not at all typical of the journals in 

which most work must be published. 

We believe it urgent that some effort be exerted to strengthen 

the interdisciplinary nature of this work, which we have characterized as seeking the answer to a large question, not merely working 

out the answers to many small questions, though that is, of course, 

the indispensable path to most progress. 

To this end we offer two recommendations, based on discussion 

and experience within this widely interdisciplinary workshop: 

1. There is no way to make continued progress unless young, 

talented research workers are steadily recruited to the work. But in 

the absence of a well-established discipline, young people are much 

more likely to seek surer and easier paths. A direct incentive should 

be provided, an incentive which would enlarge the opportunities for 

young people who wished to undertake some portion of this great 

question as their own work. 

We recommend that some funding agency - a joint effort of 

several, whether Federal or private - undertake to offer a yearly 

grant of some post-doctoral Fellowships in the Origin of Life. They 

should be grants for 2 or 3 years, perhaps, tenable at any place which 

has agreed to accept the person. Possibly, the grant should include 

not only a reasonable sum for salary and travel, but also some additional funds to encourage the host laboratory to accept the Fellow. 

The scale of the grants would of course depend on funds available; a 

substantial effect could be achieved by the U.S.A. by grants say to 

IO persons a year, as a steady-state number. The competition should 

be open to research people from any discipline and any country; the 

only requirement would be a showing of the relevance and hope of 

the study for progress towards a knowledge of the origins of life. 

It would be most appropriate to call these Fellowships by the 

name of the late Harold Urey; he was the remarkable American scientist who as much as any other man has opened the field to modern 

study. Perhaps that name would open some new sources of funding; 
125 

a memorial so constructed would be fitting in the highest degree to 

the memory of Harold Urey. 

2. Such a scheme helps meet the fundamental task of a longrange research program - careers for young investigators, given form 

within an interdisciplinary framework. There is another valuable 

device for so broad a field. It is not a response to a steady problem; 

rather, it is a means of bringing resources to bear on opportunities 

for great progress as they arise. (By good fortune, we saw it at work 

at the University of California, Los Angeles (UCLA), where a group 

organized by Dr. William Schopf out of a windfall award from the 

National Science Foundation (NSF) studies the Precambrian record.) 

The idea is simple and attractive. From time to time, set not by the 

calendar but by the state of knowledge, a group of research people 

of differing skills and approaches can be brought together to spend a 

limited time as a team. They would probably be housed at one central laboratory, bringing with them expertise, even equipment, that 

they already possess. They work jointly for a while, yet from differing specialties, at a complex of problems they recognize as ripe for a 

joint attack. One might call this a Focus Award in the Origin of Life. 

It would be given to any investigator who would persuade the reviewers that the time, place, and people were right for say a 2-year joint 

effort by from four to ten investigators. The Focus might link 

paleontology to biochemistry or astronomy, or it could span even 

wider disciplines. The award would not be made each year, or in any 

other routine way, but only on the showing that the moment had 

arrived to strengthen the dispersed and diverse research in the broad 

field by setting up a team, not for a long career, but for a limited 

time. Once every few years it seems likely that such a Focus will 

come to make sense, at a level that is overall small compared to 

the steady flow of support; it is obviously a very attractive but quite 

uncertain program. We have not addressed ourselves to the administrative problems of making such awards and making certain of fiscal 

responsibility; the task is not easy, but surely soluble. Our view is 

that the scientific merit of the proposal and the personal reputation 

of the proposers would have to be so high that difficulties can be 

overcome with relative ease. The point is to supply this important 

but diffuse field with the chance for the sort of concentration that 

bigger laboratories with well-defmed missions can now direct at their 
126 

problems. At the same time we do not propose new organizations, 

but rather to allow serious temporary cooperation at a level of real 

effectiveness. The teams ought not to be too small, nor too large; 

the tasks neither too brief nor too extensive. 

With a steady Urey Fellowship Program in the Origin of Life 

and the tempting opportunity of a Focus Award in the Origin of 

Life we feel that the tasks we have outlined, tasks which transcend 

any discipline of science but which promise answers of the highest 

importance to one of the deepest questions human beings can ask, 

can be met in the decade or so ahead. Unless some such new support 

is found outside the normal rubrics, there will not be much progress 

toward the solution of questions too profound for chemists, biologists, astronomers, or geologists to answer alone in the ordinary flow 

of the stream of contemporary science. Like all real science, this 

fundamental investigation also has foreseeable applications. For do 

not our fossil fuels represent ancient processes of organic chemistry, 

not deeply understood? And does not the ecology of the life in the 

shallow waters bear sharply on the great chemical cycles that can 

fit or spoil the Earth for human life? These are mere accidental, but 

urgent by-products of a deep study of early life and its nature. We 

thus end this book with a note of hope that these suggestions attest 

to the vigor of this field. The depth and variety of questions to be 

answered are signs of the maturation of this exciting scientific 

endeavor. If we are wise enough in this small research investment, we 

can expect real advances in knowledge and in practice of facing the 

great questions that reflective people ask: what is life, and how 

did it arise within the context of changing nature? 
APPENDIX 

MOLECULAR STRUCTURES 

OF IMPORTANT BIOLOGICAL COMPOUNDS 

The Twenty Amino Acids Found in Proteins 

GLYCINE (gly) A LANINE (ala) 

ISOLEUCINE (ile) LEUCINE (leu) 

SERINE (ser) THREONINE (thr) 

GLUTAMIC ACID (9Iu) CYSTEINE (cys) 

HOOC - CH2-CH2-CH-COOH 

I 

NH2 

VALINE (val) 

PROLINE (pro) 

CH 2- CH 2 I I CH2 CH - COOH 

"N/ 

H 

ASPARTIC ACID (asp) 

METHIONINE (met) 

127 
128 

The Twenty Amino Acids Found in Proteins (continued) 

ASPARAGINE (asn) GLUTAMINE (gin) 

ARGININE (a'9) LYSINE (lys) 

HISTIDINE (his) PHENYLALANINE (phe) 

TYROSINE (ty,) TRYPTOPHAN (t,p) 
.... c: 

o 

z 

~ 

o 

I I I 0-1>.- 0 

o 

II