AST 309
part 2:Extraterrestrial LifeThe Origin and Evolution of Life on Earth

Overview• The formation of Earth• Pre-biotic chemistry (Miller-Urey exp.)• First evidence for early life
• The evolution of life
• Extreme life on Earth: lessons for astrobiology

A timeline for the very early history of the Earth

The formation of Earth:

The Earth formed over ~50 Myr via planetesimal accretion

Earth differentiation:
The iron "drops" follow gravity andaccumulate towards the core. Lightermaterials, such as silicate minerals,
migrate upwards in exchange. Thesesilicate-rich materials may well haverisen to the surface in molten form,
giving rise to an initial magma ocean

Early Earth heats up due to radioactive decay, compression, andimpacts. Over time the temperature of
the planet interior rises towards the Femelting line.

After the initial segregation into a central iron (+nickel) core and an outersilicate shell, further differentiation occurred into an inner (solid) and outer(liquid) core (a pressure effect: solid iron is more densely packed than liquidiron), the mantel (Fe+Mg silicates) and the crust (K+Na silicates). Initiallylarge portions of the crust might have been molten - the so called magma
ocean. The latter would have cooled to form a layer of basaltic crust (such as
is present beneath the oceans today). Continental crust would have formed
later. It is probable that the Earth’s initial crust was remelted several times
due to impacts with large asteroids.

The formation of Earth:Delivery of water by icy planetesimals and comets?
After condensation of water vapor produced the earth's oceans, thus
sweeping out the carbon dioxide and locking it up into rocks, ouratmosphere was mostly nitrogen.

Kaboom! The formation of the Moon:

Currently favored hypothesis:
Earth has a gigantic grazing
collision with a Mars-size
protoplanet!

It explains the Moon’s lower density,lack of iron and oxygen isotope ratiosthat are identical to Earth’s (Apollo).

A timeline for the very early history of the EarthIn order to be able to find life outside our Earth, we have tounderstand life in our own planet. The chemistry of life and thedifferent processes during the formation and evolution of the
Earth have played a crucial role.

Is life on Earth a very special thing ?
Can life spawn spontaneously elsewhere ?

Tiny zircons (zirconium silicate crystals) found in ancient stream deposits indicate that
Earth developed continents and water -- perhaps even oceans and environments inwhich microbial life could emerge -- 4.3 billion to 4.4 billion years ago, remarkably soonafter our planet formed. The presence of water on the young Earth was confirmed when
the zircons were analyzed for oxygen isotopes and the telltale signature of rocks thathave been touched by water was found: an elevated ratio of oxygen-18 to oxygen-16.

A timeline for the very early history of the Earth

How to understand an astrobiologist (or anymicrobiologist):
Monomer: usually a small molecule that can bind chemically to form apolymer (amino acids are monomers)
Polymer: a macro-molecule composed of repeating structural units. Proteinsand nucleoacids (RNA & DNA) are polymers
Protein: a biochemical compound that facilitates a biological function
Encyme: are proteins that catalyze (e.g. increase rates) of chemical reactions
RNA: Ribonucleic acid, a macromolecule made of long chains of nucleotides (1base, 1 sugar and a phosphate group). Single strand, can carry
genetic info.
DNA: Deoxyribonucleic acid, double-helix shaped macromolecule made ofnucleotides. Carries genetic information.

The Miller-Urey experiment
In the 1930s, Oparin and Haldane independently suggested
that ultraviolet radiation from the sun or lightning discharges
caused the molecules of the primordial atmosphere to react toform simple organic (carbon-containing) compounds. This
process was replicated in 1953 by Stanley Miller and Harold
Urey, who subjected a mixture of H2O, CH4, NH3, and H2 to anelectric discharge for about a week. The resulting solution
contained water-soluble organic compounds, including severalamino acids (which are components of proteins) and otherbiochemically significant compounds.

Problem: The assumed atmospheric composition

The experiments only give large yields of interestingorganics (amino acids, nucleic acids, sugars) if the gas is
H-rich (highly reducing). If the early atmosphere was CO2+ N2 (mildly reducing), as many suspect, the yields aretiny.

Atmosphere fromvolcanic outgassing?

This would give atmosphererich in CO2, N2, and H2O.
Not the composition thatfavors Miller-Urey synthesis.

Could the original atmosphere have been delivered to the Earth fromcomets, asteroids, …? Perhaps then the composition would be H-rich.
What was the source of the early Earth’s atmosphere? Not necessarily “endogenous” (there from thestart). Outgassing from the crust due to volcanoes (top two), or planetesimal impact (lower left), orcomet vaporization (lower right)? The point here is that a major alternative is exogenous delivery oforganics by comets, asteroids, interplanetary dust…

Endogenous

Exogenous

Another alternative: irradiation of ices,
either extraterrestrial, or on a cold young Earth

Several groups have produced amino acids and other biologically-interesting molecules byultraviolet irradiation of ices meant to resemble what we think interstellar ices
are like. Munoz Caro et al. (2002) produced 16 amino acids this way. Hudson et al. (2008) et al.
recently showed that irradiation of ice with high-energy protons produces amino acids, without
any other gases present (I.e. doesn’t depend on having hydrogen-rich atmosphere.
The key compound in the ices: Nitriles. In these experiments, it was acetonitrile
You may remember it from the “amino acid-like” molecule discovered in the interstellar
medium: CH3CN. It is also detected in comets and in Titan’s atmosphere.

After condensation of water vapor produced the earth's oceans, thus
sweeping out the carbon dioxide and locking it up into rocks, ouratmosphere was mostly nitrogen.

Most amino acids have a mirror image (L and D):• L and D both found in meteorites• L only in organisms on the earth
why is D selected against?(*)
So now we have some amino acids (monomers) loosely mixed in theoceans. Liquid medium is important:• Protects molecules from UV photon disruption• Ease of transport and Interaction
Next goal is to combine Monomers into Polymers (peptide chains)(*) We believe that Earth life's "choice" of chirality was purely random, andthat if carbon-based life forms exist elsewhere in the universe, their chemistry
could theoretically have opposite chirality.

Which monomer? Which polymer?
Monomers (building blocks) polymerized into four types of polymers.

However, only two types seem crucial for primitive biological processes:
amino acids/proteins and nucleotides/nucleic acids

DNA-protein system: Too complex for first life
So which came first?
It is a temptation to think of “life" as a proteinmaking gene system. But this could not havebeen the origin of life. Not only is it far too
complex to have developed spontaneously, there
is a chicken-and-egg paradox:
No proteins without DNA to code forthem, but
No reason for DNA without proteins
to code for.

Could they somehow have developed simultaneously?
After all, nearly every DNA and RNA in today’s life
operates only in connection with protein enzymes:
protein-DNA interactions are the norm.

This protein is bending
part of a DNA, something
DNA is too stiff to do on
its own. There are
myriad other DNAprotein interactions, e.g.
repair of DNA damage.

• The “chicken and the egg” problem is obvious: Neither DNA nor
protein has any function without the other. Yet their symbiosis is far
too complex to have arisen from “nothing.”=> So what preceded the DNA/protein system?

20What came before DNA and proteins?
Almost certainly: RNARNA looks a lot like DNA, but is single stranded. The
big difference is that RNA is a molecule that can carryinformation like DNA, but can also fold itself into
complex three-dimensional shapes like proteins,
so RNAs can be their own enzymes (proteins).
Because RNA is ribonucleic acid, but can act like an
enzyme (protein), these primordial RNAs are called“ribozymes” and are the most important candidate forthe origin of life. That is why we are learning about DNA!

When naturally occurring ribozymes werediscovered in present-day organisms (including humans),
The idea that there was once an “RNA world” became
easily the most plausible scenario for the transition to life.
21

Encapsulation: Prerequisite for RNA world?
The production of RNA polymers at fast enough rate is usually considered aproblem, but there are many ways to enhance it. One is to confine the reactants to
a compartment of some kind; a lipid vesicle, forerunner of today’s lipid membranes.
Prebiotic membranes (vesicles) are easy

What followed the RNA world?
How self-replicating RNA could have led to the DNA/protein worldBut what preceeded RNA? How could an RNA be “alive”? Should we expect
the same on habitable exoplanets? How different could life be if the basic
polymer was not RNA? What if more bases, or more varied codons? What
are the chances that life would occur again if we could “play back the tape”?

The lesson we learned so far was that nearly everything that we see today inliving organisms is far too complex to have arisen spontaneously from somelifeless polymers.
That there are two ancient kingdoms, the bacteria and archaea, orthat there are prokaryotic and eukaryotic cells, or that organisms can beclassified according to their metabolic habits, are all interesting, but only shows
us that all of these are too complex: They are the products of hundreds ofmillions of years of development and evolution.
We saw a glimmer of what might have come before in ribozymes.

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A timeline for the very early history of the EarthWhen did life begin?
Stromatolites: Bacterial colonies that usedphotosynthesis
Microfossils: Difficult! Controversial….
Isotope ratios: carbon-12 to carbon-13 abundanceis affected by metabolism in living things.
When organisms ingest carbon, they preferentiallyuse 12C over 13C. (14C is radioactive, and thus
won’t remain over a long time period.) Carbon with ahigh ratio of 12C compared to 13C is therefore an
indicator of living processes. Carbon enriched in 12C
has been identified in rocks from Greenland dated at
3.85 billions of years ago. This is the earliest
evidence for life on Earth.
Best estimate: 3.5 to 4.0 Gyr ago

Establishing biological nature of fossils:
stromatolites (below), …
Stomatolites: Mats ofprevious bacterial colonies
that harvested sunlight forphotosynthesis

Stromatolites are a classic method for estimating when the Earth’s atmospherebecame oxygenated, and some think that the presence of stromatolites at suchand-such an age shows the Earth’s atmosphere was oxygenated at that time.
[Problem: Now clearer that many mat-building bacteria are not aerobic
photosynthesizers.]

Oldest stromatolites are about 3 Gyr, but photosynthesis is so complex thatit could not have been available near the beginning of life.

Ancient microfossils:The Earliest Trace of Life? This fossil from
Western Australia is 3.5 billion years oldand shows carbon traces that indicate
life. Its form is similar to that of modernfilamentous cyanobacteria (inset).

Science 8 March 2002 :“Earliest Signs of Life Just
Oddly Shaped Crud?”Where did life begin?
•Land?
Problem: No protection from intense UV,or from sterilizing impacts.
Additional reason for excluding origin on land:
Hard to imagine life not in an aqueous solution.
• Ocean?
How to concentrate the molecules so theypolymerize in a reasonable time?
One possibility: Encapsulation of molecules in cell-like membrane.
• In tidepools or lagoons?
Evaporation concentrates monomers, but unfortunately exposes to UV.
• Hydrothermal deep-sea vents?
A present-day favorite.

The Hadean/Archean biological world
Prokaryotes: Most successful organisms on Earth. The only life forover 2 Gyr, many still with us. Essentially infinite lifetime forcolonies. Note the complexity!
No organelles (eukaryotic cells only), smaller genome, no sex, butother abilities like extreme adaptation (see “extremophiles”), andhorizontal gene transfer.

Prokaryote vs. Eukaryote:So what does the geological record show?

Multicellular life (Cambrian explosion)
Eukaryotes

Photosynthesis
Earliest confirmed microfossil
Oldest purported microfossils 3.5 Gyr
Oldest isotopic evidence for life 3.8 Gyr
Oldest zircons 4.2 Gyr
Earth forms

A timeline for the very early history of the Earth

Life on Earth

The CambrianExplosion• ~ 530 Myrs ago, fossil record of animals and othercomplex organisms “explodes”• Major diversification of life on Earth• Explosion took many millions of years (organisms before580 Myrs were much simpler• Several hypotheses:– Increased oxygen levels– Earth was recovering from a Snowball event– Evolution of eyes?
– It wasn’t an explosion at all!

Life on Earth:• Earth forms over a time of 50 Myr more than 4.5 Gyrs ago• Pre-biotic chemistry somehow leads to first replicating macromolecules (maybe RNA)• RNA leads to DNA and first life form(s)• Best estimate: life on Earth is between 3.5 and 4 Gyrs old!
• For >2 Gyrs we have simple prokaryotes, they startphotosynthesis• 1.5 Gyrs ago Eukaryotes evolve• ~600 Myrs ago complex,
multi-cellular life evolves