From: sje@xylos.ma30.bull.com (Steven J. Edwards)
Subject: Biogenesis (was: ETs and Radio)
Reply-To: sje@xylos.ma30.bull.com
Organization: Bull HN, Worldwide Information Systems, Billerica, Mass., USA
Date: 29 Jul 92 18:31:35
	<1992Jul28.105743.29096@news.Hawaii.Edu>
	<rwallace.712328556@unix1.tcd.ie> <1992Jul29.162909.3574@ke4zv.uucp>
	<rwallace.712441800@unix1.tcd.ie>

In article <rwallace.712441800@unix1.tcd.ie> rwallace@unix1.tcd.ie (russell wallace) writes:
# In <1992Jul29.162909.3574@ke4zv.uucp> gary@ke4zv.uucp (Gary Coffman) writes:
#
#> When you place hydrogen and oxygen together in a 2:1 ratio and provide 
#> an energy source, what happens? Blooey! Every time? Yes, every time. 
#> That's the way it is with life chemicals, there is nothing accidental 
#> about their combination.  It's just basic organic chemistry at work. 
#> We have very good evidence that the primordial chemical mixtures of
#> early planetary bodies contain the proper precursor compounds and
#> elements in abundance. We know that solar UV and electrical discharges, 
#> lightning, will supply the necessary energy. We've done it in a test tube. 
#> We haven't yet made the step to the enclosed cell, but with 10^23 precursor
#> molecules in every cubic meter, the combination is bound to occur.
#
# Yes, I realize that amino acids are bound to form under the appropriate
# conditions. However, those amino acids are no more likely to
# spontaneously assemble into a life form than 10^6 transistors placed in
# a box and shuffled around are to assemble themselves into a working
# computer.

The current leading theory is that ribonucleic acids were the first
step ont he biogenesis chain.  A recent (early 1992.07) issue of
_Science_ had a very convincing article about self assembly of
replicating clusters of pure and nearly pure RNA molecules.  It turns
out that a regular ribosome (protein assembler) is still remarkably
functional even when almost all of its structural amino acids are
removed.  Before this research, many scientists had a deep suspicion
that nucleic acids, either DNA or RNA, got the ball started; now we
have experimental evidence.

Therefore, there is now no need to appeal to deep time and large
reaction volumes for complex protein self assemblers and self
replicators: the RNA molecules got there first.

A suggested scenario:

1) Significant quantities of single or small chain amino acids form
spontaneously, but no "wonder molecule" replicator is seen.

2) Ancestral variants of rRNA molecules form clusters spontaneously;
these have partial replicative ability in varying degrees.  Cluster
replication is irregular and probably not too accurate as far as
faithful duplication of the parent template.

3) Some rRNA clusters are capable of incorporating naturally formed
amino acid chains.  Some of these cases the amino acids enhance
replicative fidelity and productivity; natural selective pressure
begins due to limited supplies of spontaneously formed amino acids.

4) At some point an rRNA replicating cluster using amino acids becomes
capable of partial synthesis of useful amino acids.  This replicator
gains a tremendous advantage over imperfect and less efficient
replicators who must compete for the limited spontaneous amino acid
production.

5) The protein making rRNA clusters become self sufficient and the
first true ribosomes are the result.  This is a good candidate for
"first life".

6) Over long periods, lots of nifty developments occur: many different
amino acids start to be used and are assembled in complex three
dimensional shapes; these proteins have greater and more complicated
electropotential surface features for greater variety; very primitive
precursors to cells form as the natural result of polar molecules that
can spontaneously form spheres; the membranes of these spheres become
more complex and are used to mediate molecular and energy transport to
the replicators within; RNA molecule families specialize in functions
and now handle molecular transport (tRNA) along with information
transport (mRNA precursors) as well as both RNA and amino acid
synthesis; organelles form and become specialized; nucleated cells
develop and the nucleus becomes the repository for the latest variant
of RNA: deoxynucleic acid; DNA combined with replicating and
transcribing enzymes starts along the path of today's DNA->DNA
(replication) + DNA->mRNA->(tRNA + ribosome)->protein; the genetic
code starts with a single base reader, then a two base reader, and
finally today's three base reader (plenty of evidence for this as the
first two codes can be recovered from the third).

So there's none of this one chance in 10^1000 stuff.  It could have
happened that way, but it very probably didn't.

Even if the probability of planets like Earth (comparable star,
distance, mass, age, composition) is only one in 10^6 stars (a bit
pessimistic), then there still may be 10^4 life bearing planets in the
Milky Way.

I think the two big problems are: 1) the estimation of the rise of
tool using (radio capable) lifeforms, and 2) the average longevity of
such species.  Unless a signal of some sort is received by us, we will
never know the real figures until we start interstellar travel.

 [The above opinions expressed are my own; not necessarily held by others.]
      == Steven J. Edwards           Bull HN Information Systems Inc. ==
      == (508) 294-3484              300 Concord Road         MS 820A ==
      == sje@xylos.ma30.bull.com     Billerica, MA 01821          USA ==
"That Government which Governs the Least, Governs Best." -- Thomas Jefferson