EVOLUTION (11:704-486) - HISTORY
- SPRING 2011
I mentioned that evolution is not inherently limited to organisms, and that
there must have been evolution before anything we would now recognize as life. The early earth
was molten, but would have formed a solid crust as it cooled, releasing gases to form a primitive
atmosphere, which would have contained water vapor but very little oxygen.
In the Beginning
- As the planet cooled, oceans of liquid water and organics formed. By 4.0 By
ago, there may have been small proto-continents, but no large and emergent land masses, as far
as we have any way to know. The thought is that large land masses may not have emerged until
about 2.5 By ago. There is a lot of speculation and theorizing here, but very few data.
In the primordial soup, there were almost surely organic molecules of simple sorts, as a result
of all the geological and chemical changes going on since the earth was formed. In the last few
decades, chemists and physicists have tried to mimic what might have happened in those early
days. We still have a rudimentary understanding, at best, and we certainly cannot (yet) make
life evolve in a test-tube (there are experiments described by Futuyma). The evolutionary
challenges (or hurdles) that would have had to be overcome include:
the origin of simple monomeric compounds like amino acids and nucleic acids,
assembly of nucleotide bases into nucleic acids and of amino acids into polypeptides,
replication of the nucleic acids,
compartmentalization of such organic systems, protecting the molecules from dilution, and
production of systems with relatively strong fidelity, systems that could have catalyzed the
extraction of energy, either from other molecules or from sunlight, allowing replication.
What we know
– Enough experimentation has been done and theory proposed that we can
begin to make some sense of it all. What we can demonstrate is what might have happened, but
we’ll probably never know exactly what did happen, 4 billion years ago, but that’s life – or in
this case – pre life. We have figured out at least the following:
Simple organic molecules can be produced, with methane (CH
), ammonia (NH
) and water
O) vapor, using an electric discharge, and what we get is hydrogen cyanide (HCN) and
CO), which can undergo further reactions to yield simple sugars, as well as
the components of nucleotide bases and amino acids.
Natural selection can occur in simple, non-living systems. Sol Spiegelman (1970) placed
RNAs, RNA polymerase and nucleotides in a solution, where he got replication of short
strands of RNA, some of which replicate faster than others. Now, that is not quite the
primordial state, since there would have been no RNA polymerase sitting around, but the
larger point is that we don’t need a living cell for natural selection to work.