So far we have glibly bypassed a massive problem. How do we get
from the Haldane soup to even the simplest fermenting bacterium?
A long, long step exists between amino acids, sugars, and nucleosides,
and simple cells of the Fig Tree type, and this is the step about
which we know least.
From the short time span involved from the formation of the planet
to the development of these simple photocells, it can be argued
that the problem must be simpler than we think.
An equal time span occurred from protocells to eucaryotes, and
we think we have a good idea as to how this change came about. The
difficulty is that we have visible evidence for this latter process,
in microfossils and living survivors, but no such evidence has survived
for the evolution of protocells.
This is a continual problem in evolutionary history, the erasure
of the older record. Survival on this planet is based on efficiency,
and there are no museums of unsuccessful types.
Even among the bacteria, we do not have samples of all of the
ancestral chemistries, only those that enabled their possessors
to get along in odd corners where their more "advanced"
eucaryotic competitors could not survive.
We should not view the present-day bacteria as representative
of the ancestors of the main stream of development, but rather as
Nevertheless, we do have this record to study from the protocells
of the Fig Tree deposits to the first eucaryotes. For periods earlier
than this, we have nothing at all.
We know how the planet began, and how this first phase in the
evolution of life ended. The gap between must be filled by imagination
tempered by the results of laboratory experiments.
The chemical problems to be overcome are many.
How were polymers of proteins, nucleic acids, and lipids formed
in an aqueous environment, when polymer formation requires the removal
of water and is thermodynamically nonspontaneous?
How were the first reacting systems isolated from their surroundings
to avoid a lethal dilution and cannibalism by other competing systems?
How were the chemical reactions of a protocell integrated into
a coherent and efficient "metabolism" that would increase
its chances for survival?
And finally, having achieved all these things, how did the successful
protocell find a way of preserving its gains and passing them on?
These are the next questions that we must try to answer.