All of these, of course, are models and nothing more. They show
what could have happened, and what is not impossible.
Oparin has suggested an evolutionary scheme for protocells or
"protobionts" along the lines suggested by his coacervate
experiments. He proposes that in lakes or ponds with appreciable
concentrations of polymerized material, coacervate droplets would
be formed naturally by wave action, as diagrammed on the preceding
page for a lipidlike material.In general, the composition of these
droplets would differ from that of the bulk solution.
These "microenvironments" in time could develop into
enclosed systems of chemical reactions that absorb high-energy compounds
from their surroundings (like the glucose-1-phosphate experiment)
to perform protective reactions or other necessary syntheses.
The absorption of light for synthetic purposes, as in the chlorophyllascorbate
experiment, might have occurred first at this prebiological stage.
In this limited sense, "photosynthesis" might have preceded
The experiments of Fox and Oparin with microspheres and coacervates
suggest a model for how living organisms might have developed.
The first stage along the road to life would have been stable,
self-maintaining, enclosed chemical systems such as these - perhaps
growing and propagating by simple fission or division into smaller
droplets that had the same chemical abilities and growth potential.
Control of reactions in these protobionts would be effected by weak
natural catalysts, also made by the protobiont and passed along
to each daughter fragment during fission.
This would have been the era of chemical evolution, where the criterion
for success would be the ability to find or synthesize the chemicals
necessary for the continuance of the droplet, and the ability to
prevent its own materials from being cannibalized for use in a neighboring
The development of an efficient outer membrane that could exert
control on what came into and left the protobiont would be a strong
aid to survival, as would an active-transport system that could
concentrate certain substances inside the membrane.
The ability to carry out reactions quickly, and to grow to such
a size that the protobiont droplet would fall apart into many independent
daughter droplets, also would be advantageous to the survival of
one particular kind of protobiont. Enzymes or their simpler catalytic
precursors therefore would confer a great survival advantage on
The second stage in the development of a living cell would be characterized
by the ability to transfer to daughter fragments during division,
not samples of all catalytic substances, but the instructions for
making more of these pre-enzymes from simpler molecules.
This would mark the beginning of hereditary information storage,
and of evolution by genetic variation and natural selection. It
is a convenient point at which to draw a boundary between prelife
and life. This original information-storage system would have been
far simpler than today's DNARNA-protein machinery, but all traces
of it would have been erased as later improvements took over.