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 life.

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 system.

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 a droplet.

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.