The Universality of Life
ALL EXTANT LIVING organisms are descendants of a single ancestral form of life. So much is clear. But why is it so? There are several possible answers to this question.
First, we could, with the proponents of an extraterrestrial origin of life, identify the ancestor with the immigrant germ that seeded the Earth four billion years ago.
A second explanation is that no other ancestral form was possible. It is single because it is unique.
A third possibility is that the ancestral form arose among a number of competing forms by a process of Darwinian selection.
Or, alternatively, there were several forms to start with, but all the other lines are extinct.
Finally, there is the possibility that a mere accident caused the ancestral form to emerge among several that were equally possible.
Having agreed to disregard the first possibility, we are left with the other four. The central issue here is the old dichotomy between chance and necessity. How much in the common ancestor was due to contingency, how much to determinism? We have no solid clues to answer this question, only surmises based on what we know of the nature of life, and suspect of its origin.
Given the physical-chemical conditions that prevailed on our planet 3.8 billion years ago, a protometabolism leading to RNA-like molecules was bound to arise along well-defined, reproducible chemical lines. Such is the unambiguous conclusion I have drawn from a consideration of the mechanisms involved. Because of the congruence rule, this conclusion extends to all features of today's metabolism that were prefigured in protometabolism, including such key elements as electron trans-