Shadows of Forgotten Ancestors

Now in these laboratory experiments, which are being carried out in our time, imagine many generations of RNA molecules more or less identically replicating in the test tube. Mutations inevitably occur, and much more often than in DNA. Most of the mutated RNA sequences will leave no, or fewer, copies, again because random changes in the instructions are rarely helpful. But occasionally a molecule comes into existence that aids its own replication. Such a newly mutated RNA might replicate faster than its fellows or with greater fidelity. If we were uncaring about the fates of individual RNA molecules—and while they may arouse feelings of wonder, they seldom elicit sympathy—and wished only for the advancement of the RNA clan, this is just the kind of experiment we would perform. Most lines would perish. A few would be better adapted and leave many copies. These molecules will slowly evolve. A self-replicating, catalytic RNA molecule may have been the first living thing in the ancient oceans about 4 billion years ago, its close relative DNA being a later evolutionary refinement.

In an experiment with synthetic organic molecules that are not nucleic acids, two closely related species of molecules are found to make copies of themselves out of molecular building blocks provided by the experimenter. These two kinds of molecules both cooperate and compete: They may aid each other’s replication, but they are also after the same limited pool of building blocks. When ordinary visible light is made to shine on this submicroscopic drama, one of the molecules is observed to mutate: It changes into a somewhat different molecule that breeds true—it makes identical copies of itself, and not its pre-mutation ancestor. This new variety, it turns out, is much more adept at replicating itself than the other two hereditary lines. The mutant line rapidly out-competes the others, whose numbers precipitously fall.¹⁸

We have here, in the test tube, replication, mutation, replication of mutations, adaptation, and—we do not think it is too much to say—evolution. These are not the molecules that make us up. They are probably not the molecules involved in the origin of life. There may well be many other molecules which reproduce and mutate better. But what prevents us from calling this molecular system alive?

Nature has been performing similar experiments, and building on its successes, for 4 billion years.

Once even crude replication becomes possible, an engine of enormous powers has been let loose into the world. For example, consider that primitive organic-rich ocean of the Earth. Suppose we were to drop a single organism (or a single self-replicating molecule) into it, considerably smaller than a contemporary bacterium. This tiny being divides in two, as do its offspring. In the absence of any predators and with inexhaustible food supplies, their numbers would increase exponentially. The being and its descendants would take only about one hundred generations to eat up all the organic molecules on Earth. A contemporary bacterium under ideal conditions can reproduce once every fifteen minutes. Suppose that on the early Earth the first organism could reproduce only once a year. Then in only a century or so, all the free organic matter in the whole ocean would have been used up.

Of course, long before that, natural selection would be brought to bear. The genre of selection might be competition with others of your kind—for example, for foodstuffs in an ocean with dwindling stocks of preformed molecular building blocks. Or it might be predation—if you don’t look out, some other being will mug you, strip you down, pull you to pieces, and use your molecular parts for its own ghastly purpose

Major evolutionary advance might take considerably more than one hundred generations. But the devastating power of exponential replication becomes clear: When the numbers are small, organisms may only infrequently come into competition; but after exponential replication, enormous populations are produced, stringent competition occurs, and a ruthless selection comes into play. A high population density generates circumstances and elicits responses different from the more friendly and cheerful lifestyles that pertain when the world is sparsely populated.

The external environment is continuously changing—in part because of the enormous population growth when conditions are favorable, in part because of the evolution of other organisms, and in part because of the ticking geological and astronomical clockwork. So there’s never such a thing as a permanent or final or optimum adaptation of a lifeform to “the” environment. Except in the most protected and static surrounds, there must be an endless chain of adaptations. However it feels on the inside, it might very well be described from the outside as a struggle for existence and a competition between adults to ensure the success of their offspring.