Shadows of Forgotten Ancestors

We have, so far in this saga, not come within a billion years of our epoch. But even in so remote a time, many of the major themes and variations of present life on Earth had been clearly articulated. Some of the fossils of that time are indistinguishable in form from some contemporary organisms, the stromatolites being the most famous example. Others are wildly different. There has surely been a growing biochemical sophistication over the aeons, in enzyme chemistry, fidelity of DNA replication, and many other matters that must be indetectable in mere fossils; still, it seems astonishing that any organism should be unchanged—even if just in its gross anatomy—over 3.5 billion years. We can recognize again a stolid conservatism in living things. And yet quick and fundamental change sometimes happens. The picture that emerges is of a rich menu of candidate adaptations offered up by mutation for consideration by natural selection. But only under sentence of death (or what in the evolutionary perspective is the same thing, the threat of no descendants) are these mutational propositions seriously taken up and tried out. Except for cosmetic touches, new kinds of life are ordinarily discouraged. Change is grudging.

You can see the same classes of molecules used over and over again for completely different purposes. Today, for example, the same complex organic molecule is used, with minor variations, as the green pigment that sips sunlight in plants; as the red pigment that carries oxygen through the bloodstreams of animals; as the agent that makes shrimps and flamingos pink; and in a widely used enzyme that helps wheedle energy safely out of sugar. The energy is banked, against future need, in molecules nearly identical to the nucleotides A, C, G, and T of the genetic code. While these are molecules of breathtaking versatility, their repeated use and recycling reveals parsimony as a way of life.

It’s as if, for every million dyed-in-the-wool conservative organisms, there’s one radical who’s out to change things (although usually very small things); and for every one of the radicals, only one in a million actually knows what it’s talking about—providing a significantly better survival plan than the one currently fashionable. And yet the evolution of life is determined by these revolutionaries.

Given enough food, microorganisms reproduce so quickly that they can evolve in the time between putting them on a shelf for storage and retrieving them for further examination. The speed with which bacteria “acquire” resistance to antibiotics cautions restraint about prescribing them too frequently. The antibiotic does not usually induce adaptive mutations; instead, it acts as a fierce agent of selection, killing off all bacteria except a favored few that, by chance, are immune to the medicine—a strain that earlier, for other reasons, might not have competed successfully with its fellows. The fact that bacteria quickly evolve resistance to antibiotics (or insects to DDT) reflects the enormous diversity of forms and biochemistries always churning subsurface in the microbial world. There is a continuing war of measure and countermeasure, raging between host and parasite—in this case, between the pharmaceutical companies, generating new antibiotics, and the microbes, generating new resistant strains to replace their more vulnerable ancestors.

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Well-developed even by 3.5 billion years ago, we’ve argued, was the distinction between the inside and the outside, me and you, us and them, a rudimentary consciousness of self. If you’re in the habit of eating organic molecules dissolved in the primeval oceans, you’re also used to eating the molecules that make up other beings; after all, they’re the same molecules. But then you’d better take care you don’t eat yourself. You may not have pity or compassion for other organisms. That’s probably not how a microbe views the world. But you must make some fine distinctions. You may lack sentimental feelings for your chloroplasts, but if you digest them, you’re in trouble. If the distinction is too difficult for you to make—if you can’t figure out the difference between “me” and “you,” if you can’t control your digestive enzymes—then you’ll leave fewer offspring, or none. There’s not yet any thinking through. There may be no feelings of any sort. Nevertheless, organisms are beginning to behave as if they had wants, needs, preferences, emotions, drives, instincts.