Shadows of Forgotten Ancestors

In its beginning, all sex must have been fumbling, confused, haphazard, the microbial equivalent of bedroom farce. But the advantages that sex confers on future generations seem to be so great that, provided the costs were not too high, selection for improved sexual hardware must soon have been up and running, along with whatever new software was required to stiffen a resolve for sexual congress. Passionate organisms, other things being equal, leave more descendants than those of more tepid dispositions. Unenlightened on the selective advantage of new DNA combinations, organisms nevertheless developed an overwhelming compulsion to trade their hereditary instructions. Like hobbyists who exchange comic books, postage stamps, baseball cards, enameled pins, foreign coins, or celebrity autographs, they didn’t think it out; they just couldn’t help themselves. Trade is at least a billion years old.

Two paramecia may conjugate, as it’s called, exchange genetic material, and then drift apart. Recombination does not require gender. There aren’t boy bacteria and girl bacteria, and bacteria do not have sex—do not recombine segments of their DNA—with every act of reproduction. Sexual plants and animals do. However you bring it about, recombination means that every new being has two parents rather than only one It means that members of the same species—and, except during courtship, the members of most species are solitary and asocial—must arrange a centrally important act that can only be performed in pairs. The two genders might have slightly different goals and strategies, but sex calls, as an absolutely minimum requirement, for cooperation.

Once so powerful an impetus is let out into the world, it might lead, through slow and natural stages, to other kinds of cooperation. Sex brings an entire species together—not just by protecting one another from the cumulative build-up of dangerous mutations, not just by providing new adaptations to a changing environment, but also in the sense of an ongoing, collective enterprise, cross-linking different hereditary lines. This is very different from the asexual practice, where there are many parallel lines of descent, the organisms nearly identical within each line, generation upon generation, and no close relatives between lines.

When sex becomes central to reproduction, the attractiveness of each sex to the other, and the drama of choosing among rivals is moved to center stage. Associated themes include sexual jealousy; real and mock fighting; careful noting of the identities and whereabouts of potential sexual partners and rivals; coercion and rape—all of which in turn lead swiftly, as Darwin pointed out, to the evolution of strange and wonderful appendages, color patterns, and courting behavior that humans often find beautiful, even in members of distantly related species. Darwin thought this sexual selection might be the origin of the human aesthetic sense. Here is a twentieth-century biologist on what sexual selection has brought forth in birds:crests, wattles, ruffs, collars, tippets, trains, spurs, excrescences on wings and bills, tinted mouths, tails of weird or exquisite form, bladders, highly coloured patches of bare skin, elongated plumes, brightly hued feet and legs … The display is nearly always beautiful¹⁰

—especially to the bird of the opposite sex who chooses sexual partners partly on the basis of their good looks. Fashions in beauty then spread rapidly through the population, even if the style isn’t a bit of good in, say, evading predators. Indeed, they spread even if the lifetimes of those who adopt them are thereby considerably shortened, provided the benefit for future generations is sufficiently large. One promising explanation of the showy displays of male birds and fish to the females of their species is that all this is to assure her of his health and prospects.¹¹ Bright plumage and shiny scales demonstrate the absence of an infestation of ticks or mites or fungi, and females—unsurprisingly—prefer to mate with males unburdened by parasites.

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The sockeye salmon exhaust themselves swimming up the mighty Columbia River to spawn, heroically hurdling cataracts, in a single-minded effort that works to propagate their DNA sequences into future generations. The moment their work is done, they fall to pieces. Scales flake off, fins drop, and soon—often within hours of spawning—they are dead and becoming distinctly aromatic. They’ve served their purpose. Nature is unsentimental. Death is built in.

This is very unlike the far less dramatic asexual reproduction of beings like paramecia, where, pretty closely, remote descendants are genetically identical to their distant ancestors. The ancient organisms can with some justice be described as still alive. With all its manifold advantages, sex brought something else: the end of immortality.