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.
Sexual organisms do not generally reproduce by fission, by splitting in two. The big macroscopic sexual organisms reproduce by making special sex cells, often the familiar sperm and egg, that assemble the genes of the next generation. These cells survive just long enough to accomplish their task, and are hardly able to do anything else at all. In sexual beings, the parent does not evenhandedly distribute its body parts and transmute into two offspring; rather, the parent eventually dies, leaving its world to the next generation, which in its time also dies. Individual asexual organisms die by mistake—when they run out of something, or when they experience a lethal accident. Sexual organisms are designed to die, preprogrammed to do so. Death serves as a poignant reminder of our limitations and frailties—and of the bond with our ancestors who, in a way, died that we might live.
The more active the enzymes devoted to DNA proofreading and repair in big multicellular organisms, the longer the life span tends to be. When these enzymes—themselves of course synthesized under the control of the organism’s DNA—become sparse or inactive, replication errors proliferate and are compounded, and the individual cells increasingly try to implement nonsense instructions. By relaxing the extreme fidelity of its replication, DNA can arrange, at the appropriate moment, for its own death, and that of the organism doing its bidding.
Where sex mandates the death of the individual organism, it provides life to the hereditary line and the species. Still, no matter how many consecutive generations have been recorded of nearly identical asexual beings, eventually the accumulation of deleterious mutations destroys the clone. Eventually, there is a generation where all the individuals are smaller and more feeble, and then you can hear extinction knocking. Sex is the way out. Sex rejuvenates the DNA, revivifies the next generation. There’s a reason we rejoice in it.
A billion years ago, a bargain was struck: the delights of sex in exchange for the loss of personal immortality.12 Sex and death: You can’t have the former without the latter. Nature, she drives a hard bargain.
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The first living things had no parents. For about 3 billion years, everyone had one parent, and was pretty close to immortal. Now, many beings have two parents and are unambiguously mortal. There are, so far as we know, no lifeforms that regularly have three parents or more*—although it doesn’t seem much more difficult, in terms of plumbing and allure, to arrange than two. The variety of genetic recombination would be correspondingly greater. And the ability to recognize an error in the message (as the deviant sequence when the three are intercompared) would be much improved. Perhaps on some other planet …
On hearing the love call of the male, the female cowbird promptly adopts a come-hither posture, unmistakably indicating her readiness for copulation. Mature female cowbirds raised in isolation will adopt this posture upon hearing the male’s serenade for the very first time. The male, if he’s raised in isolation, if he’s never heard the cowbird love song in his life, still knows it by heart. The musical score, and information on how to appreciate it, are encoded in their DNA. Perhaps on hearing it the female, at least a little, falls in love with him. Perhaps, on seeing her fetching response to his music, the male, at least a little, falls in love with her.
In contrast to parental care and kin selection, which are so prominent among the birds and mammals, many frogs and fish eat their young. Cannibalism is a commonplace—not just in extraordinary circumstances such as overcrowding or famine, but under normal, everyday conditions: The little ones are plentiful, they’ve gone through all the effort of fattening themselves up into convenient and nutritious packages, only a few need to survive to continue the hereditary line, and an affectionate family life that might exert a restraining influence is lacking. But parental care is not restricted to the birds and mammals. It pops up here and there among fish and even invertebrates. Dung beetle mothers, who have laid their eggs in the “brood balls” they’ve skillfully rolled out of animal feces, dote on their young. And Nile crocodiles, whose powerful jaws can bite a human in two, walk about carefully carrying their little hatchlings, who peer out from between the mothers’ teeth “like sightseers on a bus.”13
Even if it is merely genetic sequences working out their self-interest, something that an outside observer might interpret as love has been building in the kingdom of the animals, especially since the extinction of the dinosaurs. With the origin of the primates, it begins its full flowering. It works to bind a species together, in effect to fashion something approaching a common loyalty.