Shadows of Forgotten Ancestors

Another proposed explanation for the persistence of sex, wonderful in its novelty, invites us to consider the age-old arms race between parasitic microbes and their hosts. There are more disease microorganisms in your body at this moment than there are people on Earth. A single bacterium reproducing twice an hour will leave a million successive generations during your lifetime. With so many microbes and so many generations, an immense number of microbial varieties are available for selection to operate on—especially selection to overcome your body’s defenses. Some microbes change the chemistry and form of their surfaces faster than the body can generate new model antibodies; these tiny beings routinely outwit at least some parts of the human immune system. For example, an alarming 2% of the plasmodium parasites that cause malaria significantly change their shapes and styles of stickiness each generation.⁷ In light of the formidable adaptive powers of disease microorganisms, a real danger would arise if we humans were genetically identical, generation after generation. Very quickly, the blur of evolving pathogens might have our number. A variety that outsmarts our defenses might click into place. But if our DNA is reassorted every generation, we have a much better chance of keeping ahead of the potentially deadly infestation of disease microbes.⁸ In this highly regarded hypothesis, sex provides essential confusion to our enemies and is the key to health.

——

Because females and males are physiologically different, they sometimes pursue different strategies, each to propagate its own hereditary line; and these strategies, while of course not wholly incompatible, introduce a certain element of conflict in the relations between the sexes. In many species of reptiles, birds, and mammals, the female produces only a small number of eggs at a time, perhaps only once a year. It then makes evolutionary sense for her to be discriminating in her choice of mates, and devoted to nurturing the fertilized eggs and the young.

The male, on the other hand, with plentiful sperm cells—up to hundreds of millions per ejaculation and the capability of many ejaculations a day in a healthy young primate—can often better continue his hereditary line through numerous and indiscriminate matings, if he can pull it off. He may be much more ardent and eager, and at the same time much more likely to drift from partner to partner—cajoling, displaying, intimidating, and impregnating as many females as possible. Moreover, since there are other males with identical strategies, a male can’t be sure that a particular fertilized egg or hatchling or cub is his; why should he spend time and effort nurturing and raising a youngster that might not even carry his genes? The investment might benefit his rival’s descendants and not his own. Better to be off fertilizing more females.

This is by no means an invariable pattern, though; there are species in which the female is eager to mate with many males, and there are species in which the male plays a major, even a primary, role in raising the young. Over 90% of the known species of birds are “monogamous”; so are 12% of the monkeys and apes, to say nothing of all the wolves, jackals, coyotes, foxes, elephants, shrews, beavers, and miniature antelope.⁹ However, monogamous doesn’t mean sexually exclusive; in many species in which the male helps raise the children and provides care for their mother, he also is sneaking out for a little sex on the side; and she is often receptive to other males. Biologists call it a “mixed mating strategy,” or “extra-pair copulation.” As much as 40% of the young reared by “monogamous” bird pairs are revealed by DNA fingerprinting to have been sired by extramural encounters, and numbers almost as large may apply to humans. Still, the motif of nurturing females, who are choosy about their sex partners, and males given to sexual adventure and many partners is very widespread, especially among the mammals.

——

There’s a good deal of plumbing, odor signaling, and other machinery in higher organisms to get the genes of one organism in contact with those of another, so the molecules can lie down next to one another and recombine. But that’s mere hardware. The central sexual event, from bacteria to humans, is the exchange of DNA sequences. The hardware serves the purpose of the software.