Sex at Dawn

Sperm competition is best understood not as a sprint to the egg, but a race over hurdles. Aside from the anti-sperm leucocytes mentioned previously, anatomical and physiological obstacles are in the vagina, cervix, and on the surface of the ovum itself. The complexity of the human cervix suggests it evolved to filter the sperm of various males. Concerning macaques (highly promiscuous monkeys) and humans, Dixson writes: “In the genus Macaca, all species of which are considered to have multimale-multifemale mating systems, the cervix is especially complex in structure.... The

evidence pertaining to human beings and macaque females,”

he continues, “indicates that the cervix acts both as a filtering

mechanism and as a temporary reservoir for spermatozoa

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during their migration into the uterus.” As with the complex penis and external testicles in the male, the elaborate filtering design of the human cervix points toward promiscuity in our ancestors.

The idea that female choice (conscious or not) can happen after or during intercourse rather than as part of an elaborate precopulatory courtship ritual turns the standard narrative inside out and upside down. If the female’s reproductive system has evolved intricate mechanisms for filtering and rejecting the sperm cells of some men while helping along those of a man who meets criteria of which she may be utterly unaware, Darwin’s “coy female” starts looking like what she is: an anachronistic male fantasy.

But Darwin may have suspected more than he let on concerning postcopulatory mechanisms of sexual selection. Any discussion of human sexual behavior or the evolutionary implications of our genital morphology would have been extremely controversial in 1871, to put it mildly. Just imagine, as Dixson does, “what would have occurred if The Descent of Man had included a detailed exposition of the evolution of the penis and testes or descriptions of the various copulatory postures and patterns employed by animals and

human beings.”²⁸

No one can blame Darwin for opting not to include chapters on the evolution of the penis and vagina in his already explosive work. But a century and a half is a long time for discretion and cultural bias to keep smothering scientific fact.

To Meredith Small, the story of the female’s role in

conception is a miniature of the overall narrative. She sees the

popular understanding of conception as “an outdated allegory

of human sexuality” featuring the male as “aggressor,