Some Thoughts on Sex and Human Evolution

My father taught human evolution and cultural and physical anthropology, and he shared his passions with his kids, which influenced my own academic interest in neuroscience. Between his influence and my own training I have a decent understanding of how the human brain and body work and how radically our mating patterns and reproductive systems differ from even our closer relatives. And I do occasionally indulge in speculations about "evolutionary psychology" in order to help people understand how human sexuality got so messed up. However, I always acknowledge the uncertainties and the speculative quality of any such discussion of the peculiar origins of human sexuality.

I find the subject irresistible because human sexuality is weird. VERY weird! Most mammals have relatively consistent mating systems. They may have bizarre customs, but all members of a single species do pretty much the same things in roughly the same ways. But human mating systems are mixed up, messed up, and incredibly diverse.

To start with, we have by far the worst reproductive systems of all the mammals:

Maternal death rates: Without modern medicine we have the highest death rates in childbirth of any mammal, rivaled only by the hyena, and far higher than any other.

Menstruation: Other mammals can reabsorb a defective embryo with no fuss, but humans have a chorionic placenta that requires a complex menstrual cycle in order to flush out defective embryos. There is nothing nice and cuddly and natural about this. It is a uniquely debilitating and wasteful system. No other female mammals suffer from anything like it.

Extremely low conception rate: Most mammals have a conception rate of 90-99% per ovulation. Because of our bizarre menstrual cycle, we have a much lower conception rate than other mammals, around 15-20% per ovulation for fertile couples having regular sex, and less than 1% for a random act of intercourse. Women who are trying to conceive may require more than a dozen ovulations and hundreds of copulations to get pregnant, even if both partners are fertile and healthy.

In addition, our bodies seem designed to be confusing:

Cryptic ovulation: To complicate the conception problem, our ancestors evolved what is called "cryptic ovulation." Unlike any of our close primate relatives, there was no way people in prescientific societies could tell when a woman was fertile, so trying to conceive required frequent copulation throughout the cycle.

Full breasts on fertile and infertile females: Unlike any other female mammal, adult women have breasts that remain enlarged and prominent even when they are not pregnant or nursing. This hides yet another possible clue that a female is not fertile.

We also have a combination of rare mating patterns that together make us unique:

Nonreproductive sex: Humans, dolphins, and pigs seek out recreational sex. Bonobos use brief genital rubbing as a way to establish social ties and resolve tension. But most other mammals have zero interest in intercourse without the specific cue of a female who is ovulating and in heat.

Sexual frequency: One consequence of these evolutionary adaptations is that humans are incredibly oversexed by mammalian standards. Even in a monogamous marriage, a woman can easily have sex 10,000 times or more if both partners remain in good health, far more sex over a lifetime than other female mammals experience.

Social monogamy: Most mammals, including our closest cousins, are polygamous and opportunistic about sex; humans are in the small fraction, about 15%, of mammals that practice some kind of pair bonding and coparenting. Even in officially polygamous human societies, the vast majority of marriages involve only two people.

(Note: This is not to say that humans don't cheat on their spouses. As in other pair-bonding species, many individuals do stray if given the chance. Also, like many monogamous species, humans often practice serial monogamy, changing partners after a period of exclusivity together.)

Extreme variation: Humans have THE most varied, conflicted, and confusing courtship rituals and mating systems in the animal kingdom. We may marvel at the peculiar things some non-human species do, but there is a great deal of consistency within each species. Among humans, different cultures set different rules for nearly every aspect of mate selection. Just the idea of a courtship process that sometimes takes minutes and sometimes takes months or years, would be inconceivable for other mammals.

Speculating about how we got so messed up is fun and can be illuminating, and I sometimes find it irresistible. So I'm no one to throw stones at someone else just because they do some "evo-devo" theorizing or make pop science points based on somewhat speculative neuroscience.

On the other hand, I'm in a good position to judge when people are stretching the research way too far and presenting wild claims and mere guesses as if they were facts. For a really good example of the difference between junk science and the responsible use of the existing research, see Sex at Dawn and Sex at Dusk. (Spoiler: Dawn is dishonest garbage; Dusk is a great read, meticulously honest, informative, and fun.)

Anyway, I hope this digression has piqued your curiosity about evolution and the weirdness of human sexuality. What strange, mixed-up creatures we are!