Eve: How the Female Body Drove 200 Million Years of Human Evolution

by Cat Bohannon

Women are more likely to die of heart attacks, even though they’re less likely to have them—symptoms differ between the sexes, so women and their doctors alike fail to catch them in time.

So, when we think about Elizabeth Shaw screaming her sci-fi head off at the misogynistic medpod, we shouldn’t just feel terror and pity and disbelief. We should feel recognition. Why is this still happening? Aren’t the sciences supposed to be objective? Gender neutral? Bound by the empirical method?

Taking the time to control for the female reproductive cycle is considered difficult and expensive;

Even if everyone actually followed the new rules, given that it usually takes more than ten years for drugs to move from clinical trial to market, 2004 was the first year any new drug approved for sale would have been tested on significant numbers of women. Drugs that were released before the new regulations took effect are in no way obliged to go back and redo their clinical trials.[*3]

Most clinical studies show that across multiple drug types, women metabolize drugs more quickly than men.[*5]

my adipose tissue is actually an organ, much less that it evolved from the same ancient organ as my liver and most of my immune system.

Most women’s bodies begin preparing for pregnancy in childhood, not because it’s a woman’s destiny to be a mother, but because human pregnancy sucks, and our bodies have evolved ways to help us survive it.

The maternal body is surprisingly resilient: battered on all sides, evolved to be so battered, and somehow, improbably, still alive.

our adipose tissue and our livers and our immune systems all came from the same primordial organ, called the “fat body.”

Bodies are basically units of time. What we call an individual “body” is a way of bounding a series of cascading events that follow self-replicating patterns until finally entropy sets in and enough goes wrong that the forces that keep you from flying apart at the seams finally let go.

It’s not your bladder’s fault that the mammalian uterus evolved to squat on top of it like Quasimodo. That only happened about forty million years ago. Actually, if we’re talking about the gravity problem, that was only four million years ago. Before then, our ancestors had the good sense not to walk on two legs, smooshing all our long-evolved organs on top of one another in our trunks (not to mention generally screwing up the spine).

We are this flesh, these bones, this brief concordance of matter.

Vertebrates still represent only about 1 percent of all living species.[*14] Thus, the majority of what you and I call evolution—what we’re debating about endlessly in litigation and fitful bursts on op-ed pages and conflicting textbooks in far-flung communities, this thing that has caused so much trouble—represents only 13 percent of the time there’s been any life on Earth at all.

bildungsroman.

Morganucodon: she who lived under giants.

For God so loved the beetles, and the furry, warm, heart-fluttering Eves who ate them.

Any biologist will tell you that the story of life is really the story of water. Our earthly cells evolved in shallow oceans, and they never got over it.

But colostrum is especially dense with immunoglobulins: antibodies tagged to respond to pathogens that the mother’s body knows to be dangerous. In fact, before we discovered penicillin, cow colostrum was commonly used as an antibiotic.[*5]

After all, giving birth isn’t just when you reproduce. It’s also a key moment for the bacteria in and on your body: the construction of an entirely new environment that’s especially suited for their survival.

Our guts are, in essence, as social as our brains—or at least as influenced by our disease-prone social nature, and that history has pressured our milk to change, too. Forget about the Paleo diet: modern Homo sapiens have already adapted to urbanization and the bacterial challenges that come with it.

it’s also tied to sexual satisfaction: no matter your gender, the more prolactin you have in your body after sex, the more satisfied and relaxed you feel. This may be because prolactin counteracts dopamine, which your body produces in buckets when you’re sexually aroused. Likewise, if you have too much prolactin in your system, you’re more likely to suffer from impotence.[*11]

Also, while human beings behave more altruistically toward members of their own group after a dose of oxytocin, they also act more defensively and aggressively against people they perceive as being out of their group—so it’s hardly the angel of our better nature. And no one really knows what oxytocin is doing in the brain: Does it make us interpret others’ social signals differently? Does it just make us pay more attention to faces? Does it simply make us feel warmer toward known things (like people we know) than toward unknown things (people we don’t know)? In the end, the only thing we’re dead sure oxytocin does is make certain kinds of tissues contract.

when many babies grow up, their brains still associate milk-related signals with healing and comfort. Eating fat-dense and/or high-carb foods, especially if they taste sweet—the sort that many humans tend to seek when feeling stressed or lonely—produces an analgesic effect in a number of different mammals. For rat and human alike, “comfort food” can dampen the body’s pain response, a kind of grown-up breast substitute.[*13]

The majority of human women favor cradling their infants and nursing them from our left breast, which also happens to line our baby up with the side of our face that is more expressive. No, really—and other primates do this, too. Among humans, the muscles on the left side of the face are slightly more adept at social signaling, and 60 to 90 percent of women preferentially cradle infants toward the left of the body’s midline, with the baby’s head more exposed to the left side of her face. This preference is strongest in the infant’s first three months of life, precisely the period when new mothers are nursing more often throughout the day. This is true across many human cultures and historical periods.

Meanwhile, the right hemisphere of the adult brain is largely responsible for interpreting human social-emotional cues, and it receives those signals dominantly through the left eye. So the mother’s left eye carefully watches the infant’s face, interpreting the baby’s emotional state, while the infant gazes intently up at the most expressive side of the mother’s face, learning how to read her emotions and respond—something that human beings spend huge portions of their childhoods learning how to do.

Milk with a lot of cortisol tends (at least in rats and mice and certain kinds of monkeys) to produce baby personalities that are less risk seeking, and those traits seem to persist through the individual’s lifetime.

These individuals explore their environment less. They’re less social with other members of their own species. They react more skittishly to unknown stimuli. They like to play it safe. Babies with low-cortisol milk, on the other hand, explore more. They’re more social. They spend more time playing with their den mates. And when they grow up, their personalities tend to have similar features. While many things go into building an individual’s personality, at least among species we’re able to study in the lab, what’s in the milk they drink is, all on its own, a strongly predictive factor.[*14]

Still, it’s not true that the best scenario is milk with no cortisol. A low and consistent amount of cortisol in a mother’s milk helps her offspring later in life. If you lace a mother rat’s drinking water with low levels of cortisol, her offspring will perform better on maze tests, have better spatial recognition, and generally be less stressed out when faced with challenges than young rats whose mothers didn’t drink water dosed with cortisol.

Researchers think that to a certain degree mildly challenging environments inoculate children against the upcoming stresses of adulthood. So maybe it’s better to have a mother’s milk “demonstrate” a moderately dynamic and challenging environment. But if a woman is stressed out all the time, with cortisol levels through the roof, her kids might likewise be more fearful, hesitating to explore new territory and learn new things. In other words, our bodies teach our children about the world, not just by actively showing them their environment, but also by what we put in their mouths.

Mothers’ bodies tailor milk’s contents for the needs of their offspring through a complex communication system between mouth and breast. Babies’ personalities are shaped by its particular makeup, are soothed by its fats and sugars and hormones, their guts purged and recolonized by friendly bacteria. Milk is something we do as much as something we make. It has evolved to be social.

Aka men are either holding or within arm’s reach of their children more than 47 percent of their time.

Among studies that try to parse modern heterosexual male desire, hip-to-waist ratio is a better predictor for whether men will find a woman attractive than the size of her breasts, and this is true across multiple human cultures.[*23]

Put it this way: The average vagina is only three to four inches deep. When a woman is sexually stimulated, hormonal changes tense the ligaments holding the uterus and cervix in place. This makes them rise relative to the vaginal opening as the vagina expands its depth considerably. But a six-inch aroused vagina does not accommodate a seven-inch erect penis. In other words, there’s nothing usefully adaptive in a long human penis when four to six erect inches will do the job. In evolutionary terms, that is probably why the average erect human penis is still only a bit over five inches long.[*24] And yet, in a number of studies, heterosexual women rate pictures of men with longer penises as more attractive. There is, in other words, a disconnect between the human penis as a sexual show trait and its functionality.

But the idea of Leonardo drawing in a vasa menstrualis that he couldn’t even see, simply because he steadfastly believed it should be there, as did everyone else at the time, is the sort of thing that keeps me up at night. You see, the ideas that human beings have about reality—what it’s made of, how it works, how we all fit into grander schemata—can change fundamentally. Sometimes, those changes are so dramatic and so far-reaching that it becomes nearly impossible to understand the world the way we did before.

That’s when Enlil and the rest of the gods stepped in. Aside from inventing mortality, to set an upper limit on the human problem, they also set down a bunch of edicts about birth control and sexuality so there would be fewer births. Women were categorized into sacred temple prostitutes with special knowledge of herbs and birth control; wives, who would be okay for sex and reproduction; and “forbidden women,” who were off limits when it came to sex. Other Sumerian cuneiform tablets lay out advice for the best herbs and methods for both aiding and hindering fertility.

Breast cancer deaths have been going down lately, largely because we’re getting better at finding them and treating them before they find their way out of the breast. But the incidence of breast cancers hasn’t been going down at all. There’s still a one-in-eight chance that I, as an American woman, will develop breast cancer at some point in my lifetime, and those stats are similar worldwide.[*35] Having breasts and making milk isn’t just socially expensive, in other words; it is, all on its own, a dangerous affair.

There is no holocaust, no natural disaster, no great terror in the history of humankind that can compare with the apocalypse we call Chicxulub. It’s fair to say that it’s unimaginable. We know that there was ash. We know, for many years, it was very cold. And we know that there, somewhere in the ashfall, is the reason women have periods. In the middle of one of life’s worst disasters, the placenta took hold. Ancient mammals gave birth to live young.[*1]

Having a vaginal birth is the biggest risk factor for bladder prolapse in women. The second-biggest factor is menopause; as the hormone balance in a woman’s body shifts, the lowering estrogen levels naturally loosen the vaginal tissue and the surrounding pelvic floor muscles. Many women will have surgery to tighten the tissue and repair the prolapse. If the prolapse is significant enough—for instance, part of the bladder actually falls out of the vaginal opening, or the uterus does, cervix slumping down past the vaginal walls—some women may even have the vaginal opening surgically closed to support their organs. Naturally, this involves being willing to never have vaginal intercourse again, but many older women are willing to accept the trade-off.

Female rats who experience clitoral stimulation also show lowered stress and better general health than rats who don’t have that sort of stimulation. In other words, clitoral stimulation is good for a lab rat’s health, much as it seems to be for human women.

atavism,[*32]

eusociality.

Human child rearing is already highly cooperative, and perhaps homosexuality—wherein, barring social pressure, an individual does not naturally produce his or her own children—is a strong case for human eusociality.

The latest numbers estimate that as many as 20 percent of humans are homosexual, and given that the majority of scientists think that homosexuality is a trait present from birth, those sorts of numbers indicate that whatever part of homosexuality is classically heritable can’t have been too strongly selected against. In highly social species like ours, the benefits of having extra hands for child rearing—hands that aren’t busy taking care of genetic children of their own—might have outweighed the evolutionary pressure against homosexuality. Homosexuality has been observed in countless species, mammal and bird alike.

Certain members of anthropology and biology departments in the 1980s and 1990s wondered, What’s with all those women synchronizing their periods when they live together? That must have an evolutionary advantage, right? One ambitious fellow (published by Yale University Press, no less, in 1991) decided this meant that ancient women somehow evolved to go on collective sex strikes by synchronizing their periods, thereby enabling/encouraging men (less distracted by the pressing desire to screw) to go out and hunt and forage. This, the author theorized, was the root of all human culture. In effect, he argues that humans build cool stuff like the Pyramids and rocket ships because women get periods and therefore don’t have sex for a set number of days per month.[*41]

That’s right: the placenta is actually made of both embryonic tissue and the mother’s tissue—one of the only organs in the animal world made out of two separate organisms.

Women’s bodies are particularly adapted to the rigors of pregnancy not simply so we can get pregnant but so we can survive it.

Ever wonder why the Centers for Disease Control is located in Atlanta? Malaria. The entire reason the United States built the CDC is that malaria was rampant throughout the American South. Malaria was finally eradicated in the United States in 1951.

And while typical masculine ears tend to lose their higher range as they age, women’s ears are better at hanging on to those pitches. Importantly, our better ability to hear the very upper end of the human register is also tied to hardwired emotional response: baby cries alarm women more than men. It’s not that men can’t hear the kid crying, but that for many adult men, their ears snip off the upper end.

For example, one recent study had subjects listen to a recording of a baby crying or a more neutral noise. Then the subjects had to play a game of whack-a-mole. The ones who’d listened to babies crying were faster and more accurate in their mole-whacking efforts—they were, in other words, more alert and focused after being exposed to the sound. Women showed this result more robustly than men.[*9] The evolutionary advantages are pretty clear. If you are tuned to the sound of a baby crying, you’ll probably be better at taking action to make it stop: flee with babe in arms, fight off predators, shove useful bits of fruit or a nipple into its mouth.

Middle-aged and older men also have more trouble following a conversation in a crowded soundscape, especially if it involves a lot of higher-pitched sibilants. That also means they have difficulty hearing women’s voices, with their characteristic higher pitches, but retain the ability to hear men’s voices and other low, rumbly things. Because social power is typically assigned to men as they age, women’s voices are literally not being heard by men in power.