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

by Cat Bohannon

The fact of the matter is that until very recently the study of the biologically female body has lagged far behind the study of the male body. It’s not simply that physicians and scientists don’t bother to seek out sex-specific data; it’s that until all too recently the data didn’t exist. From 1996 to 2006, more than 79 percent of the animal studies published in the scientific journal Pain included only male subjects. Before the 1990s, the stats were more disproportionate.

By the time a clinical trial for a new medication starts testing on human subjects, it might not have been tested on female animals at all.

Many researchers default to male subjects for practical reasons: it’s difficult to control for the effects of female fertility cycles, particularly in mammals. A complex soup of hormones floods their bodies at regular intervals, whereas males’ sex hormones seem more stable.

Taking the time to control for the female reproductive cycle is considered difficult and expensive; the ovary itself is thought of as a “confounding factor.” So, unless a scientist is specifically asking a question about females, the female sex is left out of the equation. The experiments run faster, the papers come out sooner, and the researcher is more likely to get grant funding and tenure.

As we’ve increasingly learned, female bodies aren’t just male bodies with “extra stuff” (fat, breasts, uteri). Nor are testicles and ovaries hot swappable. Being sexed permeates every major feature of our mammalian bodies and the lives we live inside them, for mouse and human alike. When scientists study only the male norm, we’re getting less than half of a complicated picture; all too often, we don’t know what we’re missing by ignoring sex differences, because we’re not asking the question.

After being struck by the stubborn reality of the male norm, I did what researchers like to do: I dug into the databases to see how big a problem it was. And, well, it’s huge. It’s so huge that many papers don’t even mention that they used only male subjects. I often had to email the authors directly and ask.

The National Institutes of Health managed to update some of these regulations in 1994, but loopholes are regularly exploited: as of 2000, one in five NIH clinical drug trials still wasn’t using any female subjects, and of the studies that did, nearly two-thirds didn’t bother analyzing their data for sex differences.

it turns out, women’s fat isn’t the same as men’s. Each fat deposit on our body is a little bit different,[*6] but women’s hip, buttock, and upper thigh fat, or “gluteofemoral” fat, is chock-full of unusual lipids: long-chain polyunsaturated fatty acids, or LC-PUFAs. (Think omega-3. Think fish oil.) Our livers are bad at making these kinds of fats from scratch, so we need to get most of them from our diet.

Meanwhile, we found out just a few years ago—again, someone finally asked the question—that a human girl’s hip fat may be one of the best predictors for when she’ll get her first period. Not her skeletal growth, not her height, not even her day-to-day diet, but how much gluteofemoral fat she has. That’s how important this fat is for reproduction. Our ovaries won’t even kick in until we’ve stored up enough of this fat to form a decent baseline. When we lose too much weight, our periods stop.

We are this flesh, these bones, this brief concordance of matter. From the way we grow our nails to the way we think, everything we call human is fundamentally shaped by how our bodies evolved. And because, as a species, we are sexed, there are critical things we should be thinking about when we talk about what it means to be Homo sapiens. We have to put the female body in the picture.

Breasts, and blood, and fat, and vaginas, and wombs—all of it. How they came to be and how we live with them now, no matter how weird or hilarious the truth is. In this book, I aim to trace what we’re finally coming to understand about the evolution of women’s bodies and how that deep history shapes our lives.

Women, like all human beings, are Homo sapiens. Because we are mammals, we make milk. Because we are placentals, we have a uterus that gives birth to live young. Because we are primates, we have big eyes with color vision and ears that can hear a wide range of sound. Because we are hominins, we are bipedal and now have giant brains.

While a lot of attention is given to the fact that women’s bodies tend to be smaller, the reason we metabolize drugs differently may actually have as much to do with our livers. One recent study comparing biopsies of male and female liver tissue showed thirteen hundred genes whose mRNA expression was significantly influenced by sex; of these, 75 percent showed higher expression in females (Renaud et al., 2011). It’s not, in other words, just a matter of how much drug distributes through how much body mass, but how the cells in a sex-typical liver go about their day. And “day” matters here, too: livers, like the rest of the body, have a circadian rhythm, and female mammals are especially sensitive to our long-evolved relationship to the sun (Lu et al., 2013). More on daylight and why it matters in the “Perception” chapter.

All living creatures, mammal or not, are mostly made of water. While the adult human body is 65 percent water, newborns are 75 percent. Most animals are essentially lumpy donuts filled with ocean. If you wanted to describe life on Earth in the simplest terms, you could say we’re energetic bags of highly regulated water.

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.

Menstrual blood has taken on all sorts of cultural significance throughout human history, most of it bad. But to think that evolutionary processes would produce such a deeply significant mutation as external menstruation just so guys would be less horny for a while misconstrues what the uterus actually has to go through to make babies.

Nowadays, in the United States, only 0.65 out of every 100,000 legal abortions will result in the woman’s death, while 26.4 American women still die for every 100,000 live births. Before Roe v. Wade, 17–18 percent of all maternal deaths in the United States were due to illegal abortions—that stat was as true in 1930 as it was in 1967. Meanwhile, as many as one in four maternal deaths in today’s malarial countries are directly tied to the disease. During our worst outbreaks, the same was true in the United States.

Before the asteroid, our planet’s forests were massive conifers and ferns.[*3] But out of the ash, in place of those ancient forests, fruiting trees and their canopies formed brand-new ecosystems. Flowering trees produced, at regular intervals, vast bounties of fruits on their terminal branches—fat bulbs of sweet and sugary flesh. Fruits. Bugs. Moss. New things that ate the fruits and bugs. New things that ate the new things.

Attention directs perception just as perception influences attention: the sensory array and its corresponding brain centers are in near-constant communication with one another and signals go both ways.

Consider that every pound of body weight normally puts an extra pound and a half of pressure on the knee joint when we walk around barefoot. It goes up to four times the pressure when we jump. Our bodies have evolved to mostly handle that. But modern, gendered footwear can pull the rug out from under us: in high heels, our center of gravity is tipped forward, meaning that instead of the buttocks and hamstrings, the quadriceps at the front of the thighs have to do the lion’s share of the work, yanking the top of the knee upward, further compromising the joint. Over time, that can damage the ligaments in the knee, wear away at cartilage, and generally wreak havoc. It’s bad for our toe joints, too: walking in heels eliminates the “roll” of normal walking and instead can mean, depending on the heel’s height, a repeated slamming of all your body weight and momentum onto the forefoot.

Generally speaking, innovation is something that weaker individuals do in order to overcome their relative disadvantage.

The symbols of our ultimate technological achievements shouldn’t be the atom bomb, the internet, the Hoover Dam. Instead, they should be the Pill, the speculum, the diaphragm.

So, if there is a Female Brain, it may be more prone to depression and anxiety and certain kinds of self-harm, but it’s far less vulnerable to catastrophic failures like suicide. With the exception of things like postnatal depression, the Female Brain doesn’t seem to be more fragile. It might even be more robust: for instance, men are more likely to wind up in the ER from severe traumatic brain injuries, but women are more likely to recover from them.

Tiny creatures called forams live on the seafloor, as they’ve done for hundreds of millions of years, long before there were mammals or dinosaurs. When they die, they leave a useful layer of microscopic skeletons. In those skeletons, traces of stable oxygen are woven into the matrix of the fossilized bone. One type is more common when the world is warmer; another when it’s cold. So, if you grind up a little pile of foram fossils, you can get a pretty good model of ancient weather.

Whether during the toddler transition, the adolescent struggle, or any of those long years in between, what children’s brains are doing the most is social learning: paying extremely careful attention to what others want, trying to predict those wants, and likewise trying to figure out fast and dirty ways of communicating their own wants to others.

This is also what my kitty does.

Men have bigger lungs than women, which means they have more oxygen still circulating while they’re talking. That’s one reason the male Clinton found it easier to deliver his acceptance speech. He simply had more hot air to work with.

But it’s not the words that are important, particularly. The real payoff is grammar—the very stuff of human thought.

Everything humans care about is possible because we have language. The human mind is made for language, yes. But it’s also made of language. The same sorts of logic paths that rule language, that combine known things into new ideas, that puzzle out the code of others’ communication into knowable thoughts and desires, also write stories and build meaning and tease out the finest, strangest features of the universe. They make us what we are.

At this point, sexism in a wide array of different cultures is starting to hurt our species as a whole. To paraphrase one famous American, modern sexism is making us less healthy, wealthy, and wise.[*31]

Being smart matters. It’s not just that it helps you make “wise” decisions; it helps you make decisions in the first place. Your ability to solve problems, your ability to form deep relationships with other people, your ability to contribute to your community, your ability to keep your kids safe—everything you might want to do with your human brain is shaped by how smart it is.

Poor nutrition in early childhood is famously linked to lower IQ, even when you control for the mother’s IQ. Behavioral outcomes suffer, too. Malnourished babies tend to become adolescents who have difficulty with self-control, long-term planning, violent impulses, and other social aggression.

The golden age of Islam, in other words, produced one of the most intellectual, egalitarian, cosmopolitan, and profoundly influential societies of its time. And women were right there at the fore, contributing to its success.[*55]

But as of 1989, many Arab nations had become incredibly wealthy and yet managed to produce only 4 frequently cited scientific papers. The United States, by contrast, produced 10,481. Why? For one, they’d systematically cut off education for half their population. Roughly sixty-five million adult Arab people are illiterate right now, of which two-thirds are women.[*56]

Where women are undereducated, entire societies eventually go fallow. If history proves right, neglecting girls’ education is a sign of a civilization’s decline. You can neglect half the brains in your community for only so long.