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

by Cat Bohannon

For one thing, when women and men are exposed to equally loud environments, on average women will feel more distressed by them. That distress may lead women to try to escape the noise more quickly than men will. After all, it’s not just what your sensory array can do. It also matters what you do in response to what it reveals to you.

All that can really be said is that queer women’s ears often behave slightly differently than straight women’s do, and that the ears of gay and bisexual men don’t reflect a similar difference.[*14]

But in humans, it’s widely known that a woman’s sense of smell heightens around ovulation, and it’s not hard to imagine why that might be adaptive. After all, ovulation is an important time for a female mammal to be discerning. Since it’s more costly for us to get pregnant and give birth than for other female animals, we need to be fairly careful about which male gets to do the job.

dendritically—that’s

If you line up fossilized skulls in chronological order, you can see the eye sockets move toward the front of the head. And as this happened, the size of the visual processing portions of the brain increased dramatically.

Also, if you live in a massively 3-D space, like a tree canopy—where up and down matter as much as back and forth and side to side—and you’re trying to catch bugs that keep flying away from you, your ability to judge depth and direction suddenly matters a lot. Your brain might have to get bigger, too, since processing a lot of 3-D visual data takes a lot of computational firepower. Indeed, when paleontologists measure primate fossils’ skulls, the more stereoscopic the eye placement, the bigger the brainpan.

you’ll suffer less and recover faster from jet lag if you adjust your mealtimes to the new schedule before you go.[*28]

So, if men make themselves sleep during the day, then their testosterone will simply shift accordingly, and their testes’ production of sperm will similarly adjust to the new normal. Because it’s so much cheaper and easier for the mammalian body to make sperm, there’s just less to screw up by turning men into night owls.

If you don’t have a red opsin, you simply can’t differentiate between red and green very easily. Which doesn’t matter when you’re nocturnal—there’s not a lot of red and green going on.

Birds can all see red. Most fish can see it, too. But not cats, not dogs, not cows or horses, not rodents, not hares, not elephants or bears.

wont

To eat all those tender green and ripe red things in the daytime required a retina with a red opsin. The genes for creating that opsin, as luck would have it, are located on the X chromosome.

The eyes, too, are built to reduce signal when needed: not only are color receptors clustered toward the center of the eye, making your peripheral vision markedly different from what your brain directs you to focus on,[*31] but eyes regularly respond to internal thought, too. If you’re a person who can see, when you’re asked to imagine or remember a vivid visual scene, your pupils will dilate, even though you’re not paying attention to the external world at that moment. When your brain is internally modeling visual information, the nerve pathways that control the muscles that contract and dilate the pupils come along for the ride.

Generally speaking, human eyes do two things: saccades and fixations. Saccades are the twitchy ways eyes move from one spot to another in a visual field, and when they linger on a spot, it’s called a fixation. There are known sex differences in these patterns when people look at human faces—adult women tend to have more saccades that move between different parts of a person’s face and eyes, whereas men tend to fixate a bit more around the nose. No one knows why. But this might be why women are famously better than men at learning new faces, and it might also be why women seem to be a bit better at accurately judging what emotion that face is conveying.

Modern humans inherited the problems that come with any sort of bad design. Our feet are, in many ways, the biological equivalent of duct-taping your car’s bumper back on when you don’t have the money to send it to the body shop.

In the mid-1990s, a group of orthopedists ran a study comparing small samples of their patients’ skeletal muscles. They found that a certain metabolic pathway (the “mitochondrial electron transport complex III”) was significantly more active in their younger women subjects than in the young men.[*11] This particular pathway has to do with using fat to give muscle cells energy. Young women’s bodies are really, really good at lipid beta-oxidation: using our mitochondria to take little molecules of fat and break them down. And while all mitochondria are able to do this, having muscles that are especially good at it may be built into our female body plan. A more recent study showed that genes related to this particular sort of fat metabolism are more expressed in young women’s muscle cells than in men’s.

Stamina’s what’s interesting in bipedal human beings—how long we can keep it up. Horses rapidly tire at their top speeds, lathering and needing to stop after only a couple of miles. Given enough time, a human could actually run a horse down. Most healthy human adults can trot at, say, five miles an hour, for hours at a time. Ultramarathoners can pretty much go for days if they get sleep breaks. Horses? They’d die.

If you’re an ancient hominin like Ardi, how do you greatly expand your range? How do you swim out into that sea of grass? You need to walk in order to carry stuff, yes. But you also need to endure. You need to be able to tap a second wind. You need to push past the wall. You need to survive in the suck. The things that let us survive in the suck are the things that make us human. Yes, our capacity to innovate, but also our ability to endure in the worst conditions. To keep at it when we’re already tired. Our ability, in other words, to not give up.

So at this point, it seems women are at least as good as men, and may even be innately better, at hard-core muscular and metabolic endurance. And there’s one more thing to consider: We may be better at dealing with muscular tissue damage than men. Women recover from exercise more quickly than men do.

There are things about mixed-sex groups that are very hard to quantify. For example, group “bonding” has far more to do with culture than anything physiological.

Part of that had to do with her psychological resilience. And maybe another part had to do with how much was on the line as a female recruit. But she also might have been able to carry the gun the rest of the way because she was a woman. Supposedly, she even did it with a smile. When the man she relieved wrote his evaluation of her (all Ranger peers have to write such evaluations), he said he was particularly struck by how enthusiastic she was in that moment. There he was, completely broken, and she was practically chipper.

It’s not that females should replace males in combat roles. Rather, it may be silly not to take advantage of what female bodies could add to a group in combat situations. The point for any military strategy is to win with as few casualties as possible. Some advantages gained by including female soldiers in combat missions could be physiological. Others could be psychological.

Still, Rehana the sniper tigress is an effective story: one of many countermyths about women’s power—tiny, brutal fairy tales—that stand in opposition to myths about women’s god-sanctioned subjugation. If the women weren’t there fighting, this story wouldn’t have been told, inspiring the troops to fight harder, weakening the enemy’s psychological reserves. It’s a weapon made from the very idea of a woman. And that, in the end, may be part of what ISIS (and certain American military figures) are afraid of. Maybe the debate about women in combat is not about what men’s and women’s bodies can or can’t do—about the strengths and weaknesses of our sexually dimorphic musculoskeletal system, our metabolism, or even our psychological grit. Maybe it comes down to the idea of women’s bodies in the world—what they’re supposed to do, what they aren’t, and how they serve as a counterpoint to the idea of Manhood.

Predictably, there’s still the usual sort of worry about “lost morale” in the military should many women find their way into attack forces. But recent studies have shown—including within the U.S. Marines, a group that especially protested the change—that mixed-sex combat groups exhibit high levels of group cohesion and loyalty. In fact, mixed-sex military groups’ feelings of “belonging” are as high as, and in some cases higher than, single-sex groups’. What’s more, the rate of sexual assault is no higher in mixed-sex groups than in male-only ones.[*19]

This is the story of Tool Triumphalism: man invented weapons, claimed dominion over his peers and the rest of the animal kingdom, and all our achievements flow from there. From bone cudgel to spaceship, from the Stone Age to now, Kubrick wasn’t the only one to tell this story: the clever ape—always male—picks up something from his environment and uses it to hunt, to murder, to dominate Earth.

When male chimps go hunting, they sometimes use spears, but their own bodies, bigger and stronger than the females’, are often weapon enough. Even if they’re injured as a result, no offspring will starve. From an evolutionary point of view, their injuries aren’t as costly, because males aren’t caretakers in chimp society. Generally speaking, innovation is something that weaker individuals do in order to overcome their relative disadvantage. As a primatologist in Kenya told me years ago, “Women do clever things because we have to.”

Mothers could help daughters, sure, but for midwifery to become widespread, collaboration between members of a wider social group would also have been key.[*13] Collaboration over competition.

Most hominins—until recently, most humans—were lucky to reach age thirty-five. That means if our Eves survived childhood, they would have spent the next decade, or at most two, having children, breast-feeding, trying to keep everyone alive—or hell, at least enough to launch two kids into adulthood—and then kicked the bucket.[*33]

Instead of twisty trapdoor vaginas, we now have the Pill and the diaphragm. Instead of the Bruce effect, we have methotrexate and misoprostol. Instead of waiting for a less dangerous birth canal to evolve, we have midwives who help our newborns squeeze through the gauntlet and the miracle of modern C-sections.[*37] When, in other species, physiological evolution would have created a newly evolved feature to enable female reproductive choice, hominins used behavioral innovations instead—some of them social and others involving new tools and pharmaceuticals. That control we have over the most powerful levers of our evolutionary fitness got us to where we are today. It allowed the early human population to finally explode, expanding into nearly every ecological niche our ancestors stumbled upon. It also improved the survival rates of every pregnant female with a too-narrow pelvis and a greedy placenta.

So maybe we need a better narrative to describe humanity’s “triumph.” Our story doesn’t begin with a weapon. It doesn’t begin with a man. 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.

In fact, after spending years digging through the literature on the subject from dozens of different angles, I can actually report that the oddest thing about our species might be that the female human brain doesn’t seem to be all that functionally different from the male. Adult human “female” brains are remarkably similar, in nearly every way one can measure, from cellular structures to outward function, to adult “male” brains.

But the whole idea of IQ is controversial. For one thing, white Americans tend to have higher IQ scores, on average, than African Americans. But if you control for family income, most of those differences disappear.[*5] Similar problems pop up in tests like the SAT, where your score determines whether you have a shot at a top U.S. school; the differences in results here also largely go away if you control for family income. That implies that the way test questions are asked gives people with certain backgrounds greater advantages. It also implies that the way children are raised shapes their cognitive development. It’s extremely stressful to be poor. It might also be stressful to be a girl in a typical test-taking environment.[*6]

when listening to recorded conversations, we’re usually pretty good at estimating how much of the total time each person speaks if both participants are the same sex. But when we listen to a conversation between a man and a woman, we usually think the woman talks more than she actually does—even if she’s reading a script with the same number of words as she’d spoken opposite another female.

Women are generally better at reading and writing. This is true at every age of testing from age five on, and the gap tends to increase with age until puberty and stays relatively steady thereafter. Large data sets from the U.S. Department of Education support this, and these sorts of differences pan out internationally as well. Across both language and cultural barriers, girls tend to out-read boys early in life and continue that trend throughout their lifetime. Men make up only 20 percent of the people who buy and read novels. The numbers improve for history and other nonfiction, but overall book publishers throughout the Americas and western Europe are selling books to women.

For reasons that are still unclear, boys are two to three times more likely to be dyslexic than girls. Furthermore, given that schools aren’t great at identifying these issues—as of 2013, less than 20 percent of students that researchers identified as having reading impairment were categorized as “learning disabled” by their schools—boys are probably not receiving help with their reading problems as they move through the education system. Does that mean we’re failing our boys in school? Unfortunately, maybe.

And unlike with math, the reading gap is pretty robust between the sexes: from infancy on, male children meet verbal benchmarks later than girls, so it could be language in general that has a sex bias, not just the cognitively odd task of reading.

So, if the Female Brain does better than the male at writing tasks—at least, in a testing environment—it may not be that women are necessarily more innately “verbal” than men. Maybe women score better on writing tasks because, for one reason or another, their brains are good at anticipating what other people want.

But fluctuating sex hormones do seem to have a direct effect on many women’s brains. There are two well-documented times in our lives when this happens: just before and during menstruation, and during pregnancy.

men end their lives dramatically more often than women. It’s not exactly the battle of the sexes you’d want to win, but in this arena men are far ahead.

being a mother makes a depressive woman far less likely to feel suicidal, and suicidal mothers are less likely to try. Unfortunately, this isn’t as strongly true for fathers, not because men care less about their children, but (in this model) maybe because they have a harder time understanding that they’re needed than mothers do.[*18]

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.

What takes that male patient a year to start doing again—say, walking, or speaking, or being able to dress himself in the morning—might take a female only six or seven months. This is true even if she suffered the same kind of injury to the same place in the head: the same amount of force, the same type of impact. This isn’t because women are better at taking a hit in general. It’s just that a female-typical brain seems to be better at repairing itself or even at preventing certain kinds of damage in the first place.

In fact, as I write, clinical trials for human beings are under way to establish whether doses of female sex hormones will help people suffering from a recent traumatic brain injury to stabilize and recover. If those trials pan out, ERs of the future will have a ready supply of female sex hormones to help heal their patients’ brains.

What’s more, in one of the rare beneficial twists of sexism, a woman’s friends and family may expect less of her after an illness or injury, since they believe her to be more fragile than a man. As a result, they may distribute her life responsibilities among themselves, giving her more time to heal, allowing her to go slowly, rather than dive headlong back into her previous life.

Every cell in the body—neuron or no—has to deal with stress. Sometimes they recover from stress, and sometimes they “choose” to die instead. If you dose both dishes of neurons with things known to stress out or even kill them, XY neurons die faster and more often.[*19] The main reason for this, as far as scientists can tell, is that male XY cells have more difficulty dealing with oxidative damage.[*20]

historically, being omnivorous is the best way to survive. In a recent study of fossilized teeth, it seems the mammals that had more diverse diets were the ones that survived a massive planetary die-off thirty million years ago.[*23] The specialized animals went extinct. Nearly all mammals are descended from Eves that were lucky enough to have mouths and guts that could make do.

That’s a theory-of-mind task. And theory of mind—building a model of another’s internal cognitive state, mapping out its potential desires, and communicating accordingly—is something human beings are extraordinarily good at.

The reason that matters for the evolution of human brains, of course, is that pregnant and breast-feeding women just so happen to have brains that are doing very similar things to what human brains do at other major transitions in our body’s life cycle: they violently rearrange themselves. A pregnant woman’s brain will, quite reliably, shrink in volume by as much as 5 percent during her third trimester, followed by a steady rebuilding during the first few months after giving birth.[*25] Similar things seem to happen in other mammalian mothers, but it’s particularly dramatic in human brains.

Thus, human women might have evolved to be capable of an extra phase of brain development, of much the sort all humans go through when we’re children: a deep pruning that precedes a massive period of social learning.

In that light, motherhood is not the completion of womanhood by any means. That is the last thing I mean to imply. But because motherhood has long required a uniquely challenging period of social learning for human women, it shouldn’t be surprising to find that human women’s brains may go through a unique phase of brain development to prepare them for those profound challenges.