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

by Cat Bohannon

This is part of why human newborns usually lose weight in the first weeks after they’re born: they gobble up their own fat reserves until their mother’s milk converts from colostrum to mature milk and they’re able to take in—and digest—a proper meal.

for instance, as of 1990, 64 percent of babies born to Black mothers in the United States were out of wedlock, where it was only 24 percent in 1965 (Akerlof, Yellen, and Katz, 1996).

Compared with humans, most marsupials are born (that is, exit the uterus and head to the pouch) at a point of development that would be roughly seven weeks into a human pregnancy—incredibly underdeveloped, in other words.

Getting a body ready for that kind of arrival in the world is the essence of the eutherian story: giraffe mothers are pregnant for fifteen months, and those pregnancies are very taxing.

Biologists call this “maternal investment”—an umbrella term for all the things a female has to do (physiologically and behaviorally) to make reproductively successful offspring and what it’s gonna cost her.

Changing one’s body plan comes with a similar risk calculus. A big part of the evolution of milk had to do with protecting newborns from bacteria.

With the development of the three-holed body plan, our ancient Eves had to evolve ways of protecting the birth canal from contamination with bacteria from feces. Like Morgie,

Though not all fetal development maps onto evolutionary time, you can actually get a window into how this might have happened by looking at what mammals do in the womb:

Note that the human embryo is, in many ways, “sexually indifferent” up to week seven or so, at least in terms of these urogenital parts:

the echidnas and platypuses retain their cloaca, and the males still have their testes inside their body instead of hanging out in a scrotum.

Modern placentals go through a complicated dance of development—flaws in urogenital development are among the most common birth defects human beings suffer.

The closer you get to the current model in our evolutionary past, the more likely you are to find mild malformations—maybe a hymen that covers too much of the vaginal opening, maybe a urethra that’s kinked or cramped somehow.

Before the asteroid hit, mammals were already working on these problems. But from what we can tell from the fossil record, the long winter after the impact killed off more of the marsupials’ ancestors than the placentals’.

For many marsupials, like the kangaroo, the ureters go between the female’s three vaginas toward the bladder. That means she can’t give birth to anything bigger than a jelly bean or she’d tear her ureters.

In principle, the more recent a feature, the more likely it is to fail—as true of smartphones as body parts.

In human women, these structures aren’t as robust as one might hope: as many as one in ten women suffer from urinary incontinence after a vaginal birth.

The other evolutionary problem with human birth and bladders, of course, is that we stand and sit upright. That means there’s a lot of downward pressure on the vagina from the organs in the pelvis.

only 25 percent of women of any age reliably experience an orgasm during vaginal intercourse.

eggs—and indeed modern vaginal walls are similarly nerve dull.

The running theory is that mate choice made it useful to get rid of the penis: when a female chicken doesn’t choose to usefully present her cloaca to a randy rooster, there’s simply nowhere for him to put his sperm.

The eutherian uterus evolved from the “shell gland”—a muscular, oozy organ that secreted all the stuff necessary to produce an eggshell.

Like finding the Eve of milk, tracking down the placental Eve is tricky given that soft tissue, like breasts and uteri, doesn’t survive in fossils.

it’s still pretty hard to narrow down where we break from the marsupial plan and start being placentals.

For instance, because of genetic dating methods, most estimate that the ancient placenta probably evolved anywhere between 150 and 200 million years ago—a long time before the asteroid, in other words.

The placenta is the organ that lets embryos attach themselves to the mother’s uterus without being wholly destroyed by her immune system—a pretty important feature for live birth.

It’s derived from the same membranes that surround embryos in eggs, but evolved into a big, fleshy, alien docking station between the...

As for dating our ancient Eves, researchers found a fossil in 2011—Juramaia sinensis, or “ancient mother,” a squirrel-like thing that ate a bunch of tree bugs roughly 160 million years ago in what became northeastern China.

Roughly 1 in every 350 human girls are born with two uteri and cervixes at the end of their normal, single vagina—a glitch in the developmental programming that clearly harks back to our evolutionary past.

I haven’t yet mentioned the 1 in 4,500 girls born every year without a uterus.

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.

So while homosexuality might not be something that’s selected for in classic evolutionary terms, it nonetheless exists commonly in the population for a number of reasons.

Generally speaking, the biggest mammals in the world usually have a single, fused uterus, with a single cervix leading to a single vagina, and they likewise have one or two offspring per pregnancy.

The smallest? Two uteri, two cervixes, and a litter.

it’s a dance between what the mother’s body needs and what her hungry offspring need, with each accommodation skirting just on the edge of killing one or both of them.

At some fairly flexible point in her adolescence—anywhere between eight and eighteen—the female Homo sapiens arrives at menarche: the rite of passage that involves uterine blood and tissue leaking out of her vagina for an average of three to seven days.

Shedding menstrual material out of a vagina is super rare. And we’ve only just come up with a good theory for why we do it.

The question is why the uterine lining starts building up before it knows a fertilized egg is barreling down the fallopian tubes toward it. Among Donna’s descendants, this trait is exceedingly rare. Yet it has evolved independently three different times: once for higher primates, once for certain bats, and once for the elephant shrew.

It turns out the mammalian uterus isn’t a lush pillow—it’s a war zone. And ours may be one of the deadliest. Human women menstruate because it’s part of how we manage to survive our bloodsucking demon fetuses.

The uterus and its temporary passenger are, in fact, in conflict: the uterus evolving to protect the mother’s body from its semi-native invader, and the fetus and placenta evolving to try to work around the uterine safety measures.

If a certain set of genetic mutations makes the offspring generally stronger, slightly better developed, and better nourished when it exits the mother’s body, those genes will be selected for.

What is clear is that the placenta lies at the center of the problem. Researchers have managed to isolate two proteins that placentas produce that seem tied to women with preeclampsia.

Normally, these proteins help increase the mother’s blood pressure just enough to help deliver a bit more blood, more often, to the placenta to supply the fetus with what it needs. But in certain concentrations, these proteins narrow the blood vessels too much, which starts the hypertension cascade of preeclampsia.

But recent research undercuts that theory: women who never give birth are less likely to develop autoimmune diseases than women who have given birth at least once.

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. That wasn’t very long ago.

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.

This is also why ovarian cancer is so dangerous: not only do the ovaries regularly undergo strong hormonal changes and cell turnover, which makes them more prone to cancer in general, but they’re small and tucked against other organs. By the time ovarian cancer is diagnosed, it has often spread around the lower abdomen, with tumors popping up on the intestines, uterus, bladder, kidneys, and/or liver.

It’s also a useful reminder that what gender essentialists seem to find so essential—the presence of one or another sort of sex-typical genitalia—is a rather small difference in fetal development that takes only a handful of weeks and frequently goes astray. Our evolutionary path is littered with glittering difference, and so it is with bodies in any population. In life on Earth, diversity is a feature, not a bug.

We haven’t the foggiest idea why so many cisgender girls are born with wonky sex organs—I mean, we don’t know the exact mechanisms—but given our evolutionary history, it’s obvious why there might be so many fail points along the developmental path.

Second, only 30 percent of wolves in the wild survive their first year.