A sallow light rises over the land at the opening of 2001: A Space Odyssey, one of the most celebrated movies of the 20th century. Stanley Kubrick’s shot pulls in on a band of furry man-apes gathering around a watering hole; no women, no children—or at least none easily discerned. The scene shifts to a young male, who pulls a large bone from a skeleton. He stares at it for a moment before beating the ground, slowly at first, then furiously. He soon runs off and uses it to bludgeon another hominin to death. Prehistoric man has invented the first weapon.

This is the story of what I call “tool triumphalism”: Man invented weapons, claimed dominion over his peers and the rest of the animal kingdom, and all of our achievements flow from there. As a culture, we still tell ourselves that this special cleverness is why we’ve succeeded as a species. And maybe that’s true—but not in the way you might think. Among our ancient ancestors, the most prolific tool creators probably weren’t male. And I propose that the most important early invention people came up with probably wasn’t a weapon, fire, agriculture, the wheel, or even penicillin. Humanity’s greatest innovation was gynecology.

Among paleoanthropologists, primatologists, comparative ethologists, cognitive scientists, and even the more ape-interested historians, there’s agreement that humanity’s predecessors were avid tool users and clever problem solvers. This isn’t uncommon: A variety of animals wield tools, octopus and crow alike. And primates frequently use tools in lab settings, although it’s true that primatologists aren’t really sure how many would do so in the wild—labs are weird places, and human scientists are hardly normal troupe-mates. But the archeological record shows that, for example, capuchin monkeys have been modifying stone tools for 3,000 years. So, in the broad sense, tool use is associated with many different clever beasts. It was certainly true of whatever human forerunner Kubrick was going for—as a researcher who’s studied the evolution of cognition, I’d guess it was Homo habilis, but it could have easily been Homo erectus. Even earlier in the hominin line, with australopithecines (like the famous specimen commonly known as Lucy), who lived roughly 3 million years ago, our ancestors’ fossilized bones are already associated with stone tools. That means that Lucy’s creations, and those of habilis after her, were hardly triumphant. Those very smart primates were using everything they could to survive.

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Describing how the set of practices one could collectively call gynecology—or how any kind of tool use—evolved is tricky, because brain tissue almost never fossilizes, nor does the uterus. We’re actually talking about a set of behaviors—not an organ, not a bone, but something our ancestors used their cognitive and physical abilities to do. The artifacts of tool use are preserved, however, and evolutionary biologists can use them to trace changes in the habits and capabilities of evolving species over vast stretches of time—particularly when those artifacts are made of rock and usefully situated near the fossilized bones of their makers. Researchers including the Smithsonian paleoanthropologist Briana Pobiner have shown that butchery marks on bones of hominins’ prey indicate how they used tools to access food other meat-eaters couldn’t—marrow, for example, or brains. But what we learn about behavior in our deep past is nearly always, by nature, inferred. What paleoanthropologists are studying when they analyze stone axes is a triangulation of sorts: the implicit (possible) relationship between an object, its user, and its environment.

This article has been adapted from Cat Bohannon’s new book, Eve: How the Female Body Drove 200 Million Years of Human Evolution.

Generally speaking, innovation is what weaker individuals do in order to overcome their relative disadvantage. From a scientific perspective, female primates have more to gain—and more to lose. Most are smaller and more vulnerable than the males. Given that their bodies are the ones that have to build, birth, and nurse children, females also have more urgent food and safety needs than males. So if our female ancestors were also good problem solvers—as higher primates are—then it makes sense for them to have been inventors who adapted around their limitations, though that’s not the picture we usually paint of early hominins. Yet feminine invention is well known in chimpanzees, some of our closest living relatives. In Senegal, chimps—more females than males—use wooden spears they’ve made to hunt prey. And the next generation? In most cases, a study in the Republic of Congo found, they learn how to do it from mom.

And in evolutionary terms, species don’t really get a harder problem than the one we have to deal with: Human beings are really, really bad at reproducing ourselves. Even today, with modern medical interventions, our pregnancies and births—and, as a result, our postpartum recoveries—are longer, harder, and more prone to crippling complications than they are for most other primates. Compared with other apes, human women have a really small pelvic opening and human babies have a really big head, because when hominins evolved to walk upright, the structure of our pelvis changed. By analyzing their fossils, many paleoanthropologists think the australopithecines shared a similar dilemma. Once brainy tool users like habilis and her peers came around, it had become a real issue. (It’s hard to fit a watermelon through a lemon-size hole.) If you can’t successfully deliver live offspring, or survive childbirth, your lineage is headed for extinction. And yet, somehow, there are 8 billion Homo sapiens on the planet right now.

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I propose that the only reason we got to that number is that our ancestors, over time, applied their deep sociality and general cognitive abilities to our biggest obstacle: Hominins had to invent gynecology. Lucy probably had a midwife. Habilis likely had even more fertility workarounds, and erectus more after her. Slowly but surely, our foremothers would have started regularly helping one another give birth and directly manipulating their fertility patterns in real time: not just at the moment of birth, but in the long on-ramp of fertility prior and the hideously long off-ramp of postpartum survival. None of our accomplishments would have been possible without it.

All gynecology is remarkably similar: It’s a set of practices and accumulated medical knowledge that try to make reproduction safer by preventing and treating excessive uterine bleeding and bacterial infection; guiding the intensity of labor efforts to match the dilation of the cervix; and enhancing, or preventing, female childbearing when desired. The contemporary version many people are familiar with, featuring a speculum and a paper gown, was pioneered in the mid-19th century, in large part through cruel experimentation and surgery on enslaved women. But we use its basic form, in some version, in every single contemporary human culture. From the records we have—and there’s a surprising number, including written accounts and ancient specula made of iron—we did it in historical cultures, too, in various ways, scaffolded by various belief systems.

Gynecology is absolutely essential for our species’ evolutionary fitness, especially if you’re talking about a reproductive system like ours in its ancient state. As populations were repeatedly hit with one ridiculous challenge after another, our ancestors would have needed to regrow a viable population to carry on their physiological or behavioral innovations. Sometimes that would have required clustering births in earlier years. Sometimes it actually meant having fewer kids over longer periods of time. No matter the strategy, the goal was clear: Have more mothers and babies survive when they might not otherwise.

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The biggest clue to our potential for gynecology is the ancient stone tools associated with protohumans. The earliest examples weren’t explicitly gynecological, but mapping those caches—how far they spread, how consistent the tech, how often they’re found with fossils—is the best way we have of tracking how early hominins were sharing complex social knowledge, because that kind of flint knapping isn’t fast or easy; it’s a skill you need to learn how to do. From what we’ve established, scientists are fairly sure that ancestors like habilis and erectus lived in highly collaborative groups, desperately trying to outlearn and outrun a world full of muscled, toothy things that were all too happy to eat them. When they weren’t running, they were, now and again—painfully, and with difficulty—giving birth. And they were surviving, in no small part because they were working together.

To invent gynecology, protohumans needed to be able to trust one another enough to be around one another at those crucial moments of vulnerability: labor, birth, and early nursing. That’s why the arrival of midwives is one of those moments in hominin history for which we can truly say, “This is when we started to become human.” It would have required a profoundly cooperative female society and a social structure that rewarded helpful behaviors. We can’t prove habilis lived like that so long ago. But there’s some evidence: As the anthropologist Sarah Blaffer Hrdy writes brilliantly in Mothers and Others, alloparenting—the concept of caring for young who aren’t your children—is both distinct in our species and vital to its ascendancy, and was instrumental in our long, slow transformation to more cooperative societies with strong female bonds. The primatologist Frans de Waal also convincingly makes the case that becoming deeply cooperative was key to our survival. And from what many primatologists have seen among extant ape communities, a more collaborative female environment would provide the sort of fertile social ground that could allow creatures such as Lucy, habilis, and erectus to invent a widespread culture of midwifery.

The fossil record tells the hominin success story—and underscores the need for these practices. When Homo sapiens migrated across the continent of Africa and eventually the globe, a time paleoanthropologists call the “Great Expansion,” we were also, simultaneously, reducing our species’ genetic diversity, because at each new site, the group would be breeding largely with themselves. In order to avoid becoming doomed to extinction because of inbreeding, each band of migratory humans would have been under even more pressure to build up and sustain a minimum viable population in that new location. No strategy would be perfect, but a reproductive free-for-all without shared knowledge (and shared child-care resources) would not be a good one. In other words, for each transition point in humanity’s ancient migrations, it’s reasonable to expect to find a group of skinny, scrappy people helping each other just barely produce enough kids to replace themselves, finding ways around the inherent problems of inbreeding, and miraculously surviving.

This article has been adapted from Cat Bohannon’s new book, Eve: How the Female Body Drove 200 Million Years of Human Evolution.