Anatomically modern humans first evolved some 200,000 years ago, but many of the traits we associate with early civilization—like bone and antler tools, grindstones, projectile weapons, fishing and the widespread use of fire—didn’t crop up until around 40,000-50,000 years ago, during a period often dubbed the “Great Leap Forward.” The reason for the gap is one of science’s most hotly debated questions, but according to a new study published in the journal “Current Anthropology,” the answer may be hormonal. In a paper titled “Craniofacial Feminization, Social Tolerance and the Origins of Behavioral Modernity,” University of Utah doctoral candidate Robert Cieri, University of Iowa professor Robert Franciscus, and Duke University’s Steven Churchill, Jingzhi Tan and Brian Hare argue that our ancestors only started looking—and behaving—like modern humans after experiencing a dip in testosterone.
Lead author Cieri and his colleagues claim that the blossoming of culture dovetailed with a period of physical change during which humanity developed a shorter and more rounded face with a less pronounced brow ridge. These increasingly “feminine” features indicate a reduction in testosterone, a male hormone also associated with aggression and antisocial behavior. As testosterone decreased, humans may have evolved a more agreeable disposition and begun working together to build sophisticated societies. “The modern human behaviors of technological innovation, making art and rapid cultural exchange probably came at the same time that we developed a more cooperative temperament,” Cieri said in a Duke University press release. “The key to our success is the ability to cooperate and get along and learn from one another.”
As part of the study, researchers measured 1,421 human skulls and analyzed the differences in brow-ridge projection, facial shape and cranial volume. The sample included 13 Middle Pleistocene-era skulls dating back to between 200,000 and 90,000 years; 41 Late Pleistocene skulls from 38,000 to 10,000 years ago; and 1,367 modern skulls culled from locations on all six inhabited continents. Cieri and his coauthors say this fossil evidence shows “a significant reduction in androgen-mediated craniofacial masculinity” between the Middle Pleistocene and Late Pleistocene skulls, suggesting humans went through physical changes prior to the explosion of culture associated with the “Great Leap Forward.” Brow ridges—one of the most reliable indicators of testosterone levels—consistently decreased in size between the Middle Pleistocene and Late Pleistocene samples, and were smaller still in the modern skulls. The researchers also found that the facial shape of the Late Pleistocene skulls was shorter and more rounded than those of their ancestors.
The effects of testosterone on behavior are especially evident in chimpanzees and bonobos, two of our closest ape relatives. Male bonobos do not have high levels of testosterone, which makes them much more easygoing than hormone-fueled chimps, who grow competitive and mercurial during puberty, when their testosterone levels spike. According to study co-author Brian Hare, the differences are also written on their faces. “It’s very hard to find a brow-ridge in a bonobo,” he said in the press release.
The researchers point to studies on other species as further proof that hormonal change can have a direct impact on skull shape and temperament. In a 50-year experiment, a captive population of Siberian silver foxes was selectively bred to increase tameness and lack of aggression toward humans. Over dozens of generations, the foxes developed a more friendly and playful disposition, but they also went through important physical changes. Their skulls reduced in size, and the male foxes developed shorter and wider heads that resembled those of female foxes more than their male counterparts in the wild. A similar effect has been noted to occur in domesticated dogs and pigs. “If we’re seeing a process that leads to these changes in other animals, it might help explain who we are and how we got to be this way,” said Hare.
The researchers are unsure if the hormonal changes in our prehistoric ancestors were the result of lower levels of circulating testosterone or a reduction in testosterone receptors, but they argue the drop most likely evolved out of the rapid population growth that accompanied the “Great Leap Forward” some 50,000 years ago. Cieri claims that increased competition for resources may have also made early humans act more civil toward one another. “If population density starts increasing, not only are there more people in your immediate environment that you have to get along with, but all land would be occupied with human groups,” he told Time Magazine. “You wouldn’t just go across to the other side of the valley to hunt bison by yourself, you’d go to the other side of the valley and maybe make a treaty with the other people who live there.”