This question is often asked: What separates humans from animals? Anthropologists, however, would rather ask—and answer—what separates humans from earlier humans.
About 50,000 years ago, homo sapiens developed the capacities for “innovation, planning depth, and abstract and symbolic thought,” as a study published in Current Anthropology earlier this month puts it. In academia, this moment in human evolution is referred to as the shift toward “behavioral modernity.” Until recently, not much was known about why our species veered toward more sophisticated sensibilities.
"As population density and social complexity increased, females may have preferred males with more feminized faces that signal a greater propensity to invest in parenting effort."
A group of anthropologists and biologists at Duke University had a theory: It’s because our skulls changed shape. This would have led to, as their study argues, a “change in average human temperament toward a less aggressive, more socially tolerant individual.”
To test their hypothesis, the team measured more than 1,400 skulls—1,367 modern ones from 30 ethnicities; 41 from between 10,000 and 38,000 years ago; and 13 ancient ones from more than 80,000 years ago—paying special attention to the brow ridge, face shape, and endocranial volume. “The study was motivated,” the researchers say, “by us trying to find a biological explanation—with evidence—of what could explain the huge explosion of culture around 50,000 years ago.”
After taking stock of their painstaking measurements, the researchers were surprised by how well their data supported their hypothesis. They found that there had indeed been a structural change in the human cranium—specifically, our brow ridges shrunk and the upper parts of our faces got shorter. It happened in the late Pleistocene era, and the shift indicated a lowered level of testosterone acting on the skeleton. This “feminization” of our heads made us less violent and more genteel.
The researchers think that sexual selection could have been what feminized our skulls. “Facial masculinity appears to be an honest signal of behavioral tendencies,” the authors explain in the paper, and “as population density and social complexity increased, females may have preferred males with more feminized faces that signal a greater propensity to invest in parenting effort.”
“Although our results don't really pertain to populations of humans living today,” the researchers say, “it's important to note that the potently biggest leap forward in human technology was likely accomplished through advances in cooperation, not intelligence.”
Rosie Spinks contributed reporting.