A study examining more than 1,400 ancient and modern human skulls suggests how lowered testosterone levels and subsequent softer -- more feminine -- skull features accompanied agreeableness, cooperation, and the “origins of behavioral modernity.”
Homo sapiens appeared in the fossil record about 200,000 years ago. But widespread evidence of modern behavior -- symbolic art and advanced technology like bone and antler tools, heat-treated flint, projectile weapons, fishing and birding equipment -- showed up just 50,000 years ago.
“Humans are uniquely able to communicate complex thoughts and cooperate even with strangers,” Robert Cieri from the University of Utah says in a news release. “New research on fossilized Stone Age humans from Europe, Africa and the Near East suggests these traits are linked, developed around 50,000 years ago, and were a driving force behind the development of complex culture.”
Cieri and colleagues measured and compared craniofacial features -- brow ridge, facial shape, interior volume -- of 13 human skulls older than 80,000 years, 41 skulls from 10,000 to 38,000 years ago, and 1,367 skulls from 30 ethnic populations in the 20th century. A trend emerged: reduction in the brow ridge and shortening of the upper face. Heavy brows were out, rounder heads were in, and these changes can be directly traced to testosterone acting on the skeleton.
“Differences between the younger and older fossils are similar to those between faces of people with higher and lower testosterone levels living today,” Cieri says. You can see this “feminization” in the composite image on the right: an ancient human with heavy brows, large upper face (left) versus a recent human with rounder features, less prominent brow (right). The work was published in this month’s Current Anthropology.
However, the team couldn’t tell from the bones whether these humans had less testosterone in circulation or fewer receptors for the hormone.
Lower testosterone is linked with social tolerance and cooperation in our closest ape relatives. "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," says study coauthor Brian Hare of Duke in a university release.
Aggressive chimpanzees and mellow, free-loving bonobos develop and respond to stress differently. Male chimps experience a flood of testosterone during puberty, but bonobos don’t. When bonobos are stressed, they don't produce more testosterone (as chimps do), but they do produce more of the stress hormone cortisol. Their social interactions and faces are different, too. "It's very hard to find a brow-ridge in a bonobo," Hare adds.
The researchers speculate that higher population densities triggered the shift towards lower testosterone levels, which led to more feminine faces. Increasingly, people had to work together, and being very aggressive became less advantageous. "If prehistoric people began living closer together and passing down new technologies, they'd have to be tolerant of each other," Cieri says. "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.”
Images: Robert Cieri, University of Utah
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