A new study has suggested that a decline in testosterone levels in modern humans feminized their features and behaviour, paving the way to modern civilization.
The study, led by biologist Robert Cieri of the University of Utah, examined over 1,400 modern and ancient skulls and found that around 50,000 years ago heavy brows started to phase out and skulls became rounder.
This coincides with what some anthropologists believe was an explosion of cultural creativity by Homo sapiens, known as the Great Leap Forward or the Upper Paleolithic Revolution, which led to finely-made tools, figurative art, long-distance networking and the development of natural languages. It also marks modern human expansion into previously unexplored environments across the planet, such as Australia and Northern Eurasia. They believe this change in behaviour is what distinguishes present day humans and their recent ancestors from other living primates and extinct hominid species, such as the Neanderthals or Homo erectus.
Cieri said: “Human fossils from after modern behaviour became common have more feminine faces, and differences between the younger and older fossils are similar to those between faces of people with higher and lower testosterone levels living today.”
Although the cause in the reduction of testosterone levels isn’t clear, Cieri suggested that its link to decreasing aggression and increased social cooperation conveyed a biological advantage to modern humans.
The research team also included animal cognition researcher Brian Hare, who says the same effect is found in other animal species, particularly in our closest ape relatives – chimpanzees and bonobos. Male chimpanzees develop a strong increase in testosterone during puberty, but bonobos do not. Social interactions among chimpanzee are markedly more aggressive than among bonobos, and their faces are different, too. “It’s very hard to find a brow-ridge in a bonobo,” Hare said.
(Composite image above shows the differences between an ancient modern human with heavy brows and a large upper face and a recent modern human with rounder features and a less prominent brow – image by Robert Cieri, University of Utah).