Cieri argues that human society advanced when people started being nicer to each other, which entails having a little less testosterone in action. "Heavy brows were out, rounder heads were in, and those changes can be traced directly to testosterone levels acting on the skeleton," according to fellow Duke anthropologist Steven Churchill.

While the team is confident that less testosterone was behind these skeletal changes, they unable to say whether they came about from lower levels of testosterone in circulation, or fewer receptors for the hormone.

Co-researcher Brian Hare says that changes driven by testosterone are evident in our closest ape relatives - aggressive chimpanzees and mellow, free-loving bonobos.

Those two apes develop differently, Hare said, and they respond to social stress differently. Chimpanzee males experience a strong rise in testosterone during puberty, but bonobos do not. When stressed, the bonobos don't produce more testosterone, as chimps do, but they do produce more cortisol, the stress hormone. "Their social interactions are profoundly different and relevant to this finding; their faces are different, too. It's very hard to find a brow-ridge in a bonobo," Hare notes.

As far as modern humans are concerned, the Duke study argues that living together and cooperating put a premium on agreeableness and lowered aggression - attributes that indicate lower testosterone. "If prehistoric people began living closer together and passing down new technologies, they'd have to be tolerant of each other," Cieri said. "The key to our success is the ability to cooperate and get along and learn from one another.

 

Source:  http://www.scienceagogo.com/