Modern humans appear in the fossil record about 200,000 years ago, but it was only about 50,000 years ago that we find widespread evidence of bone and antler tools, heat-treated and flaked flint, projectile weapons, grindstones, fishing and birding equipment and a command of fire.
     Scientists have shown that, at around the same time that culture was blossoming, human skulls changed in ways that indicate a lowering of testosterone levels. The study is based on measurements of more than 1,400 ancient and modern skulls.
     "The modern human behaviours of technological innovation, making art and rapid cultural exchange probably came at the same time that we developed a more cooperative temperament," said lead study author Robert Cieri, a biology graduate student at the University of Utah USA.
     Heavy brows were out, rounder heads were in, and those changes can be traced directly to testosterone levels acting on the skeleton, according to Duke anthropologist Steven Churchill, who supervised Cieri's early work on the subject. What they can't tell from the bones is whether these humans had less testosterone in circulation, or fewer receptors for the hormone.
     The research team included animal cognition researchers Brian Hare and Jingzhi Tan, who say this argument is in line with what has been established in non-human species. In a classic study of Siberian foxes, animals that were less wary and less aggressive toward humans took on a different, more juvenile appearance and behaviour after several generations of selective breeding.
     "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 Hare, who also studies differences between aggressive chimpanzees - our closest ape relatives - and mellow, free-loving bonobos. Their social interactions are profoundly different and, relevant to this finding, their faces are different, too.
     "If prehistoric people began living closer together and passing down new technologies, they'd have to be tolerant of each other," Cieri says. "The key to our success is the ability to cooperate and get along and learn from one another."

Edited from ScienceDaily (1 August 2014)