Modern humans appear in the fossil record about 200,000 years ago, but it was only about 50,000 years ago that making art and advanced tools became widespread.
A new study appearing Aug. 1 in the journal Current Anthropology finds that human skulls changed in ways that indicate a lowering of testosterone levels at around the same time that culture was blossoming.
"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," said lead author Robert Cieri, a biology graduate student at the University of Utah who began this work as a senior at Duke University.
The study, which is based on measurements of more than 1,400 ancient and modern skulls, makes the argument 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 Duke anthropologist Steven Churchill, who supervised Cieri's work on a senior honors thesis that grew to become this 24-page journal article three years later.
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 also included Duke 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 behavior 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," said Hare, who also studies differences between our closest ape relatives -- aggressive chimpanzees and mellow, free-loving bonobos.