The rise of modern civilization and human culture may have corresponded to a drop in testosterone linked with both male characteristics and aggression, a study suggests.

Researchers writing in Current Anthropology say a study of more than 1,400 skulls, both ancient and modern, suggests cultural shifts bringing advances in human society began at about the time when skulls showed evidence of lowering levels of the hormone.

From the heavy brows that testosterone created in our early ancestors, there was a change to rounder, softer skull features associated with less of the hormone, they said.

A loss of testosterone and a lessening of the aggression is linked to having more cooperation between people, the researchers suggest.

In other words, civilization got a boost when people started being nicer to each other.

"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," says lead study author and biologist Robert Cieri, a graduate student at the University of Utah who initiated the study while a senior at Duke University.

Cieri compared brow ridges, face shapes and interior volume of modern human skulls to older skulls, finding a reduction in the size of the brow ridge and a shortening in the upper part of the face, traits generally accepted as indicating a reduction in the effects of testosterone.

In the fossil record, modern humans made their appearance about 200,000 years ago, but evidence of a human culture in the form of art and tool-making only occurred about 50,000 years ago, the researchers said.

The change in skull shape that indicates a reduction in testosterone levels can be dated to around that same time, they said.

Duke University animal cognition experts Jingzhi Tan and Brian Hare point out that the same phenomenon has been seen in a number of animal species.

In a classic study conducted among Siberian foxes, descendants of individual animals who showed less fear of humans and less aggression began to display a more juvenile look and behavior following selective breeding for several generations.

"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.

Living together in ever-larger human groups required cooperation that placed a premium on lowered aggression and increased agreeableness, the researchers say.

"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."