The next time you appreciate a piece of music or use a “manly” seeming power tool, remember to thank lowered levels of testosterone. Researchers at Duke have found a correlation between what appears to be a reduction in testosterone levels and the explosion of “modern” human society about 50,000 years ago.

“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 evidence, they say, is in the fossil record. In examining specimens of early human skulls and comparing them to more modern examples, they found that more masculine features, like thick brows and squared jaws, began to soften right about the same time people began producing more “cultural” artifacts, like artwork and tools. Basically, with less testosterone to go around, humans started being a little nicer to one another. The eased temperaments then allowed the more cooperative society to blossom.

“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.”

It’s not entirely surprising. If amicability and cooperation quickly became preferred traits in the changing society, it stands to reason that mates would select for these traits, accelerating the proliferation of more successful (albeit less aggressive) humans.

The study compared the brow ridge, facial shape and interior volume of 13 modern human skulls older than 80,000 years, 41 skulls from 10,000 to 38,000 years ago, and a global sample of 1,367 20th century skulls from 30 different ethnic populations.

The findings are consistent with reduced levels of testosterone found in other species. Bonobos, for example, live much more cooperatively than chimpanzees, and accordingly they experience fewer spikes in testosterone during puberty and when under duress.

“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.