Kinder human beings with less testosterone emerged just as culture, art, and tools were developed in society 50,000 years ago, thus changing the pace of history. Duke University researchers found these answers in human skull sizes, and published their study in the journal Current Anthropology. The findings reveal how we became technologically advanced so quickly.

"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 the study’s lead author Robert Cieri, a biology graduate student at the University of Utah who began working on the research as a senior at Duke University, in a press release.

Researchers measured more than 1,400 ancient and modern skulls and found that when humans had lower levels of testosterone, which made them theoretically kinder and led to advancements in technology. Human beings made their first appearance in fossil records nearly 200,000 years ago, but didn’t start creating arts or tools until about 50,000 years ago.

"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 Duke researcher Brian Hare, who focuses his studies on the differences between our closest ape relatives, aggressive chimpanzees and calm bonobos, in a press release.

Lower testosterone levels gave skulls more subtle brow lines and rounder heads in humans who were busy developing weapons and artwork. Cieri compared the brow ridge, facial shape, and volume of 13 human skulls older than 80,000 years ago; 41 skulls from 10,000 to 38,000 years ago; and 1,367 skulls of 20th century humans from 30 different ethnic populations across the globe.

When Hare looked at the skulls, he saw it as a reflection of the apes and how they respond to social stress differently. When chimpanzees, which have higher testosterone levels, were stressed out, they produced more testosterone. Meanwhile, bonobos do not have high testosterone levels and don’t produce more when they’re stressed out, but they do secrete the stress hormone cortisol. 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 said.

Testosterone changes the skulls shape, and provides another clue as to why humans advanced so quickly after they had already been around for 150,000 years, despite so many other theories. The lower levels of testosterone may have made it easier for people to focus, and become more intellectually stimulated through cultural exchanges once they were calmer and less aggressive.

"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: Cieri RL, Churchill SE, Ranciscus RG, Tan J, Hare B. Craniofacial Ferminization, Social Tolerance, and the Origins of Behavioral Modernity. Current Anthropology. 2014.