Authors of the new paper -- published this week in the journal Current Anthropology -- say a reduction of testosterone some 50,000 years ago precipitated thinner, less bushy eyebrows, rounder heads, gentler personalities and flourishing culture.
"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," Robert Cieri, a biology graduate student at the University of Utah and lead author of the new study, told The Telegraph.
"Human fossils from after modern behavior became common have more feminine faces, and differences between the younger and older fossils are similar to those between faces of people with higher and lower testosterone levels living today," Cieri said.
Brian Hare, who studies animal cognition at Duke University, aided the research. He says that his work has located similar correlations among human's closest ape relatives. Testosterone-rich chimpanzees feature more pronounced brows and demonstrate more aggressive behaviors than the mellow, free-loving bonobos, who produce less testosterone and more cortisol, a hormone used to handle stress.
"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," explained Hare.
Cieri says it makes sense that less testosterone would enable more productive cultures.
"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."