Brow ridges receded and the length of the face between the eyes and the top of the jaw shortened in the skulls from after 50,000 years ago versus before, Cieri found in the study, which was published Friday in the journal Current Anthropology.
Humans didn't became more girly in order to evolve culturally and intellectually. Rather, features considered hyper-masculine began to go, he said.
"It's not necessarily that people are acting more like females, it's just kind of this overly aggressive, intolerant phenotype was maybe selected against," Cieri said.
He said it's possible testosterone began to dive as humans grew more populous and began living in more tightly packed conditions.
"It could have happened because as population density started increasing people were living closer together and had to get along," Cieri said.
Around 50,000 years ago, there is widespread evidence of better technology emerging human-made bone and antler tools, heat-treated and flaked flint, projectile weapons, grindstones, fishing and birding equipment and a command of fire.
The research team also included Duke University animal cognition researchers Brian Hare and Jingzhi Tan. Cieri began the study as his senior honors thesis while an undergraduate at Duke before entering the University of Utah as a biology graduate student.
Hare and Jingzhi said the findings of the human skull study are consistent with what's been found in animals as well, according to a Duke news release. For example, a study of Siberian foxes found animals that were less wary and aggressive toward humans took on 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," Hare said in the release.
Hare also studies chimpanzees, which are aggressive, and bonobos, which are more laid-back. Hare said it's difficult to find a bonobo with a brow-ridge.
Cieri also found that even among 20th century hunter-gatherer communities, testosterone levels were higher than in farming communities from the same time.
Cieri said it's possible testosterone is still decreasing in humans today but probably not to the same extent.
The research was supported by the National Science Foundation, the Leakey Foundation and the University of Iowa Orthodontics Department.