SALT LAKE CITY, UT.-
A study of 1,400 ancient and modern human skulls suggests that a reduction in testosterone hormone levels accompanied the development of cooperation, complex communication and modern culture some 50,000 years ago.
The research, published in todays issue of the journal Current Anthropology, uses craniofacial evidence to propose that lowered testosterone levels could explain the relatively sudden origin of modern behavior about 50,000 years ago, says University of Utah
biology graduate student Robert Cieri.
Cieri conducted the study of the feminization of human skulls and faces with colleagues at Duke University in North Carolina before moving to Utah in 2012.
Humans are uniquely able to communicate complex thoughts and cooperate even with strangers, Cieri says. New research on fossilized Stone Age humans from Europe, Africa and the Near East suggests these traits are linked, developed around 50,000 years ago, and were a driving force behind the development of complex culture.
Homo sapiens, or modern humans, first appeared in the fossil record about 200,000 years ago, but evidence of modern behavior, such as symbolic artifacts and advanced tools are only about 50,000 years old, he adds.
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 says.
He notes that lower testosterone is associated with social tolerance and cooperation in bonobos and chimpanzees, and with less aggression in humans. Cieri speculates that higher population densities could have triggered the shift towards lower testosterone levels, as people increasingly had to work together to succeed, and being highly aggressive became less advantageous.
Whatever the cause, reduced testosterone levels enabled increasingly social people to better learn from and cooperate with each other, allowing the acceleration of cultural and technological innovation that is the hallmark of modern human success, Cieri says.