About 50,000 years ago, people started developing tools. They started making art, in caves. And they started cooperating. Simultaneously, that's when our faces went from looking like the skull on the left, to the one on the right.
A group of researchers from Duke and the University of Utah are theorizing that the correlation is not coincidence; that, in fact, the changing shape of skulls signals a change in something else that would have made cooperation more likely: A drop in male testosterone levels.
"Looking mostly at the brow ridges, the area above your eyes... and the shape of your upper face" said Bob Cieri, lead author on the research, you can see when testosterone levels taper off.
How? Cieri says you can look at some of humans' closest biological relatives, bonobos and chimps, to see the effects of lower testosterone levels.
"Bonobo males kind of look like chimp females," said Robert Cieri, a graduate student at the University of Utah. "The male skulls don't look as masculinized."
Cieri says this may have started to happen as population density increased. When groups of humans started bumping up against other groups, they may have found it advantageous to cooperate with each other, rather than pummel each other into the ground (as high levels of testosterone might make more likely).
Cieri is careful to point out that the levels of testosterone we're talking about are pretty high.
"It's not really selection of men versus women," said Cieri. "It's selection against hugely dominant, aggressive males that don't maybe even exist in today's population.