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.