With all due respect to all the macho men out there, you guys might have been holding the human race back — at least in prehistoric times. Researchers at Duke University and the University of Utah say the rise of human civilization as we know it is linked to a drop in testosterone levels.
The paper, published in the journal Current Anthropology, posits that a testosterone deficit facilitated the friendliness and cooperation between humans, which lead to modern society. Study lead Robert Cieri posits "reduced testosterone levels enabled increasingly social people to better learn from and cooperate with each other."
While Homo sapiens are at least 200,000 years old, the species didn't really start acting human until at least 50,000 years ago, when widespread use of tools and ornaments first appeared. (Via PBS)
Cieri theorized that a drop in testosterone might be correlated with that renaissance, and looked to ancient skulls for evidence. Testosterone levels can significantly shape the development of certain facial features, particularly the brow ridge and upper face.
The scientists examined 1,400 human skulls from various prehistoric and modern periods. Comparing the newer skulls to the ancient ones, researchers noticed a sharp decline in features sculpted by testosterone; modern skulls have smaller brows and more rounded faces. (Via Matt Celeskey / CC BY SA 2.0, High Contrast / CC BY 3.0 DE, Thomas Roche / CC BY SA 2.0)
Based on these findings, the study concluded two things; "the fossil record of H. sapiens does reflect reductions in craniofacial masculinity," and "it seems likely that important increases in human social tolerance developed during this interval." (Via Current Anthropology)
Or as The Washington Post puts it, the decline of testosterone led to "less head clubbing and more community building, basically."
There's some precedent for these findings in the animal kingdom. The researchers cited previous studies of Siberian foxes, whose appearances became more juvenile as they were domesticated. They also noted the differences in facial structures between aggressive chimpanzees and their more relaxed cousins, the bonobos. (Via Vimeo / Tyler Cole, Psych USD / CC BY SA 3.0, Thomas Lersch / CC BY SA 3.0)
But there's only so much information researchers can glean from the ancient skulls. For instance, the study doesn't reveal whether the testosterone-lacking skulls were actually caused by a testosterone deficiency, or if the developing humans simply had fewer and fewer receptors for the chemical.