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As 'Bros' Declined Through History, Society Thrived

Evidence that humankind's 'Great Leap Forward' happened at the expense of testosterone and aggression.
Helga Esteb / Shutterstock.com

The blooming of human society into a place of thought and culture likely didn't happen because of some singular intellectual awakening—some bolt from above opening the species' eyes to gentleness and deep-thinking. It happened through gradual biological changes that saw human testosterone levels decline, with the result being less aggression and more technology, art, and culture in general.

This is according to new findings from researchers at Duke University, published in the journal Current Anthropology, who examined changing skull shapes over the course of human development.

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The skull-testosterone connection is this: different levels of the hormone lead to different skeletal features, with higher testosterone levels tending to result in more severe facial structures, including, crucially, a heavy brow.

The correlation is strong enough that it's possible to take a large number of skulls (14,000 in this case) representing different periods in human history, and determine with some degree of precision what levels of testosterone were present (or what levels of testosterone hormone receptors were present) in the human owners of those skulls. These levels can then be mapped against intellectual advances through human history in the hopes of finding a correlation.

Human intellectual advancement remains something of a mystery. We've been around for some 200,000 years, but it was only 50,000 or so ago that humans started making big advances in culture and technology. This is around when language, economic trade, cave painting and jewelry, hunting technologies like fish hooks and pit traps, and rituals all started appearing. The reason for this sudden leap, known alternately as the "Great Leap Forward," the "Upper Palaeolithic Revolution," and  "the Big Bang of human consciousness," have remained something of a mystery.

Robert Cieri/University of Utah

The last Ice Age is often thought of as the environmental stress that led to the Great Leap; as it got harder to live and the human population plunged, human development shot ahead just because it had to.

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In particular, some researchers point to mutations about that time in a particular gene with a large influence on language development, while other theories suggest that it was cooked food and the resulting sudden abundance of calories (and energy) that shoved humans forward (sorry, paleos).

The current Duke study doesn't quite eliminate anything, but it does hone in on a particular feature developed by society around that time: living together in close spaces in communities with relatively high (for humans at the time) population densities. In this environment, it would be softer faces and gentler behavior that determine survival, rather than brutish aggression. And so testosterone declines make a good bit of sense, with the result being increased "social tolerance," in the paper's words.

The phenomenon has already been observed in animals. With help from many rounds of selective breeding, a famous bit of research found that foxes with less aggressive personalities over time developed more juvenile appearances and behaviors. They became domesticated, in other words, not unlike humans did during their leap forward.

In a Duke press release, the authors note as well the differences observed between chimpanzee and bonobo apes. The latter, which lack the brow ridge common in chimps, are relatively mellow and social, while chimps are aggressive in comparison. It's been noted that chimps experience large increases in testosterone during puberty and release the hormone in stressful situations, while bonobos instead produce the hormone cortisol (like humans).

Ultimately, it's more support to the notion that modern humans are fundamentally prosocial, genetically tuned to cooperate rather than compete or fight, despite the headlines. "If prehistoric people began living closer together and passing down new technologies, they'd have to be tolerant of each other," lead author Robert Cieri said in the Duke release. "The key to our success is the ability to cooperate and get along and learn from one another."