The story of us: Lowered testosterone allowed culture & technology to blossom
|Credit: Robert Cieri, University of Utah|
A composite image shows the facial differences between an ancient modern human with heavy brows and a large upper face and the more recent modern human who has rounder features and a much less prominent brow. The prominence of these features can be directly traced to the influence of the hormone testosterone.
If you follow this blog, you've probably guessed that I'm fascinated with the history of the human animal from the earliest days. Understanding how we became us helps in our day to day lives and in our writing.
This story offers a very different view of what happened fifty thousand years ago to allow technology, culture and civilization to simply explode upon the scene in the middle east. Yes, there are all sorts of theories of us being visited by aliens from outer space to gave us the secrets of whatever. To me, these theories assume that the human animal is completely incapable of invention and creativity, like we're the dolts of the universe who would still be living in caves, clubbing each other over the heads for breeding rights.
According to this new report there is a genetic reason we stopped living in caves and clubbing each other over the head: a drop in the level of testosterone in the average male. What fascinates me is that the estimated date, 50,000 years ago, is about 10,000 years after genetic Adam and genetic Eve were known to have lived in the area between the Nile River and the swampy area now the site of the Suez Canal. We are all of us, from the Eskimos to the Bushman of the Kalahari, descended from this man and this woman.
Is this the genetic change that Adam and Eve gave us? A lowered testosterone level that allowed for more cooperative behavior that allowed survival against more aggressive brutes still over-amped on a male hormone known to cause aggressive behavior up to and including psychotic rage?
I offer no answer to this question.
Here's the story.
Modern humans appear in the fossil record about 200,000 years ago, but it was only about 50,000 years ago that making art and advanced tools became widespread.
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"The modern human behaviors of technological innovation, making art and rapid cultural exchange probably came at the same time that we developed a more cooperative temperament," said lead author Robert Cieri, a biology graduate student at the University of Utah who began this work as a senior at Duke University.
The study, based on measurements of more than 1,400 ancient and modern skulls, makes the argument that human society advanced when people started being nicer to each other, which entails having a little less testosterone in action.
Heavy brows were out, rounder heads were in, and those changes can be traced directly to testosterone levels acting on the skeleton, according to Duke anthropologist Steven Churchill, who supervised Cieri's work on a senior honors thesis that grew to become this 24-page journal article three years later.
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What they can't tell from the bones is whether these humans had less testosterone in circulation, or fewer receptors for the hormone.
The research team also included Duke animal cognition researchers Brian Hare and Jingzhi Tan, who say this argument is in line with what has been established in non-human species.
In a classic study of Siberian foxes, animals that were less wary and less aggressive toward humans took on a different, more juvenile appearance and behavior after several generations of selective breeding.
"If we're seeing a process that leads to these changes in other animals, it might help explain who we are and how we got to be this way," said Hare, who also studies differences between our closest ape relatives -- aggressive chimpanzees and mellow, free-loving bonobos.
Those two apes develop differently, Hare said, and they respond to social stress differently. Chimpanzee males experience a strong rise in testosterone during puberty, but bonobos do not. When stressed, the bonobos don't produce more testosterone, as chimps do, but they do produce more cortisol, the stress hormone.
Their social interactions are profoundly different and, relevant to this finding, their faces are different, too. "It's very hard to find a brow-ridge in a bonobo," Hare said.
Cieri compared the brow ridge, facial shape and interior volume of 13 modern human skulls older than 80,000 years, 41 skulls from 10,000 to 38,000 years ago, and a global sample of 1,367 20th century skulls from 30 different ethnic populations.
The trend that emerged was toward a reduction in the brow ridge and a shortening of the upper face, traits which generally reflect a reduction in the action of testosterone.
There are a lot of theories about why, after 150,000 years of existence, humans suddenly leapt forward in technology. Around 50,000 years ago, there is widespread evidence of producing bone and antler tools, heat-treated and flaked flint, projectile weapons, grindstones, fishing and birding equipment and a command of fire. Was this driven by a brain mutation, cooked foods, the advent of language or just population density?
The Duke study argues that living together and cooperating put a premium on agreeableness and lowered aggression and that, in turn, led to changed faces and more cultural exchange.
"If prehistoric people began living closer together and passing down new technologies, they'd have to be tolerant of each other," Cieri said. "The key to our success is the ability to cooperate and get along and learn from one another."
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Story Source: Materials provided by Duke University. Robert L. Cieri, Steven E. Churchill, Robert G. Franciscus, Jingzhi Tan, Brian Hare. Craniofacial Feminization, Social Tolerance, and the Origins of Behavioral Modernity. Current Anthropology, 2014