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Did Reduced Testosterone Levels Help Human Culture Advance?

August 3, 2014
Image Caption: 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. Credit: Robert Cieri, University of Utah

redOrbit Staff & Wire Reports – Your Universe Online

Changes in the human skull occurring approximately 50,000 years ago indicate that the rise of culture occurred around the same time as a reduction in testosterone levels, according to new research appearing in the August 1 edition of the journal Current Anthropology.

In the study, lead author Robert Cieri, a biology graduate student at the University of Utah who began this work as a senior at Duke University, argue that people started making art and using advanced tools only after they became nicer to each other. Those gentler personalities required having slightly reduced testosterone levels, they added – a condition suggested by the more feminine features found in skulls recovered from that era.

“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,” Cieri explained in a statement Friday.

The research team based their theory on measurements of over 1,400 ancient and modern skulls. Their efforts revealed that more recent modern humans had rounder features and a much less prominent brow, and those changes can be traced back directly to the impact of testosterone levels on the skeleton.

They are not certain if the bones indicate that these individuals had less testosterone in circulation, or fewer receptors for the hormone. However, fellow investigators and Duke university animal cognition researchers Brian Hare and Jingzhi Tan state that this hypothesis is in line with what has previously been established in non-human species.

For example, selective breeding of Siberian foxes was eventually able to produce creatures that were less aggressive towards humans and were more juvenile in appearance and behavior after several generations. Hare said that observing a process that leads to these types of changes in other creatures might also explain human behavior.

“It might help explain who we are and how we got to be this way,” said Hare, who researches differences between humans and their closes ape relatives, the more aggressive chimpanzees and the more laid-back bonobos. Those apes develop differently, he said, and they respond to social stress in different ways.

Male chimpanzees experience a strong rise in testosterone during puberty, but bonobos do not, the researchers explained. Under stressful conditions, chimps produce more testosterone, but bonobos do not. Instead, they produce more of the stress hormone cortisol. In addition to differences in the social activity of each creature, their faces are also said to be different as well – a finding the study authors call relevant to their work.

Cieri’s team compared the brow ridge, facial shape and interior volume of skulls from three different groups of humans: 13 belonging to modern humans more than 80,000 years old, 41 from people who lived between 10,000 to 38,000 years ago, and 1,367 20th century skulls from 30 different ethnic populations. They found an overall reduction in brow ridge and shortening of the upper face over time, reflecting a gradual reduction in testosterone action.

“There are a lot of theories about why, after 150,000 years of existence, humans suddenly leapt forward in technology,” Karl Leif Bates of Duke University explained. “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,” Bates added.

Cieri concluded that as prehistoric men and women began living closer together and passing down new technologies, it would pretty much have to be a given that they would have to cooperate with and learn from each other. His research was supported by the National Science Foundation (NSF), the Leakey Foundation and the University of Iowa Orthodontics Department.


Source: redOrbit Staff & Wire Reports - Your Universe Online



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