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.
Image credit: Robert Cieri, University of Utah
Though the fossil record shows that humans have existed for about 200,000 years, it was not until around 150,000 of those years had elapsed that human society began to take shape. Suddenly, humans leapt forward in terms of our ability to make tools.
Evidence shows that, around this time, humans began to construct tools out of bone and antler and heat-treated and flaked flint. We also began to make projectile weapons, grindstones and equipment for catching fish and birds. Plus, during that period, we also pioneered the use of fire.
However, experts have been unsure about what drove this first technological revolution. Did a brain mutation make humans smarter? Did cooked food have something to do it? Or was it because this was when language evolved?
The Duke researchers have another theory, which they unveil in the journal Current Anthropology.
For their study, the researchers measured more than 1,400 ancient and modern human skulls. The majority of these were 20th century skulls from 30 different ethnic populations, but the researchers also had access to 13 skulls that were more than 80,000 years old, and 41 skulls from 10,000-38,000 years ago.
The analysis showed that changes in the skulls' measurements correlated with the technological advance of humans. During this period, the heavy brows and long faces of the early humans were replaced by the softer, more rounded and more "feminine" features of the modern human.
The researchers say this is evidence that the more feminine-faced humans had lower levels of testosterone than their predecessors - possibly from having fewer receptors for this hormone.
From this, the team suggests that lowered testosterone levels may have led to humans becoming more sociable and co-operative, and less aggressive and competitive. These traits would have promoted teamwork and made technological and societal advances possible for the first time.
Testosterone levels influence skull shape in animals, too
To illustrate how this works in evolutionary terms, the team points to a famous study of Siberian foxes, which showed that the animals evolved a more juvenile appearance and docile behavior after several generations of selectively breeding foxes that were receptive to humans.
"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," says Duke animal cognition researcher Brian Hare.
Hare explains how this hormone-driven divergence has also occurred in apes, specifically chimpanzees and bonobos. While male chimpanzees experience a strong rise in testosterone during puberty, male bonobos do not, which leads to very different behaviors in the two species - aggression in chimpanzees and social tolerance in bonobos.
The hormone disparities also influence the facial features of the apes - with chimpanzees exhibiting a strong brow. "It's very hard to find a brow-ridge in a bonobo," Hare says.
Lead author Robert Cieri, a biology graduate student at the University of Utah who began this work as a senior at Duke University, concludes:
"The modern human behaviors of technological innovation, making art and rapid cultural exchange probably came at the same time that we developed a more co-operative temperament. If prehistoric people began living closer together and passing down new technologies, they'd have to be tolerant of each other. The key to our success is the ability to co-operate and get along and learn from one another."
Recently, Medical News Today reported on the analysis of a 100,000-year-old human skull that surprised experts by exhibiting traits thought to occur only in the skulls of Neanderthals.