Researchers believed a technology boom 50,000 years ago is linked with a reduction of testosterone that occurred in humans around the same time.
The researchers observed a change in the human skull that occurred around the same time we started making tools, Duke University reported. The study was published on Aug. 1 in the journal Current Anthropology.
"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.
To make their findings researchers measured over 1,400 ancient and modern skulls. The team found the reduction in testosterone led to rounder heads and a smaller brow ridge. There was also a shortening in the upper region of the face, which generally indicates a reduction in levels of testosterone.
The team cannot determine if the changes occurred because there was less testosterone in circulation or if there were fewer hormone receptors available to receive it, but this phenomenon has been observed in non-human species.
"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 cognition researcher Brian Hare.
Hare also studies the differences between chimpanzees and bonobos, which are both close human relatives. These two types of apes developed differently and respond to stress in different ways. Chimpanzee males experience a rise in testosterone in puberty while bonobos do not. When stressed bonobos do not produce more testosterone like chimps, but they do produce more of the stress hormone cortisol. The apes also have physical differences.
"It's very hard to find a brow-ridge in a bonobo," Hare said.
