April 9 (UPI) -- Why did early humans ditch the big furrowed brows of our ancestors in favor of smaller foreheads and more mobile eyebrows?
New research -- published this week in the journal Nature Ecology & Evolution -- suggests smaller, more mobile brows enabled the communication that made the formation of large social networks possible.
"Looking at other animals can offer interesting clues as to what the function of a prominent brow ridge may have been," Paul O'Higgins, a professor of anatomy at the University of York, said in a news release. "In mandrills, dominant males have brightly colored swellings on either side of their muzzles to display their status."
Researchers hypothesized that the large brows of early humans also signaled dominance.
"Sexually dimorphic display and social signalling is a convincing explanation for the jutting brows of our ancestors," O'Higgins said. "Their conversion to a more vertical brow in modern humans allowed for the display of friendlier emotions which helped form social bonds between individuals."
The explanation isn't exactly new, but some researchers have previously suggested the large brow of early humans was an anatomical necessity, needed to stabilize the skull for chewing and fill the gap between the brain case and eye sockets.
To test this argument, O'Higgins and his colleagues used 3D imaging technology and computer modeling to study the skull of Kabwe 1, a specimen of archaic hominin belonging to the species Homo heidelbergensis. Kabwe 1 lived between 600,000 and 200,000 years ago.
Researchers used their computer model to chip away Kabwe 1's large brow and found its reduction in size did not undermine the structural integrity of the skull.
"Since the shape of the brow ridge is not driven by spatial and mechanical requirements alone, and other explanations for brow ridges such as keeping sweat or hair out of eyes have already been discounted, we suggest a plausible contributing explanation can be found in social communication," lead researcher Ricardo Godinho said.
Scientists believe early humans' developed smaller foreheads as they abandoned the hunter-gatherer existence for life on the farm. The study's authors argue the shift aided the development of human communication.
It was our ability to communicate and form larger social networks that helped Homo sapiens colonize the globe and outlast all other hominins.
"Tiny movements of the eyebrows are also a key component to identifying trustworthiness and deception," said Penny Spikins, an archaeologist at York. "On the flip side it has been shown that people who have had botox which limits eyebrow movement are less able to empathize and identify with the emotions of others."
"Eyebrows are the missing part of the puzzle of how modern humans managed to get on so much better with each other than other now-extinct hominins," Spikins said.