Human ancestors, which include the Neanderthals and the Denisovans, are characterized by big furrowed brows but early humans evolved to have smaller foreheads and highly mobile eyebrows.
Smaller And More Mobile Eyebrows
Communicative foreheads were actually a side effect of the face getting smaller over the past 100,000 years. The process became particularly faster in the last 20,000 years and more recently when early humans abandoned their hunter-gatherer way of living for life on the farm.
Findings of a new study, which was published in Nature Ecology & Evolution, suggest that it happened because smaller and more mobile brows made possible the kind of communication that paved the way to the formation of large social networks.
Highly mobile eyebrows can express a wide range of subtle emotions particularly sympathy and recognition, which foster better understanding and cooperation among people.
Study researcher Paul O'Higgins, from the University of York, and colleagues used 3D engineering software to study the brow ridge of Kabwe 1, a fossilized skull of an individual from a species of archaic hominin.
The analysis discounted two popular theories about the protruding bridges, namely one that points at filling the space between the eye sockets and flat brain cases of hominins and another that suggests the ridge stabilized hominin skulls from the force of chewing.
"We simulated the forces of biting on different teeth and found that very little strain was placed on the brow ridge," the researchers said. "When we took the ridge away there was no effect on the rest of the face when biting."
Role In Social Communication
Because the shape of the brow ridge does not appear associated with mechanical and spatial advantage, and other common explanations have already been ruled out, the researchers believe this has something to do with social communication.
"We propose that conversion of the large brow ridges of our immediate ancestors to a more vertical frontal bone in modern humans allowed highly mobile eyebrows to display subtle affiliative emotions," the researchers said.
Subtle as they may seem, the small movements of eyebrows play an important role in conveying emotion and empathy. People who had botox have limited brow movements and they are less capable of empathy and identifying with other people's emotions.
Helped With Human Survival
The researchers also suggest the mobile eyebrows may have also played an important role in human survival. The unique ability of the Homo sapiens to communicate and form larger social networks allowed the species to outlast other hominins.