Eyebrows May Have Played an Important Role in Human Evolution, Researchers Say

If you hadn't noticed, we humans have highly expressive eyebrows which play a major role in the non-verbal communication of our feelings and thoughts.

This feature may have had significant implications for the evolution of our species, according to researchers from the University of York, England.

The ancestors of modern humans once had a very pronounced ridge on their brow, as well as relatively inflexible, inconspicuous eyebrows. However, over time, our foreheads gradually became smoother and the eyebrows more visible and versatile. This change may have affected how human social groups functioned (although the York team stress that no definitive conclusions can be drawn).

"Clearly during our evolution, we developed more complex and larger social groups with many interdependencies," senior author of the paper. Paul O'Higgins, a professor of anatomy at York, told Newsweek.

"Eyebrows play a role in communicating thoughts and feelings and so are important in allowing us to 'read each other's minds' to communicate. This ability to 'mind read' may well have been very important in building large complex social groups that are mutually supportive—a key feature of human evolution."

In their study, published in the journal Nature Ecology & Evolution, the researchers tried to answer the question of why an iconic skull fossil, known as Kabwe 1, had such large brow ridges. The skull belongs to a species of archaic hominin known as Homo heidelbergensis, which lived between 600,000 and 200,000 years ago.

Using 3D modelling software, the researchers tested two theories commonly put forward to explain the biological development of the large brow ridges. The first is that they are needed to fill the gap between a large face with eye sockets that sit in front of the forehead, and the forehead itself; while the second is that they stiffen the bones of the face to stabilize the skull from the force of chewing.

"Virtual modelling allowed us to show both are false—leaving us with several alternative explanations," said O'Higgins. "Most of these do not make sense—to keep sweat or hair out of our eyes, to act as sunshades etc."

Model of a modern human skull next to Kabwe 1. Paul O'Higgins, University of York

So, if the brow ridges were not serving a mechanical function, social factors may have dictated their shape, the researchers speculate. The large ridges were a sign of dominance and aggression in Homo heidelbergensis, which was once a desirable trait. But as evolution progressed, humans began to value the need to empathize and communicate over the desire to be aggressive and dominate, which could have potentially led to flatter foreheads.

This hypothesis is currently untested and, furthermore, there are other explanations for Homo heidelbergensis' huge brow ridge. For example, it could simply be related to higher levels of testosterone, Ashley Hammond, a paleoanthropologist at George Washington University, told Vox.

"The skeleton as a whole is a lot more robust and thicker," she said. "The thick brow ridge isn't necessarily designed for social signaling. It could just be a consequence of other differences in body chemistry."

In addition, more expressive eyberows may not have been the only factor driving more effective communication between people. "The human face has many more muscles which are are likely to have contributed substantially to the development of socially sophisticated communication, group cohesiveness and functioning, and therefore survival and progress," Rachael Jack, an expert in human social interaction at the University of Glasgow, told the Guardian. "So I'm not totally convinced that eyebrows are the missing part of the puzzle."