Empathy, concern, confusion, disapproval... There's a whole range of emotion we can communicate with a quick movement of the brow. sanneberg/Shutterstock

It has been said that a Neanderthal could ride the New York subway without kicking up much of a fuss – provided he or she was wearing a hat to hide their distinctively large brow ridge, that is. Compare modern humans to our ancient ancestors and one of the most noticeable differences is our lack of a heavy brow ridge. Instead, we have evolved to have smoother foreheads, finer facial features, and eyebrows. But why is this?

The argument put forward in a paper recently published in Nature Ecology and Evolution claims that highly mobile eyebrows help modern humans communicate with one other non-verbally, putting us at an evolutionary advantage against other human species. In contrast, the jutting brow of our ancient cousins, like the Neanderthals, was a "sexually dimorphic display" similar to the antlers on a stag.

The researchers came to this conclusion using a Homo heidelbergensis skull, Neanderthal mandible, and 3D modeling software similar to programs used by engineers to design buildings and bridges. Using virtual scans, the team were able to adjust the shape of the skull to test existing theories on why ancient species of human had such heavy brows. It was not to support the structure of skull or to improve the mechanics of biting, as previously thought. They, therefore, suspect the large brow ridge served a very particular social function – to display dominance.

"Sexually dimorphic display and social signaling is a convincing explanation for the jutting brows of our ancestors," Paul O'Higgins, Professor of Anatomy at the University of York and senior author, explained in a statement.

"Their conversion to a more vertical brow in modern humans allowed for the display of friendlier emotions, which helped form social bonds between individuals".

Roughly 100,000 years ago, social networks were starting to expand, human groups were beginning to diversify, and it made evolutionary sense to prioritize friendliness over aggression. It was also around this time modern humans developed the smaller, flatter brow ridge and mobile eyebrows we recognize today.

The brow ridge of the Homo heidelbergensis skull (right) is noticeably larger than the brow ridge of modern humans (left). Paul O'Higgins/University of York

"Eyebrow movements allow us to express complex emotions as well as perceive the emotions of others," co-author Penny Spikins from the Department of Archaeology at the University of York added.

"A rapid "eyebrow flash" is a cross-cultural sign of recognition and openness to social interaction and pulling our eyebrows up at the middle is an expression of sympathy. Tiny movements of the eyebrows are also a key component to identifying trustworthiness and deception. 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."

The study is not conclusive but does add to the idea that modern humans voluntarily self-domesticized to get on with the people around us.

Where would Dwayne "The Rock" Johnson's acting career be without strong eyebrow game? via GIPHY