Using our eyebrows, we can speak volumes without saying a single word. Raised brows signal shock; a single arched brow indicates skepticism; furrowed brows can let others know that we are angry, confused or lost in concentration. But our ancient relatives did not have expressive eyebrows that could move across smooth, domed foreheads. Their foreheads were sloped, with thick, protruding brow ridges.

As Charles Choi reports for Discover, a team of researchers at the University of York in England recently set out to learn more about why ancient humans had these distinctive brow ridges, and why they eventually lost them. In a study published in the journal Nature Ecology & Evolution, they suggest that oversized brow ridges had a social function, and as they shrunk in size, humans were able to better communicate subtle emotions.

Over the years, scientists have put forth a number of theories about why humans of millennia past had a protruding brow ridge. Most of these theories focused on structural and mechanical explanations: a thick brow bone may have protected ancient humans from blows to the head, shielded the eyes from water, or even prevented our ancestors’ hair from obscuring their vision. But the University of York team wanted to test two other hypotheses. The first posits that large brow bones protected the skull when our ancestors were chomping down on tough meals. The other suggests that brow ridges protruded to fill the gap between the forehead and the eye sockets, since early humans’ faces “were so enormous, they didn’t fit under the brain,” physical anthropologist and study co-author Paul O’Higgins tells Choi.

To test these ideas, researchers created a 3-D model of an ancient human skull from X-ray analysis of a fossilized skull known as Kabwe 1, which is currently housed in the Smithsonian. The skull came from an individual of the Homo heidelbergensis species, which lived between 700,000 and 200,000 years ago and may be a common ancestor of Neanderthals and modern humans.

The researchers then played around with the computer model, shrinking the size of the brow ridge to see if it would affect the mechanical stresses of biting. But they found that a smaller ridge did not reduce stress on the skull. The team also discovered that Kabwe 1’s brow was larger than necessary to fill in the space between the forehead and eye sockets. Maybe, the researchers thought, Kabwe’s pronounced brow ridge did not serve a structural or mechanical function. Maybe the purpose of the large brow was social.

The team thought back to the research of the rather eccentric anthropologist Grover Krantz, who once made a replica of a Homo erectus brow ridge and walked around wearing it, in the hopes of discovering its advantages. Perhaps unsurprisingly, he found that people crossed the street to avoid him.

“That got us thinking, maybe that why it's there in Kabwe—to give a signal of dominance,” O’Higgins tells Mary Beth Griggs of Popular Science.

Over time, however, human faces started to shrink—possibly due to advances in cooking methods or changes in levels of exercise, Choi of Discover explains. And as their faces got smaller, our ancestors became more social; groups of Neanderthals and other ancient humans seem to have inbred frequently, but among modern humans, there was much more contact between different groups. Perhaps, the researchers suggest, anatomically modern humans were better able to communicate and collaborate with one another—with the help of their eyebrows.

“Our mobile hairy eyebrows are crucial in subtle signaling behaviors,” the authors explain. “Mobile eyebrows without the constraints of a pronounced brow bridge allow subtle affirmative emotions to be expressed.”

Not all experts are convinced by the team’s theory. Ashley Hammond, a paleoanthropologist at the George Washington University, tells Brian Resnick of Vox that Homo heidelbergensis’ thick brow bones may have been caused by higher levels of testosterone; the species’ entire skeletons were, in fact, thicker than those of modern humans. But the new research adds an interesting layer to a body of evidence suggesting that communication and co-operation were crucial to the survival of our species.