The Evolution of the Eyebrow

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Eyebrows have become an obsession of late, tattooed or microbladed, shaped and drawn in bold dark lines, making a statement far beyond braiding or waxing.

Lifting one and not the other often signals disbelief, amusement, curiosity. Raising both can suggest surprise or dismay. But it wasn’t always that way.

Early humans had thick, bony brow ridges that were far less nimble than ours, incapable of expressing much of anything beyond, “Don’t mess with me, Thag.”

CreditPaul O'Higgins/Agence France-Presse — Getty Images

Scientists have long thought those brows served some structural purpose, like support for chewing prehistoric food. That they could also be used to signal aggression or intimidate competitors was largely dismissed as an evolutionary perk, as were the more flamboyant brows of modern humans.

But when Ricardo Miguel Godinho, an evolutionary anthropologist at the University of York, made digital recreations of a skull believed to be 300,000 to 125,000 years old, he found no evidence that its brow ridges provided any of the practical benefits suggested by earlier studies. “He tested out the different possible explanations, and, effectively, there’s no reason for it,” said Penny Spikins, an anthropologist who conducted the study with Dr. Godinho.

The findings, published April 9 in Nature Ecology & Evolution, suggest that the human brow has always been a primarily social tool, and that the smoother foreheads and expressive brows of modern humans may have evolved to accommodate our increasingly complex relationships.

“With a flatter, more vertical forehead, that whole area above the eyes becomes much more mobile, and the muscles can make some really subtle communicative gestures,” Dr. Spikins said. And those gestures, like lifting your eyebrows to show you recognize someone, she said, “tend to be more about expressing friendliness than intimidation.”

Though such a hypothesis is difficult to test without a time machine, Dr. Spikins said it emerged from the real-life observations of Paul O’Higgins, a co-author of the study. “Paul was frustrated that his daughters spent so much time in the bathroom mirror perfecting their eyebrows, and was saying, ‘What are eyebrows for?’” she recalled. “That’s when we thought maybe this is actually quite important.”

A version of this article appears in print on , Section D, Page 2 of the New York edition with the headline: Eyebrow Raisers: Brow Ridges Evolved To Aid Expression, Not Eating Mammoths, Anthropologist Finds. Order Reprints | Today’s Paper | Subscribe