From the New York Times - Everything you always wanted to know about the evolution of human eyebrows.

An interesting fact: My eyebrows are mostly gray these days.

New York Times - The Evolution of the Eyebrow

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.”

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 new study suggests humans evolved more mobile, expressive eyebrows to accommodate our increasingly complex relationships.

University of York researchers used 3-D engineering software to look at the brow ridge of a fossilized skull of an archaic hominin, Homo heidelbergensis, who lived between 600,000 and 200,000 years ago.