Quirks & Quarks

Why humans ditched the mono brow — for two eyebrows

At some point in our evolution, we traded a large brow ridge for more mobile eye brows.
Scientists think our eyebrows believe are simply serving a purpose to help us communicate. (Kapa65 / Pixabay)

Why big brow ridges

When you compare our hominin ancestors to modern humans today, they had thick eyebrow ridges compared to our relatively flat foreheads today. In us, that's given rise to our highly mobile eyebrows, which can move around a lot with our facial expressions. Until this paper came out from Dr. Paul O'Higgins from the University of York in England, it was thought that our ancestors had big brows for facial structural reasons. 

"One suggestion is that they're there to act like a great big beam, stabilizing the face for when bites are being made," explains Dr. O'Higgins. "Another suggestion is that simply because the face is so big, it sticks out in front of  where your face would be, so there's a gap between the forehead and the orbits that contain the eyes. And this is filled by the brow ridges. And there have been other suggestions. Social expression has been suggested in the past, as have things like sun shades, keeping sweat out of your face, and keeping your hair away from your eyes, as well."

Years ago, an American anthropologist walked around for a year wearing artificial brows on his forehead and he found it didn't really help much with the sweat or the hair problem. 

How scientists tested those theories

Dr. O'Higgins says they tested the two hypotheses that seemed most probable. "The spatial one, the filling in between the face and the brow ridge and the mechanical one." To look at this question, he studied the Kabwe 1 fossil, which he says is supposed to be representative of our last hominin ancestor. It has enormous eyebrows and a very large face with a long, sloping forehead. They did CT imaging, 3D virtual reconstruction, and virtual surgery where they shaved the brows away and found there was far more brow there than is required to fill the space. 

"So then we thought, maybe that's about dealing with the mechanics. And we shaved the brow away to an absolute minimum and loaded the face, simulating its biting using engineering tools and found that no matter whether the brow was larger or small, it made no difference to the face. And that left us really with the last, sort of, serious hypothesis about this that somehow these big brow ridges are involved in expressing things to other members of the group. Particularly, we think dominance and aggression would be things these brows would be good at expressing." 

Our highly mobile brows

"As our face became smaller and came to sit underneath our forehead, then there was basically the development of a vertical canvas of our forehead. And the muscles that, up to this point, had to pull the eyebrows backwards over a sloping skull now could pull the eyebrows vertically." 

Dr. O'Higgins says, "What we suggest is this development of the vertical canvas of the forehead allowed the eyebrows to move in more subtle ways and produce a more subtle pallet of expressions for communicating our feelings to other members of our group. And that's very important for us as humans because we form long, complex social groups. And we need to have complex social interactions."