First, they protect your eyes. The shape of the brow ridge and the outward-growing hairs of the brows themselves channel sweat, rain, and moisture away from the eyeballs, so your vision stays clear. They can also catch dust or shield eyes from sun glare.
Second, they’re essential for nonverbal communication. Scientists who study facial expressions say eyebrows are important for expressing happiness, surprise, and anger, a function that our human ancestors may have employed as well.
In 2018, researchers tried to decipher why the brow ridges of Homo neanderthal and Homo erectus were much more prominent that ours, and hypothesized that it had to do with accommodating the force of their bite. But when they manipulated a 3D computer model of an ancient hominin, bite pressure didn‘t justify the big brows. The researchers surmised that social communication was the more likely reason.
Over millennia, our brows have evolved to be smaller, more malleable, and useful for nonverbal communication. For example, today‘s speakers of sign language contort their eyebrows to complement hand signs.
Third, eyebrows act as an ID card. Eyebrows stand out against the forehead, can be clearly seen from a distance, and don’t change very much over time—making them perfect for identifying people. In a 2003 study at MIT [PDF], people were shown a picture of Richard Nixon with his eyes Photoshopped out, and then a picture with his eyebrows erased. The participants had significantly more trouble identifying Nixon and other celebrities when the brow was bald.
The takeaway? If you’re going undercover, forget the sunglasses. Shave your eyebrows instead.
A version of this story ran in 2014; it has been updated for 2023.