Humans lost most of the thick hair our ape cousins still carry. Yet we kept two neat little strips right above our eyes. They don't keep us warm, and many of us spend time plucking, shaping, or penciling them in. So what are eyebrows actually for?
Eyebrows have two main jobs. First, they help stop sweat, rain, and debris from running down into our eyes. Second, and perhaps more importantly, they help us communicate. Recent research suggests that communication through facial expression is their primary purpose.
Job One: A Built-In Rain Gutter
The older explanation is the practical one. Eyebrows follow the curve of the bony ridge above the eyes, and their hairs help steer moisture, mostly sweat and rain, away from the eyes. Anyone who has mowed a lawn on a hot July afternoon can appreciate that.
There's also a more unusual theory. Some have suggested that visible eyebrows, together with eyelashes, formed "eyespots" that could have helped scare off predators when early human ancestors began sleeping on the ground. It's an intriguing idea, but it's only one proposal among several.
Job Two: Talking Without Words
The newer view is that eyebrows are mainly a communication tool. Over time, humans developed a smooth forehead with visible, hairy eyebrows that can move in many ways. Those movements can express a wide range of subtle emotions, including recognition and sympathy.
Think about how much a single eyebrow can say. A raised brow can mean surprise or doubt. Lowered brows can signal anger or concentration. Pinched together in the middle, they can show worry. Your face is sending these messages all day long, often without you noticing.
The Secret Hello
One of the most charming eyebrow signals is called the eyebrow flash. It's a quick raise of both eyebrows lasting about a fifth of a second. It's usually unconscious, and it tells someone you recognize them and are ready to say hello.
People generally return the flash, unless they don't recognize the person or the sender looks away right after. A 1987 study recorded about 67 hours of everyday social interactions in three very different places, two on the island of New Guinea and one in the Upper Orinoco region of South America. In all three, people used longer eyebrow flashes when opening a conversation than in the middle of one.
How Our Brows Changed Over Time
Our faces used to look quite different. Pronounced brow ridges were common among Stone Age humans, and our closest living relatives, gorillas and chimpanzees, still have very prominent ones. In modern humans, the ridge is much smaller.
The fossil record shows that as the skull grew, the front of the brain moved up above the eyes rather than behind them. That gave humans a more upright forehead, the perfect stage for expressive eyebrows.
Brows in and out of Fashion
People have fussed over eyebrows for centuries. Starting in the 8th century, some Japanese men and women practiced hikimayu, removing their natural brows and painting new ones higher on the forehead. In Elizabethan times, high-status women sometimes removed their eyebrows altogether, and super-thin plucked brows came back into style in the 1920s and 1930s.
So whether yours are bushy, thin, or somewhere in between, give them a little credit. They keep the sweat out of your eyes and quietly tell the world how you feel. Not bad for two small patches of hair.
