What purpose do eyebrows have?
The main function of the eyebrow is to prevent moisture, mostly salty sweat and rain, from flowing into the eye, an organ critical to sight. The typical curved shape of the eyebrow (with a slant on the side) and the direction in which eyebrow hairs are pointed, make sure that moisture has a tendency to flow sideways around the eyes, along the side of the head and along the nose. The slightly protruding brow ridges of modern humans could also still play a supporting role in this process. Together with the eyebrows, the brow ridges also shade the eyes from sunlight. Eyebrows also prevent debris such as dandruff and other small objects from falling into the eyes, as well as providing a more sensitive sense for detecting objects being near the eye, like small insects. Eyebrows also have an important facilitative function in communication, strengthening expressions like surprise or anger.
