We, humans, are something special. We are mammals. But we are among the mammals basically the only ones who have eyebrows. The reason for that is quite simple. We don’t wear fur. In the meantime, we even have hardly any hair left on our bodies. Therefore, of course, we need something against sweat, wetness, dirt, and other foreign bodies. Eyebrows prevent these affecting things from coming into our eyes. Therefore, they are a support of the function for the eyelashes.
But there are even more than only these things. Brows are a good body part to make emotions with and they can protect the eyes from sunlight.
Sunlight?! Yes, brows work like small own awning screening sunlight coming from the top. Avoiding letting sun coming into your eyes because they grow above the connection of the eye sockets.
Especially during sport, you sweat a lot. Your eyebrows slow down the salty sweat coming out of your sweat glands e.g. on your forehead. Otherwise, sweat would just run into your eyes which would course an annoying and painful burn. Instead of letting the sweat come into the eyes, the eyebrows stop it like a thumb stem before it enters a body of water.
Besides this practical benefit, eyebrows also have an emotional effect.
They are important for non-verbal communication. Without eyebrows, we would find it much harder to communicate with other people. Their position and movement are part of a person’s facial expressions. Just by seeing someone from a far distance, the facial expression of the person would be still clearly readable. Especially when you are very stressed, simply annoyed, or mulling over a maths exercise, your eyebrows go automatically down in the direction of the eyes. At the same time, you signal your current emotional condition and expressive posture to the other person in your environment. Thereby you express your emotional state and feel like other gestures similar to hands or lips. The other person can read emotions from the face with the help of the eyebrows. This makes it easier to convey statements. You can also express your feelings non-verbally in this way.
But when you are very surprised about something perticular your eyebrows move intuitively. You make clear to the interlocutor your emotion as a counter-reaction. This enables the person to better understand and interpret your signal.
-by N.Horn-
Sources:
https://en.wikipedia.org/wiki/Eyebrow
https://www.geo.de/geolino/18904-rtkl-mensch-warum-wachsen-uns-augenbrauen