As we humans evolved, most of the hair on our body became sparse, or even completely disappeared. The reason we chose to discard our hairy layers is of some contention among scientists, but the reason we retained it in specific places was for very good reason – protection.
The hair on your head protects the scalp, and keeps dust and dirt from falling into your face (check out the fun fact at the bottom). Your eyelashes protect your eyeballs from dirt and other irritants (useful when out hunting on the savanna), while your nose and ear hair also have similar protective functions. But what about our eyebrows? Do they directly protect something?
The consensus seems to be that eyebrows keep moisture, like sweat and rain, from running down a person’s forehead straight into the eyes. This explanation is easy to visualize when you look at the morphology and facial features involved — most notably, the pronounced slant of the eyebrow hairs which directs water away from the eyes. This has obvious evolutionary survival advantages.
Sweat tends to be very salty, and salty liquids getting into your eyes while you’re running in an African Savannah would be a severe hindrance to hunting. It could also be a potentially deadly problem when the tables are turned and you are fleeing hungry predators. Eyebrows give those potentially deadly distractions a detour to the side of the face, instead of right into your eyes.
But eyebrows don’t just protect against moisture, they also protect your eyes against small pieces of dust, dandruff or grit that might be caught in your hair.
Today’s eyebrows generally play more of a cosmetic role. Being a distinctive feature of a persons face, men and women both groom them to enhance their overall appearance.
Fun Fact: The old saying that “you lose most of your body heat through your head” is actually completely false. If it were true, putting on a hat would be as effective at keeping you warm as putting on a pair of thick jeans or a coat. This simply isn’t the case.
The origin of this myth has roots in a vague US military study which took place in the 50s. The government was researching heat loss of soldiers in the arctic. IR cameras showed that, while bundled up in gear, most of the heat loss was from their heads. It’s only natural that this be the case due to their heads being left complete uncovered and unprotected – it was the only place heat could escape from.