Ever wonder why humans have eyebrows? Surely, it goes beyond aesthetics. Nature did not provide it to man so he could make himself busy during idle moments.
Scientists say that eyebrow hairs help keep the moisture away from our eyes when we sweat or walk around in the rain. The hairs prevent the sweat and rain from falling to our eyes, keeping it relatively dry and the vision clear. The same reason prevents eye irritation due to the salt content of sweat.
That may seem minor to modern man, but imagine humans who lived on Earth thousands of years ago. The Stone Age Man had to run from predators or run to chase animals for food, which obviously cause them to sweat.
With prominent forehead and thick eyebrow hair, the sweat is diverted away from the eyes. Otherwise, man would have been chased by its predators or would not have caught its prey.
Over the course of evolution, the forehead became less prominent, and eyebrow hair had varied amount of thickness.
It is of great importance to take care of the eyebrows. I can't imagine the modern man having no eyebrow hair but with very thick eyelashes.