There are three types of hair on humans, based on hair follicle size: lanugo (the hair that new born babies often have), vellus (the short, fine, un-pigmented hair on cheeks, necks, andsimilar places), and terminal (the thicker, pigmented hair on the head, eyebrows, eyelashes, whiskers, limbs,and genitals).

We evolved to have (or keep) hair on our eyelids because it helps keep sweat and foreign objects out of our eyes, and reduces the risk of eye infections.

For early humans to have evolved this feature, it must have given them a productive advantage. Eyelash hairs come from the same type of follicles as the hair on your head, and the hair shaft itself is very similar.

Eyelashes are different than head hair, though, in that they grow very slowly--about a tenth of a millimeter per day. The reason why their length appears to be constant (about 7.5 millimeters) is that the ends break off with the wear and tear of daily life.