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
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.
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