This question already has an answer here:
Why do hair continuously grow on human heads, while the same doesn't happen on other parts of the human body? Where there are hair, they only usually grow to a fixed length; but they seem to have no limit (or at least a much higher limit) as long as the head is concerned (and the face, in men).
Why does this only happen on humans? When did it start happening? What kind of evolutionary advantage could there be in overly-long head hair, so that humans have them while other primates and mammals don't?
I know there are various questions around about why humans have much less body hair than other mammals; mine is related but different: why are we the only species with so much hair on our head, to the point that they might even become inconvenient if we don't cut them?