Why humans have hair

The actual reason is still a matter of debate, but there are several compelling clues.

One reason is that hair would have protected, like a helmet, our ancestors’ heads from the sun’s rays, to which humans’ scalps are particularly vulnerable, because of our bipedal gait.

The rest of the body, on the other hand, could dissipate heat through sweating and enable endurance hunting, so it didn’t need hair.

The other reason lies in neoteny. That is, a process where typical juvenile traits are retained even in adulthood.

Our species has so many neotenic traits: large eyes and head in proportion to the body, small face and snout, mental plasticity, (all typical characteristics of cubs) and finally, hair:

In very young monkeys, the body is almost completely hairless with just a tuft of hair on the head and on a few other spots, in a way strikingly similar to humans.

A lot of human appearance and intelligence depends, in fact, on the preservation of neotenic traits during adulthood.