If humans evolved in hot, sunny climates away from the shady protection of trees then 'losing' our hair makes a lot of sense. Humans have an incredible ability to lose body heat through sweating which is only possible due to our lack of hair. This means that we can 'persistance hunt'. Persistance hunting is a method still used by a few african tribesmen today. The principle is very simple: you find an animal and then chase it continuously until it colapses of heat exhaustion. Humans can keep longer than other animals because they have no hair.
You could probably get the hair back. I'm not really familiar with the genetic basis for hair but I doubt it would take more than a few crucial control genes to make your normal mammalian hair thicker and darker.
Since there are people born with a mutation that makes them hairy-like-ape, this would seem likely.