|MadSci Network: Evolution|
This has been looked into before on MadSci Network. http://www.madsci.org/posts/archives/jan99/915647053.Ev.r.html As the answers says, there is much mystery in the origin of humans. I was quite compelled by the concept that a mass of hair was not necessarily needed in Africa, where man was said to have started. Other than that, I can only suggest web searches of journals and the like. In summary, I would have imagined that if hair was useful, we would not have lost it as that would have been a genetic disadvantage. Having said that, if the reason we lost the hair was due to some other, more advantageous genetic mutation, then hair just lost the race, and the other factor won.
Try the links in the MadSci Library for more information on Evolution.