I think I just figured out why human beings lost basically all of their hair (versus primates), and the answer is, we stopped selecting for it. That alone shouldn’t matter, but if you add in a hypothesis that more or less constant mutation happens, on some level, then traits that are not actively selected for, will eventually vanish. This is basically an entropy of genetics, that would require constant effort, or environmental pressure, to maintain the traits of a species. In the case of body hair for humans, we stopped selecting for it because we developed the ability to use animal pelts, and as a consequence, both the environment, and possibly the individuals in question, stopped selecting for body hair, and presumably started selecting for other things.

Given that people still have hair on their heads, and to some extent on their bodies, it must have some utility, even if it’s just aesthetic, though this doesn’t undermine the more general thesis, that traits simply vanish, if not selected for, which is superficially impossible to argue with, for the simple reason that mutation is real, and as a result, all traits will be subject to what is basically erosion. If that erosion is significant, the trait in question could dwindle and vanish.