system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why have humans evolved so far as to lose their thick coats but still grow hair on their heads and faces? What purpose does it have?

Just confused as to why we all still grow hair if it doesn't really do anything.
gpt
In short because as humans evolved from arboreal dwelling primates to plains dwelling proto humans it was more important to keep cool, and since we sweat to keep cool those who were less hairy tended to cool off better and hence survive better to pass on the less hairy genes. Hair on the top of the head remained because it was likely an effective sun block, and pubic hair probably had some part in sexual attraction or scent retention. Humans in this environment had dark skin pigmentation that evolved as a natural sun block as hair was lost. Once humans began expanding to cooler climates they had learned to use animal skins, and hairy individuals didn't survive any better than less hairy ones, so humans who moved to cold climates didn't have to be hairy to pass on their genes.

Edit: Also around 10,000 years ago, give or take, humans in colder climates who's skin had less pigmentation were more able to make use the decreased sunlight, and eventually the lighter skin became the predominate skin tone in these climates.