I’m placing this in IMHO because there is no factual answer, and it seems too mild for Great Debates. Please re-home as needed.
In the above linked column, Cecil points out that
Bolding mine. There are two mammalian traits which successfully address survival in cold weather: Hairy coats, and blubber. Whales, walruses, dolphins, seals etc all have in common not only an aquatic environment, but a very, very cold one as well.
Given the human tendency toward overweight, it seems to me that we are more than equipped to handle cold weather in this manner. While it would be terribly uncomfortable for us, it’s also known that people who grow up in cold environments are fairly adaptable and can expose their skin in much colder termps.
Combine that with the natural tendency that many humans carry to slow down considerably in the Winter "Seasonal affective Disorder" it seems that the combination of shelter and fat would be more than enough to get a population through the harshest Winter.
I think it’s the cooling hypothesis. Human sweating gives such an athletic advantage in hot weather that we can walk large game to death. A human can go further than a horse, in the heat. We aren’t as fragile as we fear, and we are awfully good at walking.
The human tendency toward obesity has really only existed in the last forty years, and it’s due to an environment filled with cheap, low nutrition calories. Prior to that, getting enough to eat was a daily struggle, and people were far more likely to be underweight than over.
Homo sapiens populations have regional adaptations to environment that are at least some help. Inuit tend to be shorter, have shorter limbs, thicker trunks, and flatter noses - all of which help retain body heat. They’re also low in melanin in order to absorb enough sunlight to manufacture vitamin D. The Maasai are extremely tall and slender with long limbs, and they have boatloads of melanin, to protect them for the much stronger sunlight at the equator. Neither of them, however, have much in the way of body fat.
My guess is that one or a few genetic mutations, or possibly even just particular genes being de-activated, caused a scattering of individuals early in the hominin line to start losing the body hair. From there, being hairless was either a neutral trait or a positive one. Maybe being hairless came with better sweat glands. Maybe hairless females were able to show off secondary sexual characteristics more obviously. Maybe there was a lower parasite load on hairless individuals. Maybe around the same time, the blush reflex developed, giving the people around those blushing, hairless individuals the ability to read their emotional state more clearly, and as a result, the hairless blushers were considered more trustworthy, leading to more progeny.
It doesn’t take much for a trait to spread through a population. It only has to be harmless, even if it’s weird (imagine how freaky the first blue-eyed kid must have been). If there’s any benefit at all, it’ll spread that much faster, and even a small shift in climate or environment could push that to become a universal trait, even if it isn’t obviously a winning trait.