Hair is therefore intimately connected with the nervous system and to the brain. Understand that while hair is an appendage of the skin, its ultimate purpose is as an agent of the nervous system whose function is to gather sensory information. It is a way of extending our awareness beyond the confines of our body. The same is true for animals. Hair is what tells their brains what is going on around them. It also helps keep them warm, makes them pretty, and a bunch of other stuff. But that is just icing on the cake. The primary purpose of hair is as a sensory organ.

So why did evolution favor humans losing their body hair? As humans were evolving and getting bigger brains, the increased resources that were needed to develop more brain neurons had to come from somewhere. The resources came from hair. It was the only part of the body that had extensive resources to draw from.

Every step of evolutionary advancement that our brain made has been accompanied by a loss of body hair. The more we dedicate our neurological resources to the brain, the less we have to dedicate to body hair. The smarter we get, the less body hair we have.

Some types of body hair have not gone away, and it is because they provide some sort of survival advantage. Pubic hair, for example, was not only retained throughout evolution, it actually increased. While the nervous system was taking energy away from body hair and feeding it to the brain, it was adding new resources to pubic hair. You think monkeys have hairy genitals like we do? Nope.

The reason? Once we started walking upright our potential mates couldn’t just come walking up to us and sniff our crotch, like a monkey can do. We had to find a way to trap our pheromone odor in a way that would make it available to those around us while we stood upright. Since pheromones are so important in sexual selection, evolution favored the retention of hair in our smelly zones. Consequently, humans have developed a hair pattern that is quite the opposite of the other mammals — we have relatively hairless bodies and hairy genitals.

Hair has many purposes, but its role as an agent of the nervous system has been largely overlooked. And its inverse relationship to intelligence is something that is quite thought provoking. Among other mammals, there are just a few that don’t have body hair. We assume that whales and dolphins don’t have hair because they are aquatic. But what if they don’t have hair because they are so smart? Maybe their nervous system has been directing energy away from hair and towards the brain just like ours?

Among land mammals, there are just a few that are considered to be nearly hairless. Elephants, hippos, and pigs are examples. Guess what? They are also all considered to be very intelligent.

And have you ever heard a description of an extraterrestrial visitor, who surely must be very smart to find his way to Earth, having hair? Nope. ETs are always hairless. That’s because having lots of hair and having lots of brains are kind of incompatible. From an evolutionary perspective, it makes a lot more sense to dedicate neurological energy to expanding the brain instead of using it for hair.

Great Apes have lost the inner fur layer due to tree-top nesting in tropical rainforests, where thermal insulation was less significant, but biting insects weren’t (so some apes have thick fur on their more-exposed back but less on their chests (they sleep curled in a ball in the bowl-nest)).

Humans, on the other hand, split from the apes 5,000,000 years ago, with the chromosome #2 translocation, which in my opinion induced altered behavior, by producing a hominin species which spent more time at waterside foraging & bathing and sleeping in inverted bowl-nests on the ground, aka geodesic dome-like huts, waterproof and insect-proof. Thus no selection for fur coat.

DDeden

I’d imagine that the primary reason was that we became able to insulate ourselves and thus its evolutionary importance was lost.

Assume men are hairier due to spending longer periods working/hunting outdoors, than women so some residual benefit still remained
.