Aesthetically, humans are somewhat standout from the rest of natural creation. We are multicellular organisms, instantly making us completely different to the vast majority of the species’ currently on earth today, and warm-blooded, differentiating us from every plant, fungi, invertebrate, fish, amphibian and reptile. We stand on two legs, but at the same time cannot fly, differentiating us from almost every species of bird and mammal. But this is only so much basic classification; the one trait that aesthetically differentiates from nearly all of these is our hairlessness. Brian May excepted.
Technically, there are other members of the order mammalia who go without fur; the simultaneously cute and horrifying naked mole rat is but one land-borne example, and no swimming mammal (whales, dolphins etc.) have fur. And yes, we technically do have a full layer of fur covering us, meaning that you have more hairs (in terms of number, rather than volume) than a chimpanzee- but our hairy covering is so small as to be practically not there, and the amount of insulation and protection that it provides is minimal. Across most of our body, the only natural protection we have against our enemies and the elements is our bare skin.
Exactly why this is the case is somewhat unclear, because fur is very useful stuff. It offers a surprising degree of protection against cuts and attacks, and is just as effective at keeping out the elements, be they cold, wind or rain. Length can and does vary widely depending on location and need, and many species (including some humans) have incorporated their fur as a form of bodily decoration to attract mates and intimidate rivals; the lion’s mane is the most obvious example.
Also confusing is why we have hair where we do; upon our heads and around the pubic regions. It is thought that hair on the head may be either an almost vestigial thing, left over from the days when our ancestors did have hair, although this theory doesn’t explain why it remained on our head. Better explanations include the slight degree of extra shielding it provides to our brain, our greatest evolutionary advantage, or because the obviousness of the head makes it a natural point for us to rig our hair into elaborate, ceremonial styles and headdresses, raising the social standing of those who are able to get away with such things and helping them attract a mate and further their genes. However, the pubic region is of particular interest to evolutionary biologists, in part because hair there seems counter-productive; the body keeps the testicles outside the body because they need to be kept slightly cooler than the body’s interior temperature in order to keep sperm count high and ensure fertility (an interesting side effect of which is that people who take regular hot baths tend to have a lower sperm count). Surrounding such an area with hair seems evolutionarily dumb, reducing our fertility and reducing our chances of passing our genes onto the next generation. It is however thought that hair around these regions may aid the release of sexual pheremones, helping us to attract a mate, or that it may have helped to reduce chafing during sex and that women tended to choose men with pubic hair (and vice versa) to make sex comfortable. This is an example of sexual selection, where evolution is powered by our sexual preferences rather than environmental necessity, and this itself has been suggested as a theory as to why we humans lost our hair in the first place, or at least stayed that way once we lost it; we just found it more attractive that way. This theory was proposed by Charles Darwin, which seems odd given the truly magnificent beard he wore. However, the ‘chafing’ theory regarding pubic hair is rather heavily disputed from a number of angles, among them the fact that many couples choose to shave their pubic region in order to enhance sexual satisfaction. Our preferences could, of course, have changed over time.
One of the more bizarre theories concerning human hairlessness is the ‘aquatic apes’ theory; it is well known that all swimming mammals, from river dolphins to sea lions, favour fat (or ‘blubber’) in place of fur as it is more streamlined and efficient for swimming and is better for warmth underwater. Therefore, some scientists have suggested that humans went through a period of evolution where we adopted a semi-aquatic lifestyle, fishing in shallow waters and making our homes in and around the water. They also point to the slight webbing effect between our fingers as evidence of a change that was just starting to happen before we left our waterborne lifestyle, and to humanity’s ability to swim (I am told that if a newborn baby falls into water he will not sink but will instinctively ‘swim’, an ability we lose once we become toddlers and must re-learn later, but I feel it may be inappropriate to test this theory out). However, there is no evidence for these aquatic apes, so most scientists feel we should look elsewhere.
Others have suggested that the reason may have been lice; one only needs to hear the horror stories of the First World War to know of the horrible-ness of a lice infestation, and such parasites are frequently the vectors for virulent diseases that can wipe out a population with ease. Many animals spend the majority of their time picking through their fur to remove them (in other apes this is a crucial part of social bonding), but if we have no fur then the business becomes infinitely simpler because we can actually see the lice. Once again, adherents point to sexual selection- without hair we can display our untarnished, healthy, parasite-free skin to the world and our prospective mates (along with any impressive scars we want to show off), allowing them to know they are choosing a healthy partner, and this may go some way to explaining why the ultimate expression of male bodily beauty is considered a strong, hairless chest and six-pack, symbolising both strength and health. Ironically, a loss of fur and our subsequent use of clothes developed an entire new species; the body louse lives only within the folds of our clothes, and was thought to have evolved from hair lice some 50,000 years ago (interestingly, over a million years passed between our African ancestors passing through the hairless phase and our use of clothes, during which time we diverged as a species from Neanderthals, discovered tools and lived through an Ice Age. Must have been chilly, even in Africa). It’s a nice theory, but one considered redundant by some in the face of another; homeostasis.
Apart from our brainpower, homeostasis (or the ability to regulate our body temperature) is humanity’s greatest evolutionary advantage; warm blooded mammals are naturally adept at it anyway, giving us the ability to hunt & forage in all weathers, times and climates, and in cold weather fur provides a natural advantage in this regard. However, without fur to slow the process of heat regulation (sweating, dilation of blood vessels and such all become less effective when insulated by fur) human beings are able to maintain an ambient bodily temperature almost regardless of the weather or climate. African tribesmen have been known to run through the bush for an hour straight and raise their body temperature by less than a degree, whilst our ability to regulate heat in colder climates was enough for scores of Ice Age-era human bones to be found across the then-freezing Europe. Our ability to regulate temperature surpasses even those other ‘naked’ land mammals, the elephant and rhinoceros, thanks to our prominent nose and extremities that allow us to control heat even more precisely. In short, we’re not 100% sure exactly why we humans evolved to be hairless, but it has proved a surprisingly useful trait.