In case you have forgotten, humans have relatively hairless bodies when compared to our other great ape relatives, even to the 5,000 or so other mammalian species out there. This is curious phenomenon — we seem to defy the very classification of being a mammal.

Sure, there’s a lot of variation among human populations. Some of us are hairier than the others, while others of us aren’t. There are even known genetic ‘mutations’ that reverse our hairlessness into a ‘werewolf syndrome’ which is known in science as hypertrichosis. Fajardo Aceves Jesus ManuelFajardo Aceves Jesus Manuel of Mexico is a modern day example of someone with hypertrichosis. This condition is caused by a genetic defect that,

“causes the hair growth cycle in victims of this rare disease to run amok. The follicles from which the body hair grows are apparently incapable of switching from the growth phase to the dormant phase, which normally ends in the new hair falling out and the cycle begining again.”

Hypertrichosis is considered an atavism — or an evolutionary throwback. An atavism is a trait that reappears which had once disappeared generations ago. This happens mostly likely thru a backward mutation but it can also happen thru transposable elements within the genome, i.e., the ‘hairy gene’ was locked away in a region of the genome that was not expressed but then was shuffled and inserted into an expressable region. Other known atavisms occur thru this mechanism. Genes that coded for a previously existing phenotype are often preserved in DNA, even though the genes are not expressed in some or most of the organisms possessing them.

But hypertrichosis is a rare genetic disorder, actually it is very rare. Only about 50 cases of the condition have been documented since the Middle Ages. That indicates there has been a strong positive selection to keep us hairless. But why? Are there any evolutionary explainations as to why humans are hairless?

Well, there are three main stories explanations (?) for why humans lack fur. Scientific American has asked an expert, Mark Pagel, head of the evolutionary biology group at the University of Reading, and he summarizes these hypotheses for us. These hypotheses vary in evidence but they all revolve around the idea of a positive selective pressure to not be hairy. In other words, it may have been advantageous for the human lineage to have become less and less hairy during the six million years since we shared a common ancestor with our closest living relative, the chimpanzee.

The first hypothesis, named the ‘aquatic-ape hypothesis’ considers that way back in the day, like 8 million year ago an apelike ancestor of modern humans had,

“a semiaquatic lifestyle based on foraging for food in shallow waters. Fur is not an effective insulator in water, and so the theory asserts that we evolved to lose our fur, replacing it, as other aquatic mammals have, with relatively high levels of body fat. Imaginative as this explanation is—and helpful in providing us with an excuse for being overweight—paleontological evidence for an aquatic phase of human existence has proven elusive.

I was taught that our hairlessness initially came about from an adaptation that occurred as apes moved down from the jungles and into the hot savanna. Hairlessness helped control our body temperature when hominids made the transition to a new ecosystem.

“Our ape ancestors spent most of their time in cool forests, but a furry, upright hominid walking around in the sun would have overheated.”

This theory seemed to make a lot of sense, when I was taught this by my professors. However, it had one major flaw. The lack of fur,

“might have made it easier for us to lose heat during the day, [but] we also would have lost more heat at night, when we needed to retain it.”

A recent theory, one that I briefly introduced on back in March, considers that ancestors to modern humans were selected be hairless as a means,

“to reduce the prevalence of external parasites that routinely infest fur. A furry coat provides an attractive and safe haven for insects such as ticks, lice, biting flies and other “ectoparasites.” These creatures not only bring irritation and annoyance but carry viral, bacterial and protozoan-based diseases such as malaria, sleeping sickness, West Nile and Lyme disease, all of which can cause chronic medical problems and, in some cases, death. Humans, by virtue of being able to build fires, construct shelters and produce clothes, would have been able to lose their fur and thereby reduce the numbers of parasites they were carrying without suffering from the cold at night or in colder climates.

Human lice infections, which are confined to the hairy areas of our bodies, seem to support the parasite hypothesis. Naked mole rats, animals that can be described as resembling “overcooked sausages with buck teeth,” also seem to support the theory: They live underground in large colonies, in which parasites would be readily transmitted. But the combined warmth of their bodies and the confined underground space probably negate the problem of losing heat to cold air for these animals, allowing them also to become naked.

Once hairlessness had evolved this way, it may have become subject to sexual selection—being a feature in one sex that appealed to another. Smooth, clear skin may have become a signal of health, like a peacock’s tail, and could explain why women are naturally less hairy than men and why they put more effort into removing body hair. Despite exposing us to head lice, humans probably retained head hair for protection from the sun and to provide warmth when the air is cold. Pubic hair may have been retained for its role in enhancing pheromones or the airborne odors of sexual attraction.”

Will we ever know why hairlessness was selected for in our lineage?

No, just like we may never really know why bipedalism was selected for in our lineage.

But the problem with hairlessness versus bipedalism, is that hairlessness is one of those wild witch hunts that we embark on, one that relies on story telling and a whole lot of what-if’s. It is full of conjecture and not much scientific evidence. Regardless of its flaws, this question remains a very thought-provoking one to me, as someone interested in physical anthropology.

After writing this, I consider hairlessness could have been due to a founder-type effect. Since no supportable physiological significance can be drawn at this time, what if hairlessness became dominant a dominant allele because people who carried the hairless recessive mutant allele replaced people carrying the dominant hairy allele after a population bottleneck?

Again another what-if, however it is a plausible evolutionary possibility… But is it supportable?

No, not at this time… unless we figure out when this allele was really prevalent in our populations after a certain time and correlate its switch from recessive mutant to a dominant after some bottleneck inducing phenomenon, like an epidemic that targeted hairy people more. Until we sequence more paleoDNA from other hominids (mind you we can only extract nuclear DNA from Neanderthals, as of now) this hypothesis is also just as outlandish & unsupported as the aquatic ape one.