Why’s There Hair There?

The evolutionary story behind our near-nakedness
With the advent of the genomic era, scientists are looking more and more closely at the genetic traits that make us human. But looking at […]

Art by Maria Demidova

With the advent of the genomic era, scientists are looking more and more closely at the genetic traits that make us human. But looking at phenotypes (the observable characteristics of an individual) alone highlights some pretty obvious differences. Why, for instance, do humans have so little hair compared to our close relatives living in warmer climes? Why are chimps and gorillas so hairy, while we’re stuck with a few discrete patches?

The oldest hypothesis is perhaps the most obvious: as our ancestors evolved from tree dwellers to roamers of the African plains around 3 million years ago, keeping cool as we ran around in the intense sun became more important. A naked body is clearly cooler than a hairier one, not least as evaporation of sweat is more efficient. Interestingly, this hypothesis could explain why we have particularly luxuriant hair on our heads: as protection for our scalp which would otherwise sizzle in the sun.

Yet, being cool during the day comes at a cost of being cold at night. Perhaps this explains the rarity of the transition to nakedness in mammals. Those that have become undressed all have some way of dealing with this problem of night-time cold. Naked mole rats keep their burrows at a fairly constant temperature, whilst the oceans provide a bath of constant temperature for whales and dolphins. For our primate ancestors, culture was critical: in absence of hair, we built fires and shelters to keep ourselves warm at night. So, perhaps the inability to keep warm overnight prevents our primate cousins from becoming naked too?

Cultural warmth was probably important even if hair loss evolved for other reasons, such as, perhaps, in response to parasites. A loss of hair would remove a habitat for ticks and lice. These still (literally) hang on to our heads, but we can’t lose this hair because of the counteracting selection pressures for scalp UV protection. Such interacting selection pressures may explain why there’s hair (or a lack of it) on certain parts of our bodies.

Parasites may also interact with sexual selection to favour hair loss. Females can more accurately and reliably determine the parasite load of a naked man than a hairy one. In the 1980s, Hamilton and Zuk at the University of Michigan, suggested a low parasite load may be maintained by immunological or behavioural adaptations. If these are heritable, it is in the interest of the female to choose a male with few parasites: her offspring may also be resistant to parasites.

So, the advantages of nakedness, in humans at least, are clear; but what of those discrete patches? Have they an important evolutionary purpose too? Again, sexual selection may provide a good explanation for why we keep hair where we do.

A well-kept beard may be an indicator of male neuromuscular condition—only well-coordinated males can keep a tidy beard—and it is these coordinated males which will be the best hunters to provide for their female. While this may seem like a rather convoluted leap, the more general case of male ‘ornaments’ being signals of their quality is well-founded.

Additionally, long head hair might bear witness to its owner’s general physiological state through its condition and lustre, and would be an especially strong signal from afar. Perhaps ancient humans in tribal family groups looked from a distance, and selected the best quality mates in other families on the basis of their hairstyles, much as we do today.

And finally, pubic hair. In both sexes, this hair may serve to attract attention to, and exaggerate, the reproductive organs. Or, could it just be there to prevent friction when we walk or reproduce, with a similar explanation being satisfactory for the existence of armpit hair? Or, as per your standard biology textbook, is this hair really there to wick away sweat from the especially active glands under our arms and/or waft around scents to attract the opposite sex? Probably the answer is a combination of these factors.

There are so many potential explanations for our relative nakedness, from obvious thermoregulatory ones to elaborate sexual ones. It seems both sexual and non-sexual selection are important. Our fire-making abilities allowed us to lose most of our hair, unlike our primate relatives, but the need to stay sexy means we must keep some of it. So perhaps we should ditch the razors and wax strips, and take pride in our gorgeous hair!

About Nigel Taylor

Nigel Taylor '11 is an undergraduate studying Biology at Jesus College.