The title of this post is a bit of a lie, humans aren’t actually furless (and I’m not just talking about the hair on our head). In actual fact humans have as much hair as chimps do, completely covering our body. The difference is that our hair is incredibly fine, to the point where most of it is basically invisible to the naked eye. As such we get none of the insulating or visual benefits of the more furry animals1.

These are fairly significant benefits. Living in the chilly north of England I can think of at least 11 months of the year where a nice furry pelt would be helpful. This is why our fine fur is almost unique in the natural world with only a handful of other land mammals (and no primates) possessing similar hair. If you must know, our furless friends are elephants, rhinos, hippos, pigs and naked mole-rats2.

So why are humans the only primate to have become, for all intents and purposes, furless? Answering this question is very difficult given that skin doesn’t fossilise. As such we can’t identify when our fur became so fine, which stops us figuring out what environmental, behavioural or technological factors may have prompted the change.

Nonetheless, scientists are an innovative bunch and through mathematical modelling and investigations of living creatures (both furred and unfurred) have developed a range of hypotheses that could explain why humans are naked1. These can be grouped into 3 broad categories: thermoregulation, parasites and sexual selection. The aquatic ape silliness could count as a fourth category3, but I like to pretend that doesn’t exist.

Thermoregulation

The thermoregulatory hypotheses basically state that our ancestors began doing something that meant they accumulated too much heat. Since fur is a great insulator, the extra heat would’ve been difficult to get rid of. This would’ve acted as a strong selection pressure; driving our ancestors to become naked so we could dump the excess heat and get on with doing whatever it was we were doing1.



The various hypotheses disagree over what the activity that resulted in all this heat was. Some suggest running over long distances, probably to hunt, was what got us hot and bothered4. Others think that merely walking out onto the hot, open Savannah was enough to tip us over the edge5. Alternatively fire and/or clothing may have rendered fur useless, or perhaps even detrimental6. If our ancestors kept a fire burning through the night to ward of predators then we might have begun to overheat if we were hairy!

In all of these cases the behaviour at the root of the problem was very beneficial, hence why there was a strong evolutionary pressure to keep doing it and stop suffering from overheating as we did so.

However, most these thermoregulatory hypotheses suffer from a critical flaw: insulation works both ways. Not only does hair help trap heat, but stops us absorbing as much heat from the surrounding environment. As such fire or walking out onto the sunny Savannah would not result in significant leaps in a hairy hominin’s body temperature2.

As such the only thermoregulatory hypotheses which actually work are those which claim our ancestors did something that significantly raised their internal body temperature for a period of time, like endurance running. Perhaps chasing down prey over long distances was what prompted our current nudity4.

This would make Homo erectus one of the earliest naked hominins. However, there is some tantalising evidence that some of the earlier, more ape-like Australopiths may have been the first hairless runners. The Australopiths have always been thought to be hairy, given that (aside from walking upright) they’re extremely chimp like. Chimps, if you’ve not noticed, are very hairy. But if running was the reason for our hair loss then we might well be descended from upright, naked chimps!

Parasites

Parasites are a problem for all primates, infecting them with all kinds of nasty diseases. Well, all primates except humans. We actually have a relatively low number of parasites on our body and as a result are less likely to suffer from an infestation.

Our low parasite populations are because they can only survive in hairy regions of the body. By becoming essentially hairless we’ve deprived of their natural habitat, keeping their populations in check. Even if their numbers do get too high, its easy to remove them by simply cutting their hair. Many hunter-gatherers mimic this practice, with Australian Aboriginals burning the hair short if someone is overly burdened by tics2.

This has led some researchers to suggest that the primary reason our ancestors began to lose their hair was to decrease the habitat of parasites. This would’ve made us less likely to suffer from parasite infestations, and have fewer parasites altogether. By cutting out these disease bearing bugs our ancestors would’ve significantly improved their overall health2.

However, most primates are capable of keeping parasite numbers in check through grooming. Why did our ancestors have to go and get naked instead? This may be linked to our large brains. There’s a lot of evidence that suggests our large brains evolved to help us live in large groups. However, these groups became so big that there was not enough time to maintain social relationships through grooming alone. This resulted in the development of language, since you can talk to a lot of people at once, making the bonding process more efficient.

But if our ancestors were nattering rather than de-gnat-ing parasites may have become a severe problem. The solution evolution settled on was hair loss! That way we can chat without picking fleas off each other and not worry what those fleas will do.

Sexual selection

Sexual selection hypotheses aren’t really distinct from parasite or thermoregulatory ones. Rather, they suggest that being naked is kind of noticeable (even if the species had invented clothes). This made hairlessness a good signal that the naked individual was a good hunter, in the case of endurance running, or was healthy and tic free. Thus being naked was a sign you were a good mate, and sexual selection would’ve taken over2.

Interestingly, Darwin was one of the first to suggest sexual selection played a role in human nakedness in his book The Descent of Man. Since 1871 the hypothesis has gone in and out of favour. Many like it, noting that pretty much everyone ever examined has a preference for a mostly hairless mate. On the other hand, loss of hair is often a sign of disease, so many seriously question whether or not nakedness would ever be an attractive trait1.

This criticism conveniently forgetting that it is viewed as an attractive trait by most modern people.

So why are humans hairless?

Apologies, but after making you read almost 1,000 words there is no final answer to be had. Until we can figure out just when our ancestors developed super-fine hair it’s difficult to figure out just why that we became naked. So we’re left with the idea it was linked to a high heat generating activity, like endurance running, or to reduce the number of parasites we have to deal with. Plus a bit of sexual selection, depending on how much credence you give evolutionary psychology.

Of course, whilst studying human evolution we’re often tempted to look for the silver bullet explanation; a single hypothesis that can explain everything. This need not be the case. It might well be that both parasite reduction and thermoregulation were strong motivators for hair loss.

But then, it might be that none of the explanations for our nakedness are correct. What if…

References

1. Rantala, M. J. (2007). Evolution of nakedness in Homo sapiens. Journal of Zoology, 273(1), 1-7.

2. Pagel, M., & Bodmer, W. (2003). A naked ape would have fewer parasites.Proceedings of the Royal Society of London. Series B: Biological Sciences,270(Suppl 1), S117-S119.

3. Hardy, A.C. (1960). Was man more aquatic in the past? New Scientist 7, 642–645

4. Ruxton, G. D., & Wilkinson, D. M. (2011). Thermoregulation and endurance running in extinct hominins: Wheeler’s models revisited. Journal of human evolution, 61(2), 169-175.

5. Ruxton, G. D., & Wilkinson, D. M. (2011). Avoidance of overheating and selection for both hair loss and bipedality in hominins. Proceedings of the National Academy of Sciences, 108(52), 20965-20969.

6. Kushlan, J.A. (1980). The evolution of hairlessness in man. Am. Nat. 116, 727–729.

7. Gowlett, J. A., & Wrangham, R. W. (2013). Earliest fire in Africa: towards the convergence of archaeological evidence and the cooking hypothesis. Azania: Archaeological Research in Africa, 48(1), 5-30.

Shoutout to Artem Kaznatcheev for having some great ideas about the link between the social brain hypothesis, parasites and nudity.