Theories for Hairlessness in Humans: Sexual Selection, Savannah / Thermoregulatory Hypothesis, Aquatic Ape Theory, Ectoparasites
Like bipedalism, relative hairlessness is one the defining and conspicuous characteristics of human beings. Explanations for why hairlessness arose are numerous, and include sexual selection (which Darwin favoured), the Savannah/Thermoregulatory Hypothesis, the Aquatic Ape Theory (AAT / AAH), as well as ectoparasites. Of these, only sexual selection offers true insight into why the loss of hair occurred, but another element to the story of human evolution needs consideration – in particular, the role of increased intelligence – to derive a fully consistent and credible account of denudation in the human line. Here, I will outline an explanation for human hairlessness based on sexual selection, derived from a paper on human sexuality entitled ‘True Nature’ which was published in the JGLMA, and which can be found at www.humansexualevolution.com.
But first, let us examine the shortcomings of the competing explanations.
The Savannah/Thermoregulatory Hypothesis says that when humans ventured out into the more open savannah environment from their arboreal habitats (shared with chimpanzees), they experienced considerable heat stress and consequently evolved the ability to sweat, whereby body heat is dissipated through sweat evaporation from skin surface. Such evaporation becomes enhanced when skin is more bare, and thus it is argued that the evolution of sweating concurrently favoured an accompanying thinning/loss of hair in humans. Here’s the death blow to this hypothesis – why were humans unique in their response to this heat stress? No other furred species of animals on the savannah have responded to the savannah heat stress in this way, and in fact, if you examine all those furred animals, you’ll find they have longer hair on the top surface – which is most exposed to the sun – appearing to invalidate the argument that humans lost their body hair due to the action of the sun.
The Aquatic Ape Theory essentially says that the human line went through an aquatic or semi-aquatic stage during evolution that required adaptation via various changes in anatomy and physiology, chief among them, the loss of hair and the development of a subcutaneous fat layer, claimed to have decreased resistance in the water, and increased streamlining and insulation, respectively. Several damning objections can be raised. First, it appears odd that if humans had indeed passed through an aquatic stage they did not retain some innate swimming ability. Secondly, the benefits of losing body hair were presumably to gain added speed through the water, or increased ability for long distance swimming. But why would increased speed have been needed? Certainly not for catching prey, because humans have no specialized adaptations for doing so – i.e. no large jaws, and no claws, etc. As for the swimming speed of humans, it is very unimpressive when compared to even a non-aquatic mammal like a dog. So, any purported increased speed could not have been of much use in escaping predation either. With respect to long distance swimming, the proboscis monkey provides ample proof that a primate can be fully furred, yet still be a proficient swimmer capable of swimming for miles – in the proboscis’s case, from island to island. Anyway, it is a fact that only trained athletes can swim long distances, and this ability is therefore not a species-wide characteristic. Lastly human beings, despite their subcutaneous fat layer would have been ill-equipped to spending most of their time in the water as the theory proposes, because the insulation it provides is not substantial, and humans would have been ever in danger of hypothermia in even relatively warm waters, which would have still been significantly cooler than their body temperature.
The ectoparasite hypothesis says that hairlessness was favoured as an evolutionary strategy to combat fleas, ticks, lice and other biting insects that commonly plague furred animals. Once again, we see a theory calling for humans to have employed a strategy that no other species has used when confronted with the same challenges, for reasons that are never specified. Additionally, this strategy itself is arguably not very effective, since leaving hair on the head, armpits, and genital areas still subject human beings to considerable infestation with ectoparasites.
So now to the sexual selection hypothesis. In a nutshell, it proposes that hairlessness was selected for because humans found it sexier. It might seem absurd to ask why hairlessness might have been considered more appealing, given that most people would tell you that the thought of a fully furred human would be repulsive. But this is preference based merely on hairlessness being ubiquitous today. This question is important to ask, because the answer is not obvious. Darwin, an early proponent of sexual selection as an explanation for human hairlessness, did not offer this needed insight.
A more reasonable hypothesis is that hairlessness in humans forced them to clothe themselves, and in the process it heightened sexual curiosity by obscuring the sex organs. The retention of hair in the armpits and genital area is indirect proof that when this change occurred humans still had a sexual instinct, since these areas develop hair beginning at puberty, and are known to produce chemicals that might have served as pheromones at one time. True Nature Theory says that humans lost the sexual instinct at some point during their evolution because their increased intelligence level made it both expendable, and advantageous to do so. But it was in the transition period – when we still possessed a sexual instinct, albeit a diminished one – that a mutation for hairlessness arose and was favoured so that the adoption of clothing could trigger increased sexual desire, and work in unison with the declining sexual instinct to still trigger heterosexual intercourse. Ironically then, nudity, which before then had never been considered sexually stimulating because it was everywhere, now became more titillating only because it became more scarce. Hence, it was not hairlessness in itself that was initially considered more attractive, but instead, it was more likely that hairlessness was selected for because of the response to use clothing it necessitated, and the consequent triggering of sexual curiosity and desire that was born from concealing the sex organs.