Humans are a bit weird, all things considered. We’re bipedal, which is already pretty strange; our brains and heads are outsized to a degree that literally causes us pain; and to top it all off, we don’t even have bones in our boners like the rest of our primate cousins.
But for something truly unique to the human experience, look no further than the top of your head. No other animal has our particular setup when it comes to hair: practically naked all over, with a few furry patches dotted here and there – and a lovely big mop top growing out of our scalp.
Evolutionarily, though, our ‘dos have remained a bit of a mystery. “Scalp hair is exceptionally variable across populations within Homo sapiens,” explains a new study, not yet peer-reviewed but published last month on the bioRxiv preprint server. “[But] neither the function of human scalp hair nor the consequences of variation in its morphology have been studied within an evolutionary framework.”
Science’s best guess, so far, was that our peculiar hair growth developed as a way to thermoregulate our bodies. All those other things that make us so particularly human – the upright gait, the big head – they’re great for developing tools and hunting, but they’re not the best if you don’t want to die from heatstroke: “the costs of overheating due to the metabolic heat production associated with locomotion were multiplied by the increased heat sensitivity of a large brain,” explains the paper. “These new thermoregulatory challenges required new solutions.”
But did those solutions definitely include hair on our scalp? For a while, the evidence was conflicting: after all, there are plenty of fur-covered mammals that have adapted to live in hot environments – cats, for example, can selectively cool different parts of their brain, protecting them from thermal damage and conserving water – so being bald clearly isn’t the only way to deal with overheating in the sun. More than that, there was some tentative evidence that having a luscious crop of locks on the top of your head might actually make thermoregulation more difficult: one study from the late 80s found that bald crania sweat two to three times more than those covered in hair.
Generally, more sweat means better thermoregulation, meaning the baldies would theoretically stay cooler in the sun – but follow-up studies put that conclusion to the question once again. “[It’s] logical… that a hairless head would be better off in terms of heat loss because it has no barrier blocking evaporation,” the new paper points out. “However, according to [a] more recent study… there may be a greater disadvantage to having no scalp hair since it also subjects the scalp to higher heat loads, mainly through solar radiation influx.”
In other words: bald people sweat more through their scalp, sure, but it’s mostly because their scalps get hotter. It seemed like thermoregulation might have been the answer all along.
But there were still questions left unanswered: specifically, what’s up with all the different styles out there? Unlike any other wild animal, humans can come topped with anything from an afro to a "Rachel" – and if head hair itself has something to do with thermoregulation, then maybe these different morphologies could tell us something deeper.
“Tightly curled hair – which is common in many African populations – […] may have an advantage in reducing heat gain from exposure to sunlight,” the researchers write. “Additionally, the ubiquity of tightly curled hair in a continent with unmatched genetic diversity suggests the role of scalp hair morphology deserves further attention.”
So the team set out to test this idea – in a brilliantly direct way. Fitting a thermal manikin with a selection of wigs in different styles – one straight, one moderately curly, and one with tight curls – and sitting the figure under heat lamps in a climate-controlled chamber, they measured the heat loss or gain from the top of the variously-glowed-up models to see what style of hair is best for keeping cool.
And it seems their hypothesis was right. “In general, the pattern we observed is that the highest solar heat gain was experienced under the nude condition,” they noted – while “straight hair, moderately curled hair, and tightly curled hair showed decreasing heat gain in that order.”
Regardless of texture, then, human hair does seem to protect us from overheating – but it’s the tight curls that have the advantage. It makes sense, the authors point out: like a long fur coat in the animal kingdom, curly hair stands away from the head, increasing the distance between the surface of the hair and the surface of the scalp.
“The conditions under which humans evolved were such that… evolution would have favored adaptations for water conservation,” the paper notes. “A plausible scenario could be the evolution of tightly curled hair that insulated against heat and reduced water loss while also extending how long individuals could engage in strenuous physical activity before needing a drink of fresh water.”
Of course, three wigs and a thermal manikin does not a theory prove, and the researchers are aware that their paper comes with some limitations. Nevertheless, they write, the results “are important for researchers trying to understand the evolution of early hominins and later human populations, since they provide insight into the specific contexts where hair, particularly tightly curled hair, may have been advantageous.”
“Though we do not yet understand the extent to which scalp hair helps regulate whole-body temperature, this paper provides some valuable preliminary findings,” they conclude. “This research represents a first step in understanding the connection between human scalp hair and thermal load to the brain and body.”
The preprint is available at bioRxiv.