To other primates, humans must look extremely odd. We are the only species with a general lack of fuzz, and yet for some reason, we have a whole bunch of hair sprouting from random spots on our body, like the summit of our heads.
Scientists still don't really understand why that is, but new evidence supports the theory that our scalp hair evolved to help us stay cool.
This built-in sun-guard might seem like an obvious benefit to a generous mop, but science needs hard data. When a thermal manikin was given a wig of human hair to wear in a climate-controlled chamber, researchers found the manikin did not absorb as much heat as when it was bald.
Researchers tested multiple different types of wigs on the manikin, including ones with straight hair, loose curls, and tight curls.
Ultimately, the wigs all performed in similar ways when under the hot lights of lamps, but tightly curled hair was the best at keeping the manikin cool from the 'solar' radiation above.
The results, which are yet to be peer-reviewed but are publically available on the pre-publish server bioarxiv, suggest that any type of barrier on the top of the head reduces heat gain from the Sun and, in turn, the need to sweat.
The findings suggest that scalp hair evolved in response to our species' upright posture and our increasingly large brains.
"[T]he emergence (or retention) of scalp hair may have struck an optimal balance between maximizing heat loss across the large surface area of the body and minimizing solar heat gain on the small surface area of the scalp, directly over the brain," researchers write.
"Tightly curled hair may provide an additional reduction in heat influx beyond the capacity of typically straight mammalian hair."
Tightly curled hair is a trait not seen in any other wild mammal. Clearly, there's something about the human experience that gives preference to this type of scalp coverage.
For years now, scientists have hypothesized that scalp hair, specifically curly hair, evolved as a thermoregulatory response. But experiments in 1988 found that bald men sweat two to three times more than men with scalp hair. At first, this was used to argue that hairless heads are better off at keeping the head cool.
A more recent study in 2010, however, found that bald heads simply absorb more heat, requiring more sweat in turn.
The current experiments are the first to explore how scalp hair affects a person's overall thermal load, not just a body's sweating response.
"Our findings confirm that, regardless of texture, hair acts as a barrier that decreases heat loss from the body (in this case, the scalp) to its surroundings," researchers write.
Tightly curled hair, however, doesn't lie flat, which means it allows the scalp to 'breathe' better, while still protecting it from the Sun.
As the curliness of a wig is increased, experimenters found less sweat evaporation would have been needed to shed heat from the scalp, thereby conserving water and energy.
A manikin in a climate-controlled chamber is, of course, not wholly realistic. Further research should be done outside with human participants to see how scalp hair might have evolved to function in a more natural setting.
In a hypothetical sense, however, the findings of the current experiment give credence to the idea that human hair evolved to adapt to a bipedal lifestyle, especially in hot and arid regions where drinking water was scarce.
In this environment, experts think curly hair might have allowed our ancestors the ability to partake in longer, "strenuous physical activity before needing a drink of fresh water."
Maybe a frizzy head in high humidity isn't such a curse after all.
The study was published in bioRxiv.