To other primates, humans must look extremely odd. We’re the only species that’s generally lacking in down hair, and yet for some reason we have quite a number of hairs that sprout from random spots on our bodies, like our heads.
Scientists still don’t really understand why that is, but new evidence supports the theory that the hair on our heads evolved to help us keep our cool.
That built-in sun protection might seem like an obvious benefit for a generous mop, but the science needs hard data. When a thermal doll was given a human-hair wig to wear in a climate-controlled chamber, the researchers found that the doll didn’t absorb as much heat as if it were bald.
Researchers tested several different types of wigs on the doll, including those with straight hair, loose curls, and tight curls.
Ultimately, the wigs all behaved in a similar way when placed under the hot glow of lamps, but tightly curled hair was best for cooling the doll from the “sunshine” overhead.
However, the results, which still need to be peer-reviewed, are publicly available on the pre-publish server bioarxivsuggest that any type of barrier on top of the head reduces heat absorption from the sun and therefore the need to sweat.
The results suggest that scalp hair evolved in response to our species’ upright posture and our ever-expanding brain.
“[T]The appearance (or retention) of scalp hair may have struck an optimal balance between maximizing heat loss across the body’s large surface area and minimizing solar heat input on the small surface area of the scalp just above the brain write.
“Tightly curled hair can provide an additional reduction in heat input beyond the capacity of typically straight mammalian hair.”
Tightly curled hair is a trait not found in any other wild mammal. There is clearly something in the human experience that favors this type of scalp coverage.
For years, scientists have assumed that scalp hair, particularly curly hair, has evolved as a thermoregulatory response. But Experiments 1988 found that bald men sweat two to three times more than men with scalp hair. At first it was argued that hairless heads are better off keeping the head cool.
One more current study from 2010found, however, that bald heads simply absorb more heat, which in turn requires more sweat.
The current experiments are the first to look at how scalp hair affects a person’s overall thermal stress, not just the body’s sweating response.
“Our results confirm that hair, regardless of its texture, acts as a barrier that reduces heat loss from the body (in this case, the scalp) to its surroundings,” the researchers said write.
However, tightly curled hair doesn’t lay flat, which means it allows the scalp to “breathe” better while still protecting it from the sun.
As a wig’s curl increased, the experimenters found that less sweat evaporation would have been required to remove heat from the scalp, saving water and energy.
A doll in an air-conditioned chamber is of course not entirely realistic. Further investigation should be conducted outdoors with human participants to see how scalp hair might have evolved to function in a more natural environment.
In a hypothetical sense, however, the results of the current experiment support the notion that human hair evolved to adapt to a two-legged lifestyle, particularly in hot and dry regions where drinking water is scarce.
Experts in this environment think Curly hair may have allowed our ancestors to engage in prolonged, “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.
