The curls on your head may have originally served as an evolutionary advantage for growing bigger human brains, according to new research that involved studying a bewigged mannequin in a climate-controlled wind tunnel.
“The brain is a large and very heat-sensitive organ that also generates a lot of heat,” explains Tina Lasisi, currently a postdoctoral researcher in biological anthropology at Pennsylvania State University. “So we figured, evolutionarily, this could be important—especially in a period of time when we see the brain size of our species growing.”
Tightly curled hair better protects the scalp from solar radiation, the new research shows, and it doesn’t lie flat against the skin while wet—a boon in hot conditions that can make humans sweat, like those encountered by our hominin ancestors in Africa millions of years ago.
A research article by Lasisi and her Penn State colleagues, published this month in the Proceedings of the National Academy of Sciences, describes their measurements of how hair regulates scalp temperature in direct sunlight, using different wigs on a “thermal mannequin.”
The mannequin, heated to the average body temperature of 95 degrees Fahrenheit, was placed in a climate-controlled chamber within a wind tunnel that enabled scientists to study the amount of heat transferred between its skin and the surrounding environment.
Three wigs were made from black human hair sourced from China—one straight, one moderately curly, and one tightly curled—so that the researchers could observe how different hair textures affected heat gain and loss on the scalp. They also calculated heat loss at different wind speeds, after wetting the wigs to simulate sweating.
The researchers then made a model of heat loss under different conditions and studied it under the typical conditions in equatorial Africa where early hominins are thought to have evolved.
They learned that all types of hair gave some protection from the sun, but tightly curled hair gave the best protection and minimized the need to sweat—a significant finding, says Lasisi.
“Scalp hair is… a possible passive mechanism that saves us from the physiological cost of sweating,” she says. “Sweating isn’t free—you’re losing water and electrolytes. And for our hominin ancestors that could have been important.”