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.
‘Sweating isn’t free’
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 windspeeds, 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.”
The mystery of human hair
Just why humans have hair on their heads is a long-standing question that few scientists agree on.
Many link it to our evolution from four-legged creatures to those that walk upright, reasoning that head hair helped regulate the body’s temperature by acting as a barrier to the equatorial sun.
Niccolo Caldararo, an anthropologist at San Francisco State University who wasn’t involved in the latest study, favors evolutionary anthropologist Dean Falk’s “radiator theory” : that hair protects large hominin brains in hot sunlight and insulates them when it’s cold.
But Caldararo notes it’s a complex subject with many variables: for example, white hair that reflects light might be better protection from the sun than black hair that absorbs its heat, he says.
The research by Lasisi and her colleagues is “provocative,” says Kurt Stenn, a dermatologist not involved with the study and the author of Hair: A Human History. He suggests that the researchers should have also considered the shape and density of human head hair.
For example, the Asian hair used in the study tends to be round in cross-section and therefore absorbs more heat than some types of African hair, where each hair is shaped like a long ribbon that curls more easily, he says.
Evolutionary biologist Elizabeth Tapanes at the University of California San Diego, who also wasn’t involved in the study, says the research is “a great leap forward about thinking why we have so much hair on our heads.”
She adds that studying the hair of other primates alongside human hair could help scientist better understand how it keeps the head cool; her own studies of lemurs called sifakas have found similar results.
Sifakas are vertical climbers and leapers, so they are usually upright with their heads facing the sun, Tapanes says; and the researchers found they had more scalp hair and less body hair in hot and humid environments.
An evolutionary advantage?
It’s even possible that curly hair might be one of the reasons why Homo sapiens supplanted the Neanderthal and Denisovan species of hominins, which died out about 40,000 years ago.
Lasisi points out that if the genetic mutations for curly hair occurred before Homo sapiens left Africa, but after our hominin ancestors did, it might have given early modern humans an evolutionary advantage.
But she doesn’t think that’s likely; and the study proposes instead that genes for curly hair arose much earlier in human evolution, perhaps around two million years ago when Homo erectus was the dominant hominin. And as hominin brains grew bigger, it suggests, the genes for curly hair that protected the scalp from the sun may have given those who had them an advantage.
As for straight hair: Lasisi says any genetic predisposition for curly hair among early hominins was probably variable. “We don’t expect that it would have been homogenous,” she says. At a later point in our evolution, curly hair may have lost its evolutionary advantage, and straight hair may have been favored by different types of genetic selection.
“Maybe once we had those larger brains, we also had all these cultural adaptations to avoid overheating, like better sources of water,” she says. “And at that point, maybe there wasn’t such a selective pressure for curly hair.”
Lasisi says the next stages of the research will be to look for genetic evidence that may support the theory.
“First, we’ll have to know more about modern humans, such as which genes are associated the hair morphology,” she says. “And the second step will be to collaborate with people who do ancient DNA work, to see if those are seen in archaic humans.”