How curly hair boosted brain growth by keeping early humans cool

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With summer heat steaming up many parts of the country, you may have already turned on your air conditioner.

Try plugging in a curling iron, too.

Researchers at Pennsylvania State University were intrigued by the fact that humans are unique among mammals with an almost hairless body but a hair-covered scalp.

"Humans evolved in equatorial Africa, where the sun is overhead for much of the day, year in and year out," Nina Jablonski, professor of anthropology at Penn State, said in a news release.

"We wanted to understand how that affected the evolution of our hair," she added.

To figure out if our hair evolved to help early humans, scientists used a thermal manikin — an electrified, temperature-controlled human model — to study how heat is transferred from the body to the atmosphere.

Then, they directed lamps onto the manikin's head under four different hair conditions: no hair, straight hair, moderately curly hair or tightly curled hair.

As expected, each of the three hair types reduced solar radiation to the scalp, but the tightly curled hair offered the best protection from the sun's radiation and heat.

And after wetting the scalp to simulate the effects of sweating, the tightly curled hair also minimized the need to sweat to stay cool, which would help conserve water and avoid dehydration.

The authors of the study, published in the Proceedings of the National Academy of Sciences, believe that tightly curled hair helps by increasing the distance between the scalp and the top of the hair.

The hair on our heads, they claim, probably evolved as a way to reduce the amount of heat gain from solar radiation, thereby keeping humans cool without the body having to expend extra resources.

This probably gave early humans an evolutionary advantage by allowing our brains to grow.

"Around 2 million years ago we see Homo erectus, which had the same physical build as us but a smaller brain size," said Tina Lasisi, who conducted the study as part of her Penn State doctoral dissertation.

"And by 1 million years ago, we’re basically at modern-day brain sizes, give or take. Something released a physical constraint that allowed our brains to grow," she added.

"We think scalp hair provided a passive mechanism to reduce the amount of heat gained from solar radiation that our sweat glands couldn't."

Lasisi also believes this research could have real-world applications.

"When you think about the military or different athletes exercising in diverse environments, our findings give you a moment to reflect and think: is this hairstyle going to make me overheat more easily? Is this the way that I should optimally wear my hair?" she said.