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Curly hair may have evolved to keep early humans cool, study suggests

Researchers believe findings may shed light how this evolutionary adaptation may have allowed human brains to grow in size

A thermal manikin wearing tightly curled, left, and straight, right, human hair wigs
A thermal manikin wearing tightly curled, left, and straight, right, human hair wigs

Curly hair may have evolved in early humans living in Africa thousands of years ago to help stay cool and protect from the sun’s harmful rays, research suggests.

Scientists found that tightly coiled hair provided a barrier against solar radiation to protect the scalp.

This type of hair texture was also found to help the human body conserve water by minimising the need for sweating to stay cool.

The team said its findings, published in the journal Proceedings of the National Academy of Sciences (Pnas), may shed light on how this evolutionary adaptation may have allowed human brains to grow in size.

Nina Jablonski, Evan Pugh University Professor of Anthropology at Penn State University in the US, said: “Humans evolved in equatorial Africa, where the sun is overhead for much of the day, year in and year out.

A thermal manikin wearing tightly curled hair
A thermal manikin wearing different types of curled hair (Professor George Havenith/Loughborough University)

“Here the scalp and top of the head receive far more constant levels of intense solar radiation as heat.

“We wanted to understand how that affected the evolution of our hair.

“We found that tightly curled hair allowed humans to stay cool and actually conserve water.”

It is thought that early humans evolved to walk upright somewhere around three million years ago.

Over time, as humans lost most of their body hair, they evolved sweat glands to keep cool.

But sweating comes at a cost in lost water and electrolytes, the researchers said.

For the study, the scientists used a manikin – a human model that uses electric power to simulate body heat – to test the effects of solar radiation on different human hair wigs.

The experiments, led by scientists at Loughborough University, involved shining lamps on the manikin’s head to mimic solar radiation and measuring heat loss in the manikin body.

They also calculated heat loss at different windspeeds and after wetting the scalp to simulate sweating.

The researchers found that while all types and textures of hair reduced solar radiation to the scalp, tightly curled hair provided the best protection from the sun’s heat.

The scientists believe having curly hair may have also helped the early humans’ brains to grow.

Tina Lasisi, who conducted the study as part of her doctoral dissertation at Penn State University, said: “Walking upright is the setup and brain growth is the payoff of scalp hair.

“Around two million years ago we see Homo erectus, which had the same physical build as us but a smaller brain size.

“And by one 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.

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

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