Tight Curls: How Scalp Hair Played an Integral Role in Human Evolution

Friday - June 9 2023, 23:17 UTC - 1 year ago

Tightly curled scalp hair protected early humans from the sun’s radiative heat, allowing their brains to grow to sizes comparable to those of modern humans. Researchers have studied the role human hair textures play in regulating body temperature, finding that tightly curled hair allowed humans to stay cool and conserve water. These findings can shed light on an evolutionary adaptation that enabled the human brain to grow to modern-day sizes.

Tightly curled scalp hair protected early humans from the sun’s radiative heat, allowing their brains to grow to sizes comparable to those of modern humans. Curly hair does more than simply look good — it may explain how early humans stayed cool while conserving water, according to researchers who studied the role human hair textures play in regulating body temperature. The findings can shed light on an evolutionary adaptation that enabled the human brain to grow to modern-day sizes.

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

The researchers used a thermal manikin — a human-shaped model that uses electric power to simulate body heat and allows scientists to study heat transfer between human skin and the environment — and human-hair wigs to examine how diverse hair textures affect heat gain from solar radiation. The scientists programmed the manikin to maintain a constant surface temperature of 95 degrees Fahrenheit (35 degrees Celsius), similar to the average surface temperature of skin, and set it in a climate-controlled wind tunnel.

The team took base measurements of body heat loss by monitoring the amount of electricity required by the manikin to maintain a constant temperature. Then they shined lamps on the manikin’s head to mimic solar radiation under four scalp hair conditions — none, straight, moderately curled, and tightly curled.

The scientists calculated the difference in total heat loss between the lamp measurements and the base measurements to determine the influx of solar radiation to the head, explained George Havenith, director of the Environmental Ergonomics Research Centre at Loughborough University, U.K., who led the manikin experiments. They also calculated heat loss at different windspeeds and after wetting the scalp to simulate sweating. They ran their results through a model to study how the diverse hair textures would affect heat gain in 86-degree Fahrenheit (30 degrees Celsius) heat and 60% relative humidity, like environments in equatorial Africa.

The researchers found that all hair reduced solar radiation to the scalp, but tightly curled hair provided the best protection from the sun’s radiative heat while minimizing the need to sweat to stay cool. They reported their findings on June 6, 2023, in the Proceedings of the National Academy of Sciences.

"Walking upright is the setup and brain growth is the payoff of scalp hair," said Tina Lasisi, who conducted the study as part of her doctoral dissertation at Penn State. Lasisi will start as an assistant professor of anthropology at the University of Michigan in the fall.

As early humans evolved to walk upright in equatorial Africa, the tops of their heads increasingly took the brunt of solar radiation, explained Lasisi. The braided, ropelike configuration of tightly curled scalp hair acts like a shade, diverting this solar radiation away from the scalp so that the head stays cool.