Straight hair is all the rage among women today – and many who were not born with it will try anything to make their locks look ironed. But if they were early humans living in Africa, they probably would suffer even more.Curly hair does more than simply look good to devotees – it may explain how early humans stayed cool while conserving water, according to Pennsylvania State University 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 are unique among mammals in having a functionally naked body with a hair-covered scalp. They evolved in equatorial Africa, where the sun is overhead for much of the day, year in and year out,” said anthropology Prof. Nina Jablonski. “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 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 types: straight, moderately curled or 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 Center at Loughborough University in the United Kingdom, who led the manikin experiments.
They also calculated heat loss at different wind speeds 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 at 30 degrees Celsius 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 have just reported their findings in the Proceedings of the National Academy of Sciences under the title “Human scalp hair as a thermoregulatory adaptation.”
“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. “Bipedal posture and a hairless body may have necessitated the development of scalp hair to minimize heat gain from solar radiation, particularly in hominins with large brains.”As early humans evolved to walk upright in equatorial Africa, the tops of their heads increasingly took the brunt of solar radiation, explained Lasisi.
Curly hair is more effective at reducing solar radiation
The brain is sensitive to heat, and it generates heat, especially the larger it grows. Too much heat can lead to dangerous conditions, like heat stroke. As humans lost much of their body hair, they developed efficient sweat glands to keep cool, but sweating comes at a cost in lost water and electrolytes. Scalp hair 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, said Lasisi.
“Around two million years ago, there was Homo erectus that had the same physical build as we do today but a smaller brain size,” she said. “And by one million years ago, we’re basically at modern-day brain sizes. Something released a physical constraint that allowed 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.”
The multidisciplinary research provides important preliminary results for bettering our understanding of how human hair evolved without putting humans in potentially dangerous situations, Jablonski said. The study also shows that evolutionary anthropologists have an extra tool in the thermal manikin – normally used for testing the functionality of protective clothing – for quantifying human data that is otherwise very difficult to capture, added Havenith.
“The work that’s been done on skin color and how the pigment melanin protects us from solar radiation can shape some of the decisions that a person makes in terms of the amount of sunscreen needed in certain environments,” said Lasisi. “I imagine that similar decision making can occur with hair. 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?”