Tightly curled scalp hair protected early humans from the radiant heat of the sun, allowing brains to grow to the size of modern humans. — Science Daily

“Humans evolved in equatorial Africa, where the sun is overhead most of the day, all year round,” said Nina Jablonski, professor of anthropology at Evan Pugh University of Pennsylvania. talk. “Here, the scalp and top of the head receive a much more stable level of intense solar radiation as heat. It turns out that tightly curled hair keeps humans cool and actually saves water.”

The researchers used thermal manikins (humanoid models that use electrical power to simulate body temperature and allow scientists to study heat transfer between human skin and the environment) and human hair wigs to create a variety of We investigated how hair texture affects 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 placed it in an environmentally controlled wind tunnel.

The research team made a rudimentary measure of body heat loss by monitoring the amount of electricity the manikin required to maintain a constant temperature. A lamp was then shone onto the mannequin’s head to mimic sunlight under four conditions of scalp hair (none, straight, moderately curled, and tightly curled).

George Hebnis, director of the Center for Environmental and Ergonomics Research at Loughborough University, UK, who led the mannequin experiment, said the scientists calculated the difference in total heat loss between the lamp measurements and the base measurements to determine the heat loss to the head. He explained that he determined the inflow of solar radiation. . We also calculated heat loss at different wind speeds and after wetting the scalp to simulate sweating. To study how diverse hair textures affect heat gain in environments with 86 degrees Fahrenheit (30 degrees Celsius) heat and 60% relative humidity, such as those in equatorial Africa. , and ran the results into the model.

The researchers found that while all hair reduces solar radiation to the scalp, tightly curled hair is best protected from the sun’s radiant heat, minimizing the need to sweat to stay cool. I have found that it can be suppressed. They reported their findings yesterday (June 6). Proceedings of the National Academy of Sciences.

“Walking upright is preparation, and brain growth is the boon of scalp hair,” said Tina Lasisi, who conducted the study as part of her doctoral dissertation at Pennsylvania State University. Lasisi will join the University of Michigan as an assistant professor of anthropology in the fall.

As early humans evolved to walk upright in equatorial Africa, the top of the head became increasingly exposed to the brunt of solar radiation, Rasisi explained. The brain is sensitive to heat, especially as it grows. Excessive heat can lead to dangerous conditions such as heat stroke. Since humans have lost much of their body hair, they have developed efficient sweat glands to keep them warm, but sweating comes at the cost of water and electrolyte losses. Scalp hair likely evolved as a way to reduce the amount of heat we get from solar radiation, allowing the body to keep us cool without expending extra resources, Lasisi said.

“About two million years ago, we saw Homo erectus, which had the same body as us, but a smaller brain size,” she said. “And by a million years ago, we were basically more or less the size of our modern brains. We believe that the hair on the scalp provided a passive mechanism to reduce the amount of heat we get” due to solar radiation, which our sweat glands cannot. ”

Jablonski said this interdisciplinary study provides important preliminary results for a better understanding of how human hair evolved without putting humans in potentially dangerous situations. said to do.

The study also provides evolutionary anthropologists with thermal manikins (typically used for functional testing of protective clothing) with additional tools to quantify human data that would otherwise be very difficult to capture. It also shows that there are, Hebnis added.

“Studies done on skin color and how melanin protects us from solar radiation may shape some of the decisions people make about the amount of sunscreen they need in a given environment. Yes,” said Rasisi. “I suspect similar decisions will be made about hair. When considering that military personnel and different athletes exercise in different environments, our findings give us time to reflect and think. Will this hairstyle make me prone to overheating and how to best wear my hair?”

James Smallcomb, Loughborough University, and the University of Australia also contributed to this research. Larry Kenny, Pennsylvania State University Professor of Physiology, Kinesiology, and Marie Underhill Knoll Professor of Human Performance. Mark Shriver Professor of Anthropology. Benjamin Zidney is a former undergraduate research assistant and current alumnus of Pennsylvania State University.

The National Science Foundation and the Wenner-Gren Foundation supported this research.