初期の人類を涼しく保ち、水分を補給する

深くカールした頭皮の毛は、初期の人類を太陽の輻射熱から守り、脳を現生人類と同様の大きさに成長させることができました。

人間の髪の質感が体温の調節に果たす役割を研究した研究者らによると、巻き毛は単なる見た目の美しさだけではありません。これは、初期の人類がいかに水分補給を保ちながら涼しさを保っていたかを説明するかもしれません。 この発見は、人間の脳が現代のサイズに成長することを可能にした進化的適応を明らかにする可能性がある。

ペンシルバニア州立大学の人類学教授ニーナ・ジャブロンスキー氏は、「人類は毎年、一日中太陽が頭上にある赤道直下のアフリカで進化した」と語る。 ここでは、頭皮と頭頂部は、より安定したレベルの強い太陽放射を熱の形で受けます。 私たちは、これが髪の進化にどのような影響を与えたのかを理解したいと考えました。 私たちは、髪をしっかりとカールさせることで人間は冷静さを保ち、実際に水分を節約できることがわかりました。」

しっかりカールした人毛ウィッグ (左) とストレートの人毛ウィッグ (右) を装着したサーマル マネキン。 トロールは電気エネルギーを使用して体温をシミュレートし、科学者が人間の皮膚と環境の間の熱伝達を研究できるようにします。 人間の髪の質感が体温の調節に果たす役割を調べた新しい研究では、しっかりとカールした髪が太陽の輻射熱から最もよく保護され、涼しさを保つために汗をかく必要性が軽減されることがわかりました。 クレジット: George Haveneth、ラフバラー大学

研究者らは、サーマルマネキン（電気エネルギーを使用して体温をシミュレートし、科学者が人間の皮膚と環境の間の熱伝達を研究できる人型モデル）と人間の髪の毛のかつらを使用して、さまざまな髪の質感が太陽からの熱取得にどのように影響するかを調べました。エネルギー。 放射線。 科学者たちはこの小人を、表面温度を95度の一定に保つようにプログラムした[{” attribute=””>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.

A graphic showing how scientists used a thermal manikin and human hair wigs to measure heat transfer from the scalp. Credit: Melisa Morales Garcia

“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 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 likely 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 2 million years ago we see Homo erectus, which had the same physical build as us but a smaller brain size,” she said. “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. 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, said Jablonski.

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 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?”

Reference: “Human scalp hair as a thermoregulatory adaptation” by Tina Lasisi, James W. Smallcombe, W. Larry Kenney, Mark D. Shriver, Benjamin Zydney, Nina G. Jablonski and George Havenith, 6 June 2023, Proceedings of the National Academy of Sciences.
DOI: 10.1073/pnas.2301760120

Also contributing to the research were James Smallcombe, Loughborough University and the University of Australia; and from Penn State Larry Kenney, professor of physiology, kinesiology and Marie Underhill Noll Chair in Human Performance; Mark Shriver, professor of anthropology; and Benjamin Zydney, previously an undergraduate research assistant and now a Penn State alum.

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