Headshot of Nina Jablonski
Nina Jablonski, a biological anthropologist and paleobiologist, studies primate and human evolution at Pennsylvania State University. 
Nina Jablonski

Luscious fur coats insulate many animals from the cold and protect them from sunlight, insects, and sharp objects in their environments. Yet, somehow humans evolved to be relatively hairless. While this may appear to be a case of selection against a highly desirable trait, Nina Jablonski, who studies the evolution of human skin and skin pigmentation at Pennsylvania State University, said that our relative hairlessness arose just like other traits did: it offered evolutionary advantages.

The origins of human hairlessness began nearly two million years ago, driven by environmental changes in locations where human ancestors lived. As wooded landscapes in equatorial Africa gave way to open grassland areas, human ancestors had to spend more time outdoors to find food and water. For walking and running long distances, early members of the genus Homo developed a modern human skeleton with long legs and shorter arms. “Around this time, humans lost most of their body hair,” said Jablonski. 

Shedding body hair was a key adaptation since, unlike most other mammals, primates lack a key mechanism for cooling the blood around the brain when it’s hot outside or after exercise. This means that the temperature of the brain increases when the body heats up, which can affect brain functions. Evolution of human hairlessness was accompanied by the development of more sweat glands and darker skin pigmentation. Sweat glands helped them dissipate heat from the skin more effectively, while darker skin pigmentation protected their mostly hairless skin from the damaging effects of solar radiation.

According to Jablonski, the idea of whole body cooling and heating, or thermoregulation, seems like the most likely explanation for human hairlessness based on physical evidence and our knowledge of comparative anatomy and physiology. “We were shooting in the dark decades ago. Now, we can be much, much clearer on what the likely courses of evolution were,” she said. 

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