Hair Texture Helped Ancient Human Brains Expand 2-Million-Years-Ago

A team of scientists has unveiled fascinating insights into the evolutionary significance of human hair texture. By testing various wigs on a specially designed dummy exposed to solar radiation in a wind tunnel, researchers have unraveled a hidden connection between hair texture and the expansion of the human brain, dating back approximately 2 million years. These findings shed light on the role of head hair in protecting, insulating, and stimulating our senses, revealing a captivating tale of human evolution.

Exploring the Purpose of Head Hair

Anthropologists generally agree that humans evolved hair on their heads for three primary reasons: protection, insulation, and sensory function. Hair acts as a barrier against the harmful effects of ultraviolet (UV) radiation from the sun, helping to regulate body temperature. And because the scalp is rich in nerve endings, hair enhances the sensitivity of these nerve endings for feeling light touches, movements, and changes in the environment.

Now, a team of researchers have deep-dived the insulation properties of hair, and their study sheds new insights about how human hair textures affect regulation of body temperature. A new study suggests hair growth sparked “an evolutionary adaptation” that helped the human brain to expand to the size it is today, beginning around 2 million years ago.

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Adapting A Clothing Dummy For Hair Science

The research team was led by Professor George Havenith, Director of the Environmental Ergonomics Research Centre, at Loughborough University. In the new paper, published June 6 in the Proceedings of the National Academy of Sciences, the scientists said they set out to assess how “diverse hair textures affect heat gain from solar radiation.”

Professor Havenith said that to achieve their goals the researchers adapted a dummy normally used for testing the functionality of protective clothing. Essentially, the scientists built an electric, thermal, human-shaped model that produced body heat. The human-model was then programmed to maintain a constant surface temperature of 35 degrees Celsius, similar to the average temperature of skin.

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The specially prepared dummy was tested in the climate-controlled wind tunnel. (Prof. George Havenith/ Loughborough University)

The prepared dummy was finally placed in a climate-controlled wind tunnel with heat lamps directed on its head, to mimic exposure to solar radiation. The scientists analyzed the heat transfer rates through various wig styles: none, straight, moderately curled and tightly curled hair. The results were drawn from calculating how much electricity was required for the model to maintain a constant 35 degrees Celsius temperature, with each hair (wig) type.

Sweating For Accuracy

Professor Havenith explained that to more accurately determine the influx of solar radiation absorbed by the head, with different wigs on, the team calculated the difference in total heat loss between the lamp measurements and the base measurements. Furthermore, heat loss rates at different wind speeds were monitored. And in some experiments, the researchers even wet the head, to mimic and monitor the effects of human sweat on insulation, and heat dispersal.

A computer model analyzed the heat gain properties of the four hair types, under an environment set at 30 degrees Celsius with 60% relative humidity, mimicking the conditions in equatorial Africa. The researchers concluded that while all four hair types reduced solar radiation by the scalp, “tightly curled hair provided the best protection from the sun’s radiative heat while minimizing the need to sweat to stay cool.”

Infographic explaining the elements of the testing procedure. (Lasisi et al. 2023/ PNAS)

Our Changing Relationship with The Sun

In a press release, Nina Jablonski, a Professor of Anthropology at Penn State, said humans evolved in equatorial Africa where the sun is overhead for much of the day, year in and year out. The researcher added that in Africa, the scalp and top of the head received a lot of solar radiation, and the team questioned how these heat conditions affected human evolution.

In the press release, Tina Lasisi, who was undertaking her doctoral dissertation at Penn State, says the results determined that tightly curled hair “allowed humans to stay cool and actually conserve water.” Lasisi added that when early humans began walking upright in equatorial Africa, the tops of their heads were increasingly sun burnt, and they would have suffered from heat stroke which is caused by the head hyper-heating and affecting brain functionality.

Analysing The Gland/Hair Trade Off

Lasisi said that when humans began losing their body hair, they developed sweat glands to keep cool, but sweat reduces body water, and therefore electrolyte levels. It is thought that 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," concluded Lasisi.

Lasisi, speaking for the team, said that around 2 million years ago Homo erectus had a similar physical build as we do today, but had a smaller brain. She added that by 1 million years ago humans had developed a brain size similar to the modern brain, and this is why the researchers suggest scalp hair provided “a passive mechanism to reduce the amount of heat gained from solar radiation that our sweat glands couldn’t".

Top image: Two of the wigs on the test dummy used to test the performance of different hair in heat control.  Source: George Havenith/ Loughborough University

By Ashley Cowie