Curly hair in early humans may have acted as a safeguard from the Sun's radiative heat, allowing for better water retention in the body and aiding an evolutionary adaptation that supported the growth of the human brain to its present-day size.
Nina Jablonski, one of the study authors, explained in the news release that humans developed in equatorial Africa, where the Sun shines directly overhead for extended periods throughout the year. This constant exposure to intense solar radiation as heat prompted the researchers to investigate the impact on the evolution of human hair.
Heat From the Sun Affects Hair Texture
To investigate the impact of hair texture on heat gain from solar radiation, the researchers conducted experiments using a thermal mannequin and human-hair wigs. As per ZME Science's report, the mannequin was maintained at a temperature of 35 degrees Celsius and was placed in a wind tunnel to simulate real-life conditions.
Baseline measurements were established by monitoring the mannequin's heat loss through electricity consumption. Solar radiation was replicated by directing lamps toward the mannequin's head, representing various hair conditions: no hair, straight hair, moderately curled hair, and tightly curled hair.
By comparing the heat loss between the lamp and the baseline measurements, the researchers could estimate the influx of solar radiation and its impact on the head. They also considered the effects of wind speed and wetting the scalp, simulating wet conditions. Climate conditions in equatorial Africa were incorporated into the study using a model.
The findings of the study, titled "Human Scalp Hair as a Thermoregulatory Adaptation" published in the Proceedings of the National Academy of Sciences, indicated that tightly curled hair provides the most efficient protection against Sun radiation to the scalp over other kinds of hair types.
It means that curly hair texture not only minimized heat gain but also reduced the need for sweating as a cooling mechanism, contributing to improved heat regulation.
Curly Hair Helped in Human Brain Evolution
Tina Lasisi, the study's author, noted in the news release that walking upright was a crucial development in early humans, and brain growth was the ultimate advantage of scalp hair.
The crowns of humans' heads were more exposed to severe Sun radiation when they developed to walk erect in tropical Africa, SciTech Daily reported. The brain, being sensitive to heat and generating heat itself, required protection.
The reduction of body hair in humans led to the development of efficient sweat glands for cooling, but sweating resulted in water and electrolyte loss. Scalp hair likely evolved as a passive mechanism to minimize heat gain from solar radiation, allowing humans to stay cool without expending additional resources.
According to the findings, Homo erectus had a comparable physical form to contemporary humans but a lesser brain size roughly 2 million years ago. Brain sizes, however, had reached modern-day dimensions by 1 million years ago.
The researchers propose that scalp hair played a role in releasing the physical constraint that allowed the brain to grow. The study highlights the importance of scalp hair in reducing heat gained from solar radiation, complementing the cooling effect of sweat glands.
The research not only enhances understanding of human hair evolution but also offers practical implications. The use of thermal mannequins provides valuable data that is otherwise challenging to capture.
The study contributes to a broader understanding of the role of hair in human adaptation and highlights the potential applications of thermal mannequins in anthropological research.
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