Curly hair played a role in the evolution of early humans, aiding in their ability to stay cool and conserve water, according to a new study in PNAS.
This adaptation also allowed for the development of larger brains seen in modern humans, thanks to the heat protection.
Early humans inhabited equatorial Africa, where the sun remains directly overhead throughout the day, year after year. In such an environment, the scalp and top of the head are subjected to intense solar radiation and heat.
As early humans evolved to walk upright in equatorial Africa, their heads increasingly faced intense solar radiation, which could be harmful to the brain. The development of tightly curled hair helped reduce heat gain from solar radiation, enabling brain growth without the risk of overheating.
To investigate the impact of this intense heat on hair evolution, scientists conducted experiments using thermal manikins. They discovered that all hair types provide some degree of protection from solar radiation to the scalp. However, tightly curled hair offered the most effective shield against the sun's radiative heat while minimizing the need to sweat to cool down.
The brain is particularly sensitive to heat, and its size corresponds to the amount of heat it generates. Despite humans possessing sweat glands to regulate temperature, these glands are inadequate in preventing potential overheating of the brain. Researchers propose that scalp hair likely serves as a passive mechanism, reducing heat absorption from solar radiation and compensating for the limitations of sweat glands.