https://pubmed.ncbi.nlm.nih.gov/37279270/

Humans are unique among mammals in having a functionally naked body with a hair-covered scalp. Scalp hair is exceptionally variable across populations within Homo sapiens. Neither the function of human scalp hair nor the consequences of variation in its morphology have been studied within an evolutionary framework. The evolution of human scalp hair might be explained by thermoregulation pressures experienced in hot and arid environments. Bipedal posture and a hairless body may have necessitated the development of scalp hair to minimize heat gain from solar radiation, particularly in hominins with large brains. Indeed, humans evolved in equatorial Africa, where the sun is overhead for much of the day; here the scalp and top of the head receive more constant levels of intense solar radiation.

The study, published in Proceedings of the National Academy of Science investigated how that affected the evolution of human hair and scalp morphology.

Researchers used thermal manikin — a human-shaped model that uses electric power to simulate body heat and allows scientists to study heat transfer between human skin and the environment —and human hair wigs at different wind speeds in a temperature and humidity-controlled environment, with and without simulated solar radiation.

They collected data on the convective, radiative, and evaporative heat fluxes to and from the scalp in relation to properties of a range of hair morphologies (none, straight, moderately curled and tightly curled), as well as a naked scalp.

Evidence was found for a significant reduction in solar radiation influx to the scalp in the presence of all type of hair. Maximal evaporative heat loss potential from the scalp is reduced by the presence of hair, but the amount of sweat required on the scalp to balance the incoming solar heat (i.e., zero heat gain) is reduced in the presence of hair. Particularly, the results showed that tightly curled hair provided the most effective protection for the scalp against solar radiation, while minimizing the need for sweat to offset heat gain.

Scalp hair likely evolved as a passive mechanism to reduce the amount of heat gain from solar radiation, that our sweat glands couldn’t, thereby keeping humans cool without expending extra resources.