The question of why humans possess such sparsely distributed body hair is a central puzzle in human evolution. Our closest genetic relatives, chimpanzees, are covered in a dense coat of dark terminal hair despite sharing approximately 98% of our DNA. This dramatic difference suggests a powerful evolutionary force acted on the hominin lineage after our ancestors moved onto the open savanna. Understanding this transition requires examining the selective pressures that favored its functional disappearance.
The State of Human Hair
Humans are often mistakenly thought to be hairless or to possess fewer hair follicles than great apes. Studies comparing skin samples show that humans possess a density of hair follicles comparable to that of chimpanzees and other primates. The fundamental difference lies in the type of hair those follicles produce.
Apes produce thick, long, pigmented terminal hair that forms a dense coat. Human follicles, by contrast, predominantly produce vellus hair, which is fine, short, and lightly pigmented, making it nearly invisible across most of the body. This evolutionary shift established the biological condition that allowed for subsequent human adaptations.
Thermoregulation and Endurance Hunting
The most supported hypothesis for hair reduction is the need for efficient thermoregulation in the hot, open African savanna. As early hominins moved out of forests and adopted bipedalism, they faced greater solar radiation and higher ambient temperatures. Dissipating heat generated by physical activity became a major factor in survival.
Losing the thick hair coat allowed for evaporative cooling through sweating. A dense coat traps moisture, dramatically reducing cooling efficiency and potentially leading to overheating. Reducing hair thickness allowed sweat to evaporate directly from the skin surface, cooling the blood beneath. This mechanism was advantageous during endurance hunting, a strategy unique to early Homo where prey were chased over long distances until they collapsed from exhaustion.
Alternative Evolutionary Pressures
While thermoregulation is the primary driver, other selective pressures contributed to hair reduction. The “ectoparasite avoidance” hypothesis suggests a hairless body provided a hygienic advantage in social groups. Less hair meant fewer places for external parasites (fleas, lice, and ticks) to live, reducing disease transmission within the group.
Sexual selection is another theory, where hairlessness signaled health and fitness to potential mates. Smooth, unscarred skin advertised low parasite loads and good health, making individuals with less hair more reproductively successful. The largely discredited “Aquatic Ape Hypothesis” proposed hair loss occurred during a semi-aquatic phase, but this theory lacks substantial scientific support. Multiple, smaller selective advantages likely reinforced the initial hair reduction driven by heat dissipation.
Co-Evolution of Skin and Sweat
The loss of a thick hair coat necessitated the simultaneous evolution of two major physiological changes: an enhanced sweating apparatus and protective skin pigmentation. Profuse sweating required a massive increase in eccrine sweat glands. Humans possess an eccrine gland density that is on average ten times higher than that of chimpanzees or macaques, allowing for the rapid secretion of water onto the skin surface.
This cooling system, however, left the skin vulnerable to the intense ultraviolet (UV) radiation of the African sun. The consequence was the rapid selection for increased permanent skin pigmentation, driven by the molecule eumelanin, to protect the exposed skin. Eumelanin acts as a natural sunscreen, preventing UV radiation from destroying folate, a B vitamin necessary for reproductive success, and protecting DNA from damage. The co-evolution of hair loss, high eccrine sweat gland density, and dark skin pigmentation represents a complete evolutionary package that enabled early hominins to thrive in the harsh environment of the open savanna.