Around one million years ago, Homo erectus had already mastered fire. This silent révolution, perhaps one of thé most profound in all of human évolution, did not merely transform our dietary habits or social organization. It left its mark directly on our bodies, down to thé surface of our skin.

Variation in human skin pigmentation in response to UV radiation
Human skin pigmentation varies according to UV exposure, an adaptation directly linked to thé loss of protective body haïr. Crédit: Wikimedia Commons

A 2021 study published in thé Proceedings of thé National Academy of Sciences by Tina Lasisi and colleagues helped revive this fascinating debate. Thé researchers demonstrated that thé tightly coiled structure of Afro-textured scalp haïr provides superior heat dissipation compared to straight haïr, protecting thé brain from overheating under thé African sun. This result sheds new light on thé multiple evolutionary pressures that shaped thé haïr and skin morphology of hominins over hundreds of thousands of years.

Thé paradox of fire and body haïr

Gréât apes are covered in fur. Yet our lineage progressively lost most of its body haïr. Why? Thé dominant theory invokes thermorégulation: walking upright across thé African savanna, early Homo had to travel and run long distances under a brutal sun. Dense fur, effective in shaded forests, became an obstacle to rapid body cooling in open environments.

Fire adds an additional dimension to this hypothesis. A very hairy individual sitting near a hearth faces an increased risk of burns, inflammation, and infection. Flames, sparks, and embers, thèse hazards may have, over millennia, systematically favored less hairy individuals. Biologists Mark Pagel and Walter Bodmer proposed this sélection pressure as a factor that accelerated haïr loss in Homo erectus. In a world without medicine or antibiotics, burn injuries were often lethal, and bare skin heals more cleanly than haïr-covered skin prone to secondary infections.

Homo erectus lantianensis skull at the Natural History Museum London
Homo erectus lantianensis skull at thé Natural History Muséum in London. This species is associated with thé earliest known hearths, dating back nearly one million years. Crédit: Wikimedia Commons

This theory is consistent with thé fossil record. Thé first solid évidence of homininHomininMember of the subtribe Hominina, comprising the human lineage (Homo, Australopithecus, Paranthropus…) but excluding orangutans and gibbons. The term progressively replaces "hominid" in its narrow sense.→ fire use, Wonderwerk Cave in South AfricaAfricaThe cradle of humankind: the continent where the first hominins appeared, then Homo sapiens around 300,000 years ago, before the expansion to the rest of the world.→ and Gesher Benot Ya'aqov in Israel, dates to between 790,000 and one million years ago, when Homo erectus was already fully established across two continents. Burned bones and charcoal fragments recovered from thèse sites attest to progressive domestication of fire, not merely opportunistic use of natural blazes.

Thé eccrine sweat gland révolution

Thé loss of body haïr was accompanied by another remarkable transformation: thé massive prolifération of eccrine sweat glands. Humans possess between 2 and 5 million, distributed across thé entire body surface, a figure unmatched among thé gréât apes, which rely primarily on apocrine glands far less efficient for rapid heat dissipation.

This intense sweating system allows modern Homo sapiensHomo sapiensThe present-day human species, which emerged in Africa around 300,000 years ago, the only surviving human lineage after the extinction of Neanderthals and Denisovans.→ to shed heat with extraordinary efficiency. During intense physical exertion or prolonged heat exposure, particularly near a fire, a less hairy individual richly equipped with eccrine glands would survive and reproduce more successfully than a hairy individual prone to overheating. Computer simulations published in thé Journal of Human Évolution have shown that thé combination of bare skin and abundant sweating supports sustained physical activity far beyond what ancestral fur would permit.

Diagram of human skin layers showing epidermis, dermis, and hypodermis
Thé layers of human skin house eccrine sweat glands, vestigial haïr follicles, and melanin-producing melanocytes. Crédit: Wikimedia Commons

Melanin: when bare skin faces UV radiation

Losing body haïr created a new problem: bare skin exposed directly to ultraviolet radiation. Thé dark fur of our primate ancestors absorbed a portion of harmful UV rays. Without this natural protection, thé epidermis had to develop its own défenses.

This is where melanin comes in. This pigment, produced by melanocytes in thé epidermis, filters UV-B radiation that would otherwise damage cellular DNADNAThe molecule carrying genetic information, used to reconstruct kinship between species.→ and destroy folate, a molécule essential for reproduction and embryonic development. Populations living near thé equator evolved naturally darker skin, rich in eumelanin, precisely to compensate for thé loss of protective fur and maximize solar protection. In their landmark 2000 study in thé Journal of Human Évolution, Nina Jablonski and George Chaplin showed that skin pigmentation intensity closely follows thé global map of annual UV irradiation.

It is remarkable that this adaptation occurred in tropical Africa, thé same région where Homo erectus most frequently used fire, and where solar intensity is highest year-round. Thé two sélection pressures, fire burns and UV radiation, thus acted in concert to shape thé skin characteristics of our direct ancestors.

Campfire at night representing ancestral use of fire
Fire, mastered for nearly one million years, has profoundly shaped human biological évolution far beyond its practical uses. Crédit: Wikimedia Commons

A body built around thé flames

Thé picture emerging from this research is of a human body partly shaped around thé hearth. Not deliberately, of course, but through thé relentless action of natural sélection that, génération after génération, favored individuals whose morphology best adapted to this particular thermal environment.

Hairless skin, abundant sweating, pigmentation calibrated to local light intensity: three characteristics that distinguish modern humans from all other primates, part of whose explanation may lie in thé ancestral hearth where our forebears warmed themselves, cooked their food, and slept in safety for hundreds of thousands of years.

This hypothesis is not universally accepted. Other researchers emphasize thé primary rôle of long-distance running and thermorégulation in open environments, independently of fire. But thèse explanations are not mutually exclusive. Human skin, in all its complexity, is likely thé product of multiple simultaneously acting sélection pressures, of which thé mastery of fire may be among thé most significant.