Why did humans lose most of their body hair?

Scientists do not definitively know why humans lost thick body fur, or exactly when it happened. The most dominant view is the body-cooling hypothesis: as early humans became active in hot, open environments, losing fur let them sweat and cool down more efficiently. A rival idea holds that going furless reduced the burden of parasites. Humans did not lose hair follicles entirely. We have roughly five million, but almost all produce short, fine vellus hair that no longer insulates in any useful way. [1]

This view, also called the savannah hypothesis, ties fur loss to a rising need to regulate body temperature. Humans shed heat mainly by sweating, and a review in the British Journal of Dermatology states that sweat evaporates two to three times more efficiently from bare skin than from fur-covered skin. The same review notes that humans have thermally induced sweat rates four to ten times higher than chimpanzees. It describes the thermoregulatory explanation as the most well accepted. At [bbc.com](https://www.bbc.com/future/article/20230310-why-dont-humans-have-fur), the endurance-hunting version is described: Homo erectus pursuing prey over long distances risked overheating, so less fur would have helped. [2] [1]

A heat-balance model published in PNAS supports the thermoregulatory case for hair loss. It suggests that progressive hair loss was selected because it let hominins stay active in hot, open environments, first around dawn and dusk and later through more of the day as sweating improved. The same model concluded that upright posture probably evolved for non-thermal reasons. [3]

Hair does not mineralize, so there is no fossil record of it. Researchers instead infer timing from skin. A variant at the MC1R gene that produced darker, eumelanin-rich skin arose around 1.2 million years ago, and it is believed to have followed fur loss, since bare skin would have needed protection from the sun. Tina Lasisi, a biological anthropologist, combines this with genetic evidence linking sweat gland and hair follicle development to guess that humans probably lost their body hair between 1.5 and 2 million years ago. This is an estimate, not a firm date. [2] [1]

Mark Pagel, an evolutionary biologist at the University of Reading, argues that the cooling explanation falls short. Humans lose more heat at night than is ideal, leaving us in a kind of constant energy deficit. He also points out that many populations have not done endurance running for tens of thousands of years, yet none have regrown fur, even in very cold regions. [1]

Lasisi replies that overheating would have been a stronger pressure than cold in equatorial Africa. She adds that traits like this can become canalised and hard to reverse, and that by the time people reached cold places they had fire, clothing and other adaptations such as brown fat. [1]

In 2003 Pagel and Walter Bodmer proposed the ectoparasite hypothesis: a furless ape would carry fewer parasites, and biting flies and other pests specialise in living and laying eggs in fur. Pagel says nothing since has led him to question it. The British Journal of Dermatology review also lists sexual selection and the aquatic ape idea among other proposals, but treats thermoregulation as the best accepted. [1] [2]