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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Body hair | 3/4 | https://en.wikipedia.org/wiki/Body_hair | reference | science, encyclopedia | 2026-05-05T15:31:24.069114+00:00 | kb-cron |
== Evolution == Determining the evolutionary function of androgenic hair must take into account both human evolution and the thermal properties of hair itself, as well as the variation in androgenic hair between populations, with some populations having none at all. The thermodynamic properties of hair are based on the properties of the keratin strands and amino acids that combine into a 'coiled' structure. This structure lends to many of the properties of hair, such as its ability to stretch and return to its original length. This coiled structure does not predispose curly or frizzy hair, both of which are defined by oval or triangular hair follicle cross-sections.
=== Evolution of less body hair === Hair is a very good thermal conductor and aids heat transfer both into and out of the body. When goose bumps are observed, small muscles (arrector pili muscle) contract to raise the hairs either to provide insulation, by reducing cooling of the skin by air convection, or in response to central nervous stimulus, similar to the feeling of "hairs standing up on the back of your neck". This phenomenon also occurs when static charge is built up and stored in the hair. Keratin however can easily be damaged by excessive heat and dryness, suggesting that extreme sun exposure, perhaps due to a lack of clothing, would result in perpetual hair destruction, eventually resulting in the genes being bred out in favor of high skin pigmentation. It is also true that parasites can live on and in hair thus peoples who preserved their body hair would have required greater general hygiene to prevent diseases. Markus J. Rantala of the Department of Biological and Environmental Science, University of Jyväskylä, Finland, said humans evolved by "natural selection" to be hairless when the trade off of "having fewer parasites" became more important than having a "warming, furry coat". Peter E. Wheeler of the Department of Biology at Liverpool Polytechnic said that quadrupedal savanna mammals of similar volume to humans have body hair to keep warm, while only larger quadrupedal savanna mammals lack body hair, because their body volume itself is enough to keep them warm. Therefore, Wheeler said that humans, who should have body hair based on predictions of body volume for savanna mammals alone, evolved no body hair after evolving bipedalism, which he said reduced the amount of body area exposed to the sun by 40%, reducing the solar warming effect on the human body. Loss of fur occurred at least two million years ago, but possibly as early as 3.3 million years ago judging from the divergence of head and pubic lice, and aided persistence hunting (the ability to catch prey in very long distance chases) in the warm savannas where hominins first evolved. The two main advantages are felt to be bipedal locomotion and greater thermal load dissipation capacity due to better sweating and less hair.
=== Sexual selection ===
Markus J. Rantala of the Department of Biological and Environmental Science, University of Jyväskylä, Finland, said the existence of androgen dependent hair on men could be explained by sexual attraction whereby hair on the genitals would trap pheromones and hair on the chin would make the chin appear more massive. A beard also reduces the risk of a broken jaw during a fight by half.