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Back hair is a vestigal remnant of the early evolution of humans, in which all primates were covered head to toe with hair. As early humans lived in Africa, there was evolutionary pressure to have a fine, wispy coat of hair that didn't hold in as much heat.

As early humans started to dress themselves in the skins and pelts of animals, and later in the woven fibers of plants (think cotton and linen), the evolutionary pressure to maintain a dense coat of hair was removed. Since growing hair is an expensive biological process, humans as a whole evolved towards having less and less body hair. Having hair on your back is simply the expression of a minor gene set that dates back to when early humans were completely covered in hair.

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12y ago

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