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Are there hypotheses or conjectures that postulate any selection pressures or benefits that account for the patterns of non-pubic human hair? Particularly:

  1. Humans do not grow notable (i.e., non-vellus) body hair until puberty.
  2. Terminal body hair growth in adults is not only sexually dimorphic but also peculiar in its patterning:

Hair patterns on adult humansSource

As with any trait: it is quite possible that the evolution of human hair patterns had no selective significance. It could also have been the result of a sexually-selective feedback loop, and therefore are an arbitrary sex trait.

I am looking for hypotheses or conjectures that suggest some objective benefit to terminal body hair. I struggle to imagine one particularly because of the sexual dimorphism in adult hair patterns: E.g., if facial hair provides some benefit to men, why would it not provide the same one to women?

If we stipulate that the pattern is only fully expressed in males then it is still peculiar: For example, I can't see any functional characteristic that would make hair on the front torso beneficial while not conferring a benefit if present on the shoulders or top of the torso.

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At least some of it is a side effect of the way human skin develops. Humans actually have the same number and density of hair follicles as great apes, it is merely that our hair develops as short vellum hair rather than the thicker, longer hair of great apes. This is a side effect of the same genes that control eccrine sweat gland development, turning on the genes causes the skin to develop more sweat glands at the cost of reducing hair thickness across the body. For humans, a primarily tropical species, having sweat glands was evolutionarily more useful than being hairy, so we largely traded hair for sweat glands. As a result, the distribution of hair across our bodied may be at least in part a side effect of where it would be most useful to have sweat glands. Hair is typically retained in the armpits and groin because it helps trap odors that may elicit sexual signalling.

Hormones may also play a role. Testosterone has been linked to changes in hair development, and may actually be related to male-pattern baldness.

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