Head hair is much longer than hair elsewhere on the body. This lengthening has involved several evolutionary changes: faster rate of growth, longer growing phase, increased density, and greater resistance to physical damage (Khumalo, 2005; Loussouarn et al., 2005). The multiplicity of these changes is consistent with sexual selection: the selective pressure seems to have acted on an overall visual effect rather than on one incidental factor. In some non-human primates, head hair has lengthened for apparently similar reasons, perhaps because visual attention tends to focus, as in humans, on the face and its surrounding frame (Darwin, 1936 , p. 906).
Head hair has lengthened only in those human populations that have lived in the temperate and Arctic zones, including some that have back-migrated to the tropical zone, e.g., Austronesians in Southeast Asia and Oceania, Amerindians in the tropical New World. Darwin noted "the extraordinary difference in the length of the hair in the different races; in the negro the hair forms a mere curly mat; with us it is of great length, and with the American natives it not rarely reaches to the ground" (Darwin, 1936 , p. 906).
Because while I've still got some of it... I'm gonna grow it. :)
Old metal head, what do you expect.
Seriously though... I don't know the answer to your question. Evolutionarily speaking... the women seem to love my long hair so maybe it's like a lion's mane... the guy with the best hair gets the girls?
In "The Descent of Woman", Elaine Morgan's book covering her aquatic ape theory among other things, she claims that we lost our fur once we went into the water and gained a layer of subcutaneous fat. Once the fur was gone the babies needed something to hang onto, so our head hair (the one part of our body that was out of water the most) grew long.
I forget the book it was, but the unusual human topiary was described as an early species signal, in addition to the sexual connotations. You can imagine it would be very helpful to identify early humans when we still typically had more substantial body fur. It identifies (clearly) when an adult is past sexual maturity (baldness/turning white). It's since been co-opted to signify all manner of social markers and identifiers, the most basic of which perhaps is gender identity.
Why do horses have manes? It's probably an aesthetic thing, like feathers on birds and bright colors on fish used to attract mates (I have no science on this, just thinking).
On the flip side - thick beards combined with long hair could have a legitimate defensive evolutionary purpose.
I have a chow dog. He's got a think mane of hair like a lion around his neck. I've seen another dog attack him and go for his throat - all they got was a mouthful of hair and he was pretty able to twist out of the bite. Other dogs, like pits, have a lot of extra loose folds of skin around the neck area that provide a similar defense / flexibility when something is trying to bite the throat. I suspect a male lion's mane is pretty effective in protecting their throats during fights.
This is just a simple personal observation. I have had short hair for many years, and decided to grow it out a few years ago. My hair now rests just past my waist and thanks to my Paleo diet, is lustrous and healthy. I get hundreds of second glances from guys (and girls!) compared to a glance every once in a while back in the old days. There's something about long hair that stimulates the sexual appetite of most people, I think. Not sure what the evolutionary advantage is, except perhaps to snare a mate (in one's hair ha ha).
think about where we still have hair? our ape ancestors lost their fur but kept it in vital areas, head hair to keep the head/brain warm, pubic hair to keep our reproductive organs warm, and probably to hold in some scents/feramones on the hairs (released through sweat), just like in our arm pits aswell. We lost our fur because it over heated us and we kept hair where it would be usefull. Like in the ice age, having hair on your vital areas would have been a great natural deffence against the elements. Just some ideas of mine. As for the "Decsent of woman" comment, its all hypothetical but does make sence, i think it requires more evidence though.
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