Most mammals seem fine with shorter hair (usually denser though). The only other case that comes to mind of very long hair restricted to a specific body area is that of horses manes, which to be fair I’m also not totally sure what purpose it serves. Many equines seem fine with shorter manes and tails that don’t have such long hair, like zebras. They swish flies away all the same.
Some animals have long hair but it covers most of their bodies (like llamas or yaks maybe), it’s not restricted to a small area. Other animals have denser or longer hair in some areas, like lions, but this serves a purpose (protection of the neck and head) and even then the length ratio between these longer hairs and the rest of the fur isn’t as skewed as that of humans.
So, why? I get that hair on the head helps protect it from the elements and sun, but why so long? Some humans can grow hair longer than their own body length, which is remarkable, and without doing any fact check I’d say we are probably among the top 5 species with longest hairs ever. Is it just a showoff feature like a peacock’s tail feathers, an indicator of overall health? Or does it serve another function as well?
I didn’t mean to type this wall of text…, thanks for coming to my TED talk
There are two different things coming into play here.
First of all, hair length evolved before long hair did.
Modern humans originally evolved tightly curled hair. Basically like we still see in many African populations. It’s thought that this was an adaptation to protect against the heat of the sun. Basically like an insulated sun hat. The longer this curly hair grew, the more protection our natural hat provided.
As homo-sapiens populations moved further North, this protection was no longer needed - in fact there was the opposite problem, it was cold and rainy.
Greasy straight hair offers an advantage over curly hair in this kind of environment. It acts like a waterproof blanket, preventing the skin beneath getting wet, and it dries more quickly. Heat is lost through wet skin significantly faster than dry skin, and in situations where energy sources might be hard to come by in winter months, this can be a disadvantage.
We already had hair length sorted, so it was simply a matter of reacquiring the straight hair shape.
Europeans got a leg-up in this regard - Neanderthals appear to have had straight hair, and interbreeding definitely occurred. At this point it’s worth remembering that by the time anatomically modern humans evolved in Africa, earlier hominids were already living all over Africa, Europe and Asia. Neanderthals were very similar to modern humans and shared an extremely close common ancestor.
And this is how, and why, some homo sapiens populations have long straight hair.
Aha, this answer is satisfactory. You introduce the concept of greasy long hair. At first I thought, well why not just greasy fur then? Why only on the head? But then I remember humans and probably early hominids like to wrap themselves with things like other creatures pelts and the head is probably the only place you can actually grow hair from. And if greasy it’s like a sort of raincoat thing.
I can sleep now thanks