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
I think you are looking at this from the wrong perspective, I think you are discounting sexual selection as a basis of why humans have long hair. I suspect the advantage is less this helps me to survive, and more this helps me to find a mate.
Another response pointed out the advantages of having dense curly hair and how and why we evolved with that, I think our ancestors used that starting point as a means of signaling health, youth, and vitality. And from there different hair types evolved as an expression of sexual selection.
We evolved to have heads of long thick hair because our ancestors found it attractive. And to this day it continues to be used (subconsciously or consciously) as a means of determining the sexual health of a potential mate.
If you think about it, when we get old or sick the hair is one of the first things to falter, not having a head of long thick hair doesn’t necessarily mean you are unhealthy, but it is also difficult to maintain a head of full shiny head of hair without having a level of health and vitality.
Tldr: I think to this day hair is used as an indicator of sexual health from a potential mate, having long, thick hair can signal that health from a distance, and would be a distinct advantage in spreading those genes.
This is essentially my take. A large part of human morphology is driven by sexual selection.
If you (ie, your genetic code) are looking for a partner to reproduce with, health is likely the biggest factor to consider. You don’t just want someone who is healthy, but someone who has a long history of health. Having long hair allows your potential mates to check health history. So as other humans hair gets longer, if yours stays short, you are essentially saying “I don’t wanna talk about my health history”, which is a turn off.