Many people can grow extremely long head hair and facial hair. Are there evolutionary theories as to why this is the case? It seems like having long hair could be a disadvantage, and extremely long hair seems to be rare in other mammals.
A hair follicle is a mammalian skin organ that produces hair. Hair production occurs in phases, including growth (anagen), cessation (catagen), and rest (telogen) phases. Stem cells are responsible for hair production.
Ans: Basic cause of such situation is due to the genetics differences in humans. Thus, the maximum length the hair can grow is determined by the genetic make up.
The hair doesn't keep growing!
It's important to notice that there isn't a set point for the actual hair length. The hair doesn't know that it's been cut for example. Rather, there's a set point for time. A single strand of hair, in humans, on the scalp, will be in the growth phase for several years. The growth cessation takes a few weeks, and then the rest phase will last for another couple of months, until finally the hair strand exits.
I don't think there's a natural advantage for hair of as long a length as many humans can grow it, it may just be little or not at all disadvantageous. The selective advantage, if there is one, probably eminates from sexual selection, similar to why lions grow manes. A display of health and surplus of resources. Hair has a pretty central role in beauty ideals. In some cultures, women even cover up their hair.
There are examples of traits that are disadvantageous to survival in the organism's environment, but shows advantages when it comes to reproduction, to a degree that the genes do not become extinct. I don't remember the name, so take it for what you will, but there's a fish species where the males are either small and stealthy, or they are very flashy. The females prefer the flashy males, but at the same time those males have a harder time avoiding getting eaten by other, larger fish. Thus both phenotypes are passed on to the next generation of fish.
How humans evolved to have head / beard hair that continues to grow longer than other animals is a topic that many anthropologists & biologists are still not sure about & there is no general consensus as to "why" yet.
The three main views that I am aware of however are :
1) Evolution of the "Aquatic Ape." (Ingram, 2000: Morgan 1997; 1982)
2) No real benefit, but used as a tool for "mate selection." (Darwin, 1871; Cooper 1971)
3) Practical evolutionary benefits for the human species specifically
I personally lean 90% "Evolutionary benefits" & 10% "Mate selection" as to reasons why humans' hair continues to grow longer than other animals. Beard & head.
Hope this info was a little helpful. There are copious amounts of other respectable articles on each point, but I only referenced a few.