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Why do hair continuously grow on human heads, while the same doesn't happen on other parts of the human body? Where there are hair, they only usually grow to a fixed length; but they seem to have no limit (or at least a much higher limit) as long as the head is concerned (and the face, in men).

Why does this only happen on humans? When did it start happening? What kind of evolutionary advantage could there be in overly-long head hair, so that humans have them while other primates and mammals don't?

I know there are various questions around about why humans have much less body hair than other mammals; mine is related but different: why are we the only species with so much hair on our head, to the point that they might even become inconvenient if we don't cut them?

Massimo
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    There is a limit on hair growth - http://biology.stackexchange.com/questions/1267/why-can-hair-grow-without-limit-while-eyebrow-cannot/1273#1273

    As for the rest of your question, we still haven't found the "missing link" in human evolution, so we can't really say "when" because we don't know the stages in-between. As for advantages and disadvantages, well, that seems like it might be more opinion based as I don't know if we can ever know for sure "why".

     – SolarLunix Jun 26 '15 at 17:28
  • Thanks for pointing that out; however, even if waist-long hair is the limit for most people (and I know for sure there are exceptions to this rule), the question still stands: why does this only happen on human heads? – Massimo Jun 26 '15 at 17:30
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    @SolarLunix there is no such thing as the "missing link". It's a fallacy dating back to the ancient Greeks, and used by creationists to attempt to disprove evolution. Look it up. – MattDMo Jun 26 '15 at 17:32
  • @Massimo Actually, it doesn't really only happen in humans. If you look at a lot of long hair breeds of dogs, there are some that need CONSTANT attention or their coats will become too long and get in their way. As for why it started, that's more of an opinion based thing, it could've been a mutation, or it could've started and continued due to a mating preference. – SolarLunix Jun 26 '15 at 17:34
  • @MattDMo I wasn't aware, thanks for pointing that out. Is there any research/evidence on how humans evolved? I mean I've read a bit about Neanderthals and other species in the phylum Homo (such as ones who were amazing runners), but I haven't read much more than that. – SolarLunix Jun 26 '15 at 17:36
  • @SolarLunix check out https://en.wikipedia.org/wiki/Timeline_of_human_evolution and https://en.wikipedia.org/wiki/Human_evolution for starters. And, BTW, Homo is a genus, not a phylum. – MattDMo Jun 26 '15 at 17:53
  • @MattDMo sorry, I'd change it but it won't let me edit that comment. Sometimes I get my terms mixed up (thanks for the two corrections) – SolarLunix Jun 26 '15 at 17:55
  • This post should give some insights on the mechanism of hair length control. – WYSIWYG Jun 26 '15 at 18:08
  • Suggested edit by user @JustinMasters: "I would like to suggest taking a different approach towards asking questions that have to do with science. Rather than ask why something happens, try to ask "how" or "what" questions that can provide you with the the information you seek. "Why" implies reason or purpose in such a way that one can get comfortable thinking about things the wrong way, especially when you ask "why" in a physics or chemistry class. In zoology, most why question about an animal's traits can be answered with fitness to compete for resources or mates." [cnd..] – fileunderwater Mar 20 '18 at 21:06
  • [cnd..]: "The more interesting information comes from trying to figure out the how. How do the genes / gene expression for hair and fur differ such that hair will continue growing unchecked while fur will grow to a specific length?" – fileunderwater Mar 20 '18 at 21:07
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    The above comments were a suggested edit to the Q, but I think they fit better as comments, since the edit wasn't a rewrite of the Q but rather suggestions or an alternative perspective. – fileunderwater Mar 20 '18 at 21:10

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