Why does human head hair keep growing?

[philiplaos]

What is the evolutionary/biological reason for human hair continuing to grow? This site has suggested biological reasons for straight hair, curly hair, hair loss, pubic hair, etc., but not for very long hair. Presumably, early humans would not have been easily able to cut their hair and so it would have continued to grow to the point where it would, one would think, become an encumberance and even dangerous.

 

The only guess I can make is that it has something to do with sexual selection in the same way as peacocks with more and bigger feathers attract more mating partners than others with fewer, shorter feathers, even though the larger feathers make it almost impossible for the bird to fly. Longer hair in humans, therefore, somehow indicating better genes.

 

But that's probably wrong because it doesn't 'feel' right. Any ideas?:(

[stevevall]

This is just off the top of my head, no pun intended. But before hair was combed and cut, over time, a form of dreadlocks or matted hair would have been the result. Also, hair would probably have broken at some point due to the lack of care. I read once that it was speculated that matted hair would have protected the head, kept it warm, as we loose most body heat that way, and would have been a natural pillow. Of course, it would have been a heaven for lice, but people were walking ecosystems for all sorts of critters.

[CraigD]

Answer “why?” questions about evolution is notoriously tricky, other than the safe but uninformative “because it did” answer. To the best of my knowledge, answers to the question of why humans, alone among the great apes, have long head hair, are at best educated guesswork.

 

I’d say the most popular theory – though not IMHO better supported than several others, and not very scientifically respected – is the aquatic ape hypothesis. The gist of its explanation of long head hair and short, fine body hair is that it was selected for during a period when humans spent a lot of time wading and swimming. Some of that explanation has to do with the long head hair/short body hair providing sun protection and insulation of heat-intense head while still allowing good wading and swimming performance. Another it that the long hair allows infants to cling to adults in the water (people who have swum carrying very young children will know how this works).

 

A personal theory of mine – to the best of my knowledge, unique to me – is that early human tools and clothing used a lot of long hair, the safest and easiest to get supply being on our own heads. People with longer hair could make better twine, better tools, better cloths, etc., so were more reproductively successful.

 

Lots of other theories seem about as likely. I don’t think any reasonable theory is much better supported or accepted than another. Likely, the correct explanation is a combination of many explanations.

[Illiad]

So we would look like japanese ghosts and could scare off predators. Sorry, couldn't help it

[Boerseun]

Human hair isn't the same all over. We have follicles that stop growing after a few centimeters, for instance leg & arm hair, chest & pubic hair. They grow to their set length, and then stop. Some of them malfunction, however - I've found a hair on the back of my leg that simply didn't stop growing.

 

So, I suppose that head hair also stopped growing at some convenient length, and body hair was thicker and grew to longer lengths than today. But, possibly as a result of our starting to wear other animal's skins when an Ice Age caused temperatures to drop lower than what our body coverings catered for, the environmental selection pressure for long and thick body hair disappeared - we can now regulate our own temperatures via clothing. And similarly, when technology allowed for cutting hair, long hair suddenly wasn't a problem anymore and faulty follicles that ignored the growth limit wasn't selected against. Matter of fact, they could have been selected for, simply because with extreme temperature change, the head is the hardest part to effectively cover, where the biggest thermal losses take place.

 

This is not to say that it wasn't sexual selection, however. It's merely another possibility. But whatever the case might be, I believe that the selection (be it environmental or sexual) selected for faulty follicles, the only in the mammal order (the only animals with hair) to ignore the growth limit.

 

Funny enough, we have also done the same with sheep. Commercial grade wool-producing merino sheep have been artificially selected by farmers for their faulty follicles also ignoring the growth limit. A merino sheep that goes wild will die in a few years if not shaved, literally choked to death by its wool. And wool farmers have done that to the merino sheep in only a very few centuries.

[Turtle]

as some have hinted, human head hair does not perpetually grow in length. while the terminal lengths differ among individuals it is rather constant for an individual.

 

while head-hair's insulating properties have been mentioned in regard to keeping warm, it also has cooling properties when wet. B)

 

many say prostitution is the oldest profession; i say it's barbering. B)

[Boerseun]

many say prostitution is the oldest profession; i say it's barbering. B)

That's why hairy hookers are so scarce.

[philiplaos]

"the long hair allows infants to cling to adults in the water (people who have swum carrying very young children will know how this works)."

 

Infants would also be able to cling on to hair when our ancestors spent most of their time in trees.

 

B)

[Boerseun]

"the long hair allows infants to cling to adults in the water (people who have swum carrying very young children will know how this works)."

 

Infants would also be able to cling on to hair when our ancestors spent most of their time in trees.

 

:rolleyes:

I don't think hair as something to hold on to has anything to do with it. Monkeys do it all the time; their infants have a perfectly fine grip on their mothers' hair and they hold on when their mothers go swinging through the branches, yet their hair stays short.

[CraigD]

I’m not endorsing the AAH – without paleo-anthropological technology near magical by present day standards, I don’t think such hypothesis can be more than speculation – only interpreting it, but here’s my interpretation of an AAH response to this question.

 

Hominids with modern ape-like hair moves to a coastal habitat, begin spending much of their day wading and swimming. Shortened body hair individuals thrive due to reduced skin/fur problems, but suffer due to increasing infant-losing-its-grip-and-drowning problems. Shortened body hair and lengthened head hair ones, however, get the no-wet-fur advantage without the lost-infants disadvantage.

 

In short, perhaps we have long, extra-graspable head hair to compensate for our lack of graspable body hair.

[Boerseun]

I’m not endorsing the AAH

...neither am I, but it makes for some nice speculation and fireside talk!

Hominids with modern ape-like hair moves to a coastal habitat, begin spending much of their day wading and swimming. ... In short, perhaps we have long, extra-graspable head hair to compensate for our lack of graspable body hair.

Also, it could be the reason why we sweat so much - in order to dump excess salt from a coastal aquatic lifestyle. I do not know if other apes sweat as profusely as humans, if at all.

 

For mammals, we are very sweaty creatures, and rather counter-productively, we lose a lot of electrolytes through sweating. Rather off-topic, but I think that plugs in nicely into the whole Aquatic Ape business. Nothing set in stone, but I do have a bit of a soft spot for this particular hypothesis.

[carlton-temple]

Camouflage. Our early ancestors ability to grow long hair on their heds was a result of natural selection. Those with the longest hair could squat on the ground surounded by their dirty matted hair and thus fool the predatrory dinos into thinking they were a sort of over grown mole hill while the short haired lot got eaten. The millitary still use this technique nowadays but have added rather unconvincing bits of grass and the like. (If soldiers didn't cut their hair so short they could also pretend to be mole hills as of old).

[Illiad]

Camouflage. Our early ancestors ability to grow long hair on their heds was a result of natural selection. Those with the longest hair could squat on the ground surounded by their dirty matted hair and thus fool the predatrory dinos into thinking they were a sort of over grown mole hill while the short haired lot got eaten. The millitary still use this technique nowadays but have added rather unconvincing bits of grass and the like. (If soldiers didn't cut their hair so short they could also pretend to be mole hills as of old).

 

interesting idea LOL

won't the smell give you away anyway?