b9 wrote:
i have no idea why humans have little hair on their bodies, yet their head hair (if left unchecked) will grow to a ridiculous degree and eventually weigh them down to the point of immobility. if humans did not possess exceptional intelligence (the ability to calculate that it should be cut, and the technology(blades) to do so) compared to most animals, then they would be extinct.
why does people's head hair grow continuously ?
Here's a question I actually have an answer to. I took 3 semesters of biochemistry classes in college from a professor whose previous job was doing research on hair for a shampoo company. He addressed every subject in biochemistry from the perspective of hair research. I learned so much about hair and some about biochemistry.
Somebody in class did ask why human head hair grows continuously unlike hair anywhere else on humans and unlike animal fur. This question has nothing to do with biochemistry but if a professor talks about hair for 3 semesters, the question inevitably comes up. He said that human head hair does not actually grow continuously. It just seems that way because the lifespan of each hair is so long and in some people the lifespan is long enough for the hair to reach a couple feet. But actually each hair has a lifespan and the individual hair will fall out when its lifespan is over and a new hair will grow in that same hair follicle. The lifespans are not synchronized so that all the various hairs on a head are in different phases of the lifespan which gives the illusion of continuous growth.
Every person has a different lifespan for their hair strands. This means that for every person there is a length that their hair will grow to and will not exceed because the strands will fall out in an unsynchronized manner. If a person never cuts their hair (and some people never do) it will reach the length that reflects each hair's lifespan and get no longer. With most people this turns out to be somewhere between halfway and all the way down their back.* There have been a few people here and there who have grown hair down to their feet but hair strand lifespan that long is pretty unusual. Hair scientists (like my former biochemistry professor) have been able to chart typical hair lengths because from time to time it is the fashion for women and sometimes men to grow hair until it reaches the end of its' lifespan. This was a common hippie fashion in the US in the late 60's and into the 70's. I remember some men who ceased cutting their hair entirely but couldn't get it past their middle back. One man got it down to his lower back.
He said that it was sexual selection that drove the length of human head hair. Lots of head hair has been considered sexually attractive for millenia, apparently.
*edited to add that for people with kinky hair, the hair grows longer and longer to its lifespan but will seem to grow out rather than down, as in the giant afros of the 60's and 70's.
14 Sep 2012, 10:19 am
