Tuesday, January 12, 2010

The Role of Human Hair in Evolution

  • Why is a bearded lady so strange that it warrants being in a circus freak show?  Those with one more x chromosome don't get the same attention.
  • Why when apparently about half of all men go bald are we so ready to spend thousands of dollars on creams, plugs, inplants, toupees, wigs, etc.?
  • Why are we so largely hairless as a species (as opposed to our current day relatives and conceivably our ancestors)?
  • Why do we have hair on our scalp that doesn't stop growing?  Why is that hair so distinctly segregated from the rest of our face?
  • Why do we have little tufts of hair in strategic spots on our post-pubescent bodies?
  • Why is there such variance of hair colors, textures and straightness in different ethnic groups?

To get to the 'why' we'll have to first ask 'how.'  How could they have evolved?

As a review, we'll need to look at this through the lenses of natural selection (it made our ancestors survival advantages) and sexual selection (it gave them reproduction advantages).  With that in mind, let's list out some different hair attributes and consider their potential origins.

Baldness--Allow me to give separate but related example.  Nightingale birds can have as many as one thousand distinct sings.  It's the variety that makes it sexy to the females.  One possible reason that may have come about is that it takes a long time, often years, for a Nightingale to learn that many songs, so a large repertoire of songs means the bird is old.   Old equals sexy.  Why?  Essentially an older bird by displaying age is also displaying how well his genes have sustained him up to that point.  Health+age=good genes.  Isn't it possible that baldness could have evolved as a similar indicator of age/genetic virility?  Just maybe.  Also, as the link below points out, the farther back the hairline the more masculine the appearance of the face.  Could it have been a selection for men that with age become have the appearance of increased masculinity?  Perhaps you've heard that men that go bald have higher testosterone levels.  Baldness could have been used as an indicator to females of men in their status and resource prime that have an accompanyingly high level of testosterone.  
Also, as a runner I know that I sweat the most profusely from my forehead.  Why would I do that?  Because without doing any tests I'd very readily care to bet that it's the site that I shed the most heat from.  Could it be that baldness was a means to shed heat and increase endurance in men for hunting and fighting?  I don't think that's so far fetched.

Forehead Hairline--check out <http://www.bbc.co.uk/science/humanbody/mind/surveys/faceperception2/index_6f.shtml#> for a very interesting test that points out that a low hair line is feminine and a high hairline is masculine.  Could this explain the existence of bangs?  

Pubic hair--puerile joke from my middle school days: what do you get when you turn 3 blonds upside down?  Three brunets.
I might suppose there are really three things we have to explain here: the location of the hair, the color of the hair (that's what the joke was referring to if you didn't get it) and the curl of the hair.  Location--could be related to moisture aeration and creating space between skin on skin contact, which would could help prevent fungi and other nasties from growing.  It also, clearly, shows reproductive maturity (duh) and could be a through back to the preclothing days.  It also interesting to consider how this might be related to the phenomena of the "little black dresses."  Could it be that the dark color similar to body hair triggers an unconcious romantic response?  Who knows.  Or, could pubic hair have evolved to conceal STDs that might have otherwise prevent copulation?

Glabrous Skin--it's probably just a subjective preference that's been reinforced, but it could also have to do with a preference for youthfulness (or just the appearance of youthfulness), which can be concomitant with less genetic mutations.

Facial Hair--I don't have to tell you of its connection with testosterone levels, which can be paired with higher levels of sperm and muscularity, which can be an aid for strength and therefore defense and hunting.  It could also be something similar to the commonly known 'silverback' status.  Not only indicating a certain hormonal balance (and the beneficial phenotypes therein), but also a maturity as well and concomitantly the experience and stability to provide.    Also, a full beard could be conceived to act has a "false jaw."  The shape and jaw size can actually be used to sex a skull and the larger the jaw the more likely it is to be male and to be perceived as masculine.  Increasing the apparent size of the jaw by covering with the prosthesis of a beard can increase the perceived size and therefore masculinity of the face.

Eye Brows--interesting to think that these evolved to emote.  The ones that had more expressive faces got the date.  It also may be to direct sweat away from the eyes as it descends.  It makes sense that only we have eyebrows since we're natures greatest sweaters.

Gray Hair--is gray hair just a break down of pigment production or could it actually be a sign of age.  In going out on a limb here, but there are times when age can be prefered.  Why?  Well, if an organism has lived to a ripe old age this can be an indication of superior genes.  It isn't easy to survive in the wild for long.  If you've done it, then you must have some pretty good genes.  This can be seen in Nightinggale birds that accumulate songs over time have a corresponding amount of appeal increase.  The balance that sexual selection must strike is the attraction of youth and its lack of mutations which can accumulate over time and the attration of age (think Sean Connery, George Clooney, Harrison Ford) to show superior fitness over time. 

Long Hair-- Did you know that hair keeps growing indefinitely and fur stops at a certain length?  So, why are we the only primate that has hair and in only one spot on their body?  ya, it protects from the sun, but I think it is funner to think about how it could have been used as sort of a sanity test.  In the book The Mating Mind Miller talks about how that the human mind can be analgous to a peacocks tail.  It's a costly, difficult to fake, complex structre that demonstrates a number of things--purity of genes (since, for example, half our genome is brain genes--if a huge chunk is good, mostly likely the rest is, too) and ability to overcome what is known as the 'handicap principle,' which means they can sacrifice resources and still remain healthy (the human brain can burn 6-7k calories a day during intense thought, such as chess grandmasters).  It'd work like this: sanity means a good brain, which means good genes and those that are able to keep their hair either long and unknotted or short and well trimmed it would show a proclivity to better brain and therefore better genes and groups that had longer hair on the top of their head had a superior means to other groups for distinguishing quality from poor genes.   It very easily could have turned into a status symbol for healthiness, cleanliness, sanity,  and culture saviness.

Nappy Hair--I’ve heard that it offers lice protection.  They aren’t able to easily move through the dense mass of curcly hair.

Body Hair--I wonder if a certain amount of body hair might actually catch enough sweat to aid cooling.  Meaning, without any body hair our sweat might simply poor off our body without having the evaporative cooling effect.  Hairs catching the sweat a little might cause more evaporation which might cause more cooling.

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