Before getting into this one, I want to make it clear that this is not an invitation to debate the merits of the theory of evolution. For the purposes of this post, it is assumed that evolution, specifically that humans evolved from ancient apes (NOT modern apes--humans are not descended from any modern ape). As for the "it's only a theory" argument, a theory is as close to fact as you can get in science. We don't discount Einstein's theory of relativity or Newton's gravitational theory because they are "only theories", so neither should we reject the theory of evolution on those grounds. If evolution wasn't as close to fact as science will get then it would be the conjecture of evolution, not the theory.
Anyway, continuing on now from the assumption that evolution is a fact:
Do you ever wonder why, if our distant ancestors were hairy apes, we have lost almost all of our body hair? Even stranger to me is how long the hair on our heads grows; I know of no other animal that grows the hair on its head to such ridiculous lengths. (If there is one, please enlighten me.)
In pondering this, we must understand that natural selection does not select for any traits. In fact, it selects against traits that are reproductively disadvantageous. Two big words--maybe I should clarify: suppose there are two possible traits in a critter, a red chest or a brown chest. Further suppose that, all other things being equal, those critters with a red chest have a much worse time hiding from predators and, therefore, tend to die younger (some even before they have a chance to reproduce at all) than those with a brown chest. Natural selection is at work here, selecting against red-chested critters, giving you more descendants of brown-chested critters, possibly until there are no more red chests in the critter population.
Going back to body hair, the question I find myself asking is, "why was it a disadvantage for our ancestors to have thicker body hair?" All other land mammals seem to get along just fine with more body hair in all environments. What was different about human ancestors? There is the possibility that the mutation that causes less body hair just never occurred in any other evolutionary line, but that seems unlikely to me. And why is it that we still have thick hair in certain places? What's special about an arm pit that the "less body hair" gene left that alone?
Along the same vein, why do women tend to have less body hair than men (even when they don't shave it off)? Clearly, some evolutionary force selected against women with much body hair. Nowadays, that could be seen because most men (in our culture, anyway) are more attracted to women with little body hair, but is that what started it all? If that was the selective factor, where did that attitude come from in the first place?
A similar question goes for head hair: "what was the disadvantage for not being able to grow long hair on the head?" Those who have had long hair can tell you the disadvantages of same: it takes a long time to dry; it gets tangled and is hard to maintain; it can get caught on things (which seems like a distinct disadvantage to a primitive ape to me); etc. Perhaps the two things are related; longer head hair had to come with less body hair for some reason (or vice versa).
Finally, why do men tend to grow facial hair, but women don't? (I'm speaking in generalities here, and not talking about a little "peach fuzz" on the upper lip.) What possible evolutionary factor could there have been to select against males with no facial hair, but against females with facial hair? Perhaps, again, we're looking at what specific mutations did or did not occur, but why affect males and females differently? Maybe it has something to do with the second X chromosome, seeing as the Y chromosome doesn't have as many genes on it. (I do know that this is the source of some male-female differences, such as pattern baldness, which is a recessive gene but occurs on the X chromosome and not on the Y. A woman with the gene might well have the dominant gene against pattern baldness on the other X chromosome. That's an oversimplification, I'm sure, but it's the basic idea as I understand it.)
I'm afraid I don't have the answers to any of these questions, but they have been occurring to me off and on for years. Please feel free to educate me if you know, or even share if you have a theory. I'd love to know what other people think, especially about the reduced body hair question.
Hey, not all my posts can be deep and meaningful. :-)
And even though I teach this stuff, I do wonder how the genes for baldness got selected?
ReplyDeletewow, this one makes my day !
DeleteAlex, baldness is an indicator of having more testosterone. Maybe bald dudes were better providers?
ReplyDeleteScott, it's been suggested that heat has driven some aspects of human evolution. We did start out in some very hot places (Africa). Humans stand upright and expose planes (chest and back) to the environment to help keep cool.
In Descent of Man Darwin noticed that head hair lengthened in human populations in northern climates, including some that have migrated back to tropical zones but those who remained in the tropics had shorter hair.
"...the extraordinary difference in the length of the hair in the different races; in the negro the hair forms a mere curly mat; with us it is of great length, and with the American natives it not rarely reaches to the ground..."
Crinkly (nappy) hair acts as baffles and wick heat away from the head faster than straight hair.
Darwin also noted that head hair in some non-primate species have also lengthened apparently also for similar reasons.
So...
Individuals who could stand upright may have been naturally selected as being better providers (can walk faster and simultaneously carry more stuff if you're upright) and loss of body hair may have evolved as a way to keep the upright walkers/runners cooler. Growing longer hair on the head seems to be a selected sexual characteristic (longer hair frames the face better?) in populations who lived for extended lengths of time in temperate and arctic regions.
That's my blathering for now.
The idea that we lost our body hair to keep cool has its merits, especially when I remember that sweat glands are essentially converted hair follicles (which isn't most of the time, for some reason). But why didn't any other animal living in the same warm environment do the same (or at least something similar)? It seems strange to me that such a mutation would occur in the genetic pool that led to humans and not in any other species.
ReplyDeleteThe head hair explanation makes perfect sense. I can even see longer hair in colder climates as a survival characteristic, though many humans are able to grow their hair longer than even that would require. As I think about it, perhaps it became a selected characteristic because it was a survival trait and whatever culture existed at that point saw it as attractive as a result, leading to the hair getting longer than strictly required.
Just a thought after about 3 hours' sleep. I hope it makes sense to those of you outside my head.
This is what I think (any evolutionary biologist out there please correct me if I'm wrong):
ReplyDelete1) Early humanoids evolved in warm areas.
2) We were gatherers.
3) To better gather and carry food to supply our growing brains, we eventually started to stand up and walk on two feet. Those who could stand up better and walk farther were naturally selected because they were better providers.
4) We began losing hair as a means to keep cool our upright bodies that could now walk long distances, especially in the hot savannas of Africa.
5) If other primates in hot areas with constant direct sunlight got up off their knuckles and began going on extended hikes or sprinting like Usain Bolt, maybe over a long time they'll lose their hair too.
Oh, here we go!
ReplyDeleteSeveral hypothesis including keeping upright bodies cool to reduction of parasites when living in fixed locations.
http://www.the-scientist.com/?articles.view/articleNo/31478/
Interesting article, Kay. I like the fact that it also seems to explain why other animals didn't lose their hair, which is something that has bothered me whenever pondering this particular question.
DeleteI'm not sure I buy the "parasites" theory--many animals live in large groups in relatively fixed locations and have to deal with these parasites. It is true that parasites are easier to spot and pick off (and, in fact, are less likely to be picked up) when you don't have a body covering of thick hair, but that seems more like a side-effect to me. It might be a reason for not regaining the hair in northern climes, especially if other ways of staying warm were in effect by the time our ancestors migrated to northern Europe and Asia, but I doubt it's a primary reason. Then again, what do I know? :-)