Why are humans hairless? An article in the journal of Zoology: Evolution of nakedness in Homo sapiens by Rantala puts forth quote a few ideas and their associated weaknesses. You can access the whole article in full here. Here are some of the ideas in the article:
The cooling device hypothesis:
We lost our hair to help us cool down in dry savannas. However Rantala argues that in the day time exposed skin actually receives more solar energy. Also increases perspiration which may cause dehydration which is very bad in savanna. He points out that savanna monkeys actually have increased hair coverage which is what we should expect.
The hunting hypothesis (otherwise known as running hypothesis)
This one says that carnivorous hominids would need to go hairless because they have to chase down their prey. Other primates (presumably these savanna monkeys) are vegetarians and don’t have to move as fast. But again, this hypothesis once again assumes that naked skin is really a good way to cool down (there must be evidence that it is if so many accept this.) Also, if this were true, and males were the hunters, why are females even more hairless?
Allometry hypothesis:
Not all organs increase in the same proportions as animals get bigger. Since humans evolved from smaller apes, the hair got sparser and as it did, sweat became the new coolant and fur became useless. The problem with this one is that gorillas hairs are further apart but they have a thick luxurious coat.
The vestiary hypothesis:
Hairlessness evolved alongside bigger brains and culture which allowed us to use clothing to regulate heat. Rantala doesn't like this one either because he believes that it's also based on faulty cooling factor.
Neoteny hypothesis:
Humans are juvenilized apes and hairlessness was just part of that package. Richard Dawkins points out in his new book "The Greatest Show On Earth" that adult humans share many features with juvenile apes. Perhaps hairlessness came with that package? This is unlikely because not all juvenile ape features are beneficial and natural selection has only retained the beneficial ones.
Adaptation against ectoparasites hypothesis:
One problem with a parasite hypothesis is that all apes have problems with parasites. What makes humans so special? Well the answer is that As humans began to establish base camps, fleas and other parasites became a bigger problem. Diseases caused by these parasites would have had strong selective pressure toward naked skin. Alan Rogers (University of Utah) did some research on the evolution of skin color (MC1R gene) and dates darker skin to about 1.2 million years ago. This is consistent with the time we started to occupy base camps.
The parasite idea is the one that the author likes best. It makes sense and I am sure that there is at least some truth to it. However, I believe that Rantala is mistaken about naked skin being a bad way to cool the body. Dennis Bramble at the University of Utah has been researching on this topic for a while. Here is a paper on The Evolution of Marathon Running. Bramble's research shows that on a hot day, a human could out compete a horse in a marathon. I think the problem with Rantala is that he does not account for our specialized sweat glands which are much different than other primates.
I think that there is probably truth to the parasite idea and the running idea. However, I don't think that these things themselves are the whole story. In my previous "tying it all together" post I have talked about Lane's idea that perhaps humans have increased mitochondrial capacity which was selected for to increase lifespan because of the knowledge advantages that elders provide. Raising internal heat generation lowers free radical formation at rest which in turn increases lifespan. If this is true, then this could make overheating a bigger problem. So far, I can't find any literature linking increased mitochondrial capacity to human hairlessness but I think it is certainly possible. This is an exciting possible angle for my thesis!
No comments:
Post a Comment