Summer World_ A Season of Bounty - Bernd Heinrich [84]
Thanks to the logic of evolution and DNA technology, we know that DNA accumulates gene changes, generally at a steady rate. Therefore, by comparing the number of gene changes between two animals, we can use DNA to determine relatedness, and we can use the changes as a “clock” that tells us when the divergence occurred. The data indicate that head and body (Pediculus) lice had a common ancestor about 114,000 years ago. Logically, these lice would not have diverged to become two species unless they had two different habitats to adapt to. The choice pubic spot had already been taken much earlier by a more distant relative, Pthirus pubis. Stoneking deduced that when we were still fully furry—long before we were human—we had only one other louse habitat, the fur covering the rest of our body. We became naked under the heat in the tropical African savanna, and only the head hair (which has special significance, as I will discuss later) remained as the second ecologically suitable louse habitat. When we came out of Africa about 150,000 years ago, we were probably still naked, but we would not have progressed very far north without wearing clothes. The lice took up residence in those clothes, and they needed different behavior to prosper in that new and different habitat, next to our warm body rather than on the head. A population of the original colonists stayed in our head hair, but the clothes-loving lice diverged and perfected to live in their new habitat. As they adapted, their offspring would have been disadvantaged by being saddled with the lifestyle of their old haunts. Similarly, the head lice would also be disadvantaged by inappropriate lifestyles, so isolating mechanisms evolved, and eventually the two lice could no longer interbreed and the line split into the two species.
More interesting, perhaps, is the obvious question: why were we naked in the first place? If we came out of Africa naked or nearly so, and if the apes’ and our common ancestor was probably hairy, as all apes still are, then why did we become naked? I think the best hypothesis to account for our nakedness is that we derived from a very special ape-man, an endurance predator who depended on rapid and prolonged locomotion in the heat in order to compete with other predators, primarily sprint specialists. We can still compete with cheetahs, lions, and leopards in running down antelope, but we can do it only in the midday heat. And the reason is that we have the mental capacity to pursue a goal that we can neither see nor smell but that we can imagine. Additionally we have a unique suite of adaptations to deal with internally generated body heat under the blazing sun. They include our nakedness, our ability to route blood to the surface of our extremities so that our veins bulge at the surface of exposed skin, and our ability to sweat profusely over the skin. These are capacities needed by hunters who get their edge through endurance in the heat.
A recent review article (Rantala 2007) argues that the cooling hypothesis “does not bear close scrutiny.” Perhaps, if one discounts the zoological perspective: that the predecessors of H. sapiens differed from other hominids in being erect and needing to hunt at noon under the direct overhead sun in order to compete with the large carnivores who rest then. Although feathers and hair on the dorsal surface insulate other desert animals from direct solar radiation, most have “thermal windows”—areas of very thin hair or no hair, as on the bare bellies and flanks of desert antelope, the areas less exposed to the direct rays of the sun. Other examples include the naked thighs and necks of ostriches and the large, heavily vascularized ears of desert jackrabbits and elephants.