Human skin pigmentation as an example of adaptive evolution.

HUMAN SKIN is remarkable and, in several ways, unique. It is mostly naked, it is sweaty, it is tough yet sensitive, and it comes in a range of colors. The plentiful sweat glands in human skin secrete watery fluid that evaporates and helps to cool us when we are hot, and the outer surface of our skin is rich in specialized keratin proteins that help the skin to resist physical and chemical insults while remaining exquisitely sensitive to touch. The pigmentation of human skin is of greatest interest here. The array of colors found in human skin is greater than that seen in any other mammalian species.People with darkly and lightly colored skin differ in their skin reflectance, the amount of visible light that is actually reflected from the surface of their skin. This difference can be visually striking and has spurred questions for several centuries as to how variation in skin color came about. One of the problems in studying the evolution of human skin is that it is absent from the fossil record. It generally decays soon after death, leaving no traces of its composition or color behind. Evolutionary biologists seeking to understand the evolution of skin must thus resort to other means, including comparative anatomy, physiology, and molecular genetics.The range of skin colors found in humans contrasts with the condition seen in our closest primate relatives, which have light skin covered by dark hair. This condition is typified by our closest living relatives, chimpanzees (fig. 1), which are born with uniformly pale skin. Glabrous skin on the face and hands darkens after exposure to the sun so that, after a few years of life, the infant's face becomes mottled and eventually becomes uniformly dark like that of its mother. The skin of the rest of its body, protected from the sun by hair, remains pale. The ability to produce melanin pigment in specialized cells called melanocytes in the skin in response to ultraviolet radiation (UVR) is a characteristic shared by humans and their closest relatives, the apes and Old World monkeys (Jablonski and Chaplin 2000). We can infer that this condition was present in the common ancestor of chimpanzees and humans six million years ago (Ma). Reconstruction of the nature and likely appearance of the skin of the fossil relatives of humans - generically referred to as hominins - is possible using a wide range of methods and techniques available to comparative biologists, including comparative and functional genomics. One of the best-known hominin species is Australopithecus afarensis, a species that lived in eastern and northeastern Africa between 3.6 and 2.9 Ma and became widely known through the well-preserved fossil partial skeleton nicknamed "Lucy" (AL 288-1) from the site of Hadar in Ethiopia. Reconstruction of the posture and locomotion of the species from its limb proportions and joint anatomy indicates that, although bipedal, A. afarensis was more of a deliberate walker and climber than a long-distance strider and runner (Jungers 1988). Its normal range of activities was probably closer to that of modern chimpanzees than to that of modern humans, and this generally would not have resulted in individuals' building up excess heat as the result of prolonged activity. We can infer that members of this species possessed skin similar in anatomy and appearance to that of modern chimpanzees (fig. 2). The timing of the loss of body hair in hominins can be inferred by similar means: examination of the skeleton and reconstruction of the mode and intensity of locomotion. The uncommonly well-preserved skeleton of an early member of the genus Homo from the site of West Turkana (WT-1 5000), aged about 1.5 Ma, indicates that functional modernity in gait and activity pattern had been achieved by this time, specifically, that they were engaging in extended bouts of vigorous exercise, which would have resulted in the build-up of excess body heat (Bramble and Lieberman 2004). These hominins lived in hot equatorial environments, and had to cope with high external heat loads in addition to those generated by elevated metabolism. …