DARK SKIN AS A NATURAL SUNSCREEN

One of the unique characteristics of human skin is that it varies in colour. The concentration of darker hues closer to the equator and increasingly lighter ones towards the poles suggests a relationship to solar processes. That skin colour is somehow related to the intensity of the sun has been appreciated since Aristotle formalised the climatic theories of Hippocrates and Herodotus in the fourth century BCE. People with darkly and lightly coloured skin differ in the amount of visible light that is actually reflected from the surface of their skin, referred to as skin reflectance. The colour and reflectance properties of human skin are mostly due to pigments, with melanins being the most important of these. Eumelanin is an intensely dark brown pigment in its pure form, and is responsible for most of the range of colouration seen in human skin. Phaeomelanin is the yellow-red form of the pigment that is present in smaller amounts in the skin of most people, but is most evident when concentrated in the freckles of people with lightly coloured skin.

The range of skin colours 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, which are born with uniformly pale skin. After a few years of life, the glabrous skin on the face and hands darkens after exposure to the sun so that an infant's face becomes mottled and oft en becomes uniformly dark. The skin of the rest of its body, protected from the sun by hair, remains pale.

The ability to produce eumelanin pigment in specialised cells in the skin, called melanocytes, in response to ultraviolet radiation (UVR) is a characteristic shared by humans and their closest relatives, the apes and Old World monkeys. We can infer that this condition was present in the common ancestor of chimpanzees and humans approximately seven million years ago (Ma).