Melanin, the pigment that gives black or dark skin its colour, has a powerful antimicrobial effect that can kill germs as they invade the body.
James Mackintosh, an independent biologist in Sydney, identified flaws in the notion of how dark skin evolved. Although melanin provides some protection against ultraviolet A radiation (UVA), it is ineffective against the more dangerous UVB variety.
Many parts of the body that are rarely exposed to sunlight, such as the genitalia and inner surfaces of the nose and throat, are also heavily pigmented. Dark, melanin-rich skin is found in animals such as gorillas, which are already protected from sunlight by fur and shady forest habitats.
In a study to be published in the Journal of Theoretical Biology, Dr Mackintosh noted that melanin was known to protect against infection in other animals, particularly insects, and sought to discover whether it might have a similar effect in mammals. The first details of his research appear in New Scientist today.
In the laboratory studies, human skin sacs containing melanin inhibited microorganisms, he said. “Melanin is a sticky molecule. The bacteria and fungi get all tangled up and it stops them from proliferating. This effect could explain why darker skinned people are less prone to some skin diseases than those with pale complexions.
Melanin is made from tyrosine, an amino acid needed for many crucial digestive proteins. In cool, wet climates, where food was scarce, tyrosine was needed to enhance levels of these proteins, and not enough was left to produce large quantities of melanin, Dr Mackintosh suggested. In the tropics, with abundant food, tyrosine could be diverted to producing melanin.