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New theory on why
dark skin protects in the tropics
A paper by Sydney-based biologist James Mackintosh, will soon appear in the Journal of Theoretical Biology. According to this week's New Scientist, if Mackintosh's hypothesis is true, then evolving skin colouration should correlate with past temperature and humidity rather than latitude or exposure to sunlight. A popular theory on why darker skin prevailed in some areas and lighter skin in others, is that the extra melanin in darker skin protects against cancer and sunburn from ultraviolet radiation. But, New Scientist reports, some parts of the body which are hardly ever exposed to sunlight, such as genitalia, throats and nasal passages, are packed with melanin cells. And animals such as gorillas have dark skin even though they are covered in fur and live in shady forests. What's more, melanin has been shown to be a poor sunscreen that doesn't protect well against UVB radiation. The magazine reports that James Mackintosh realised that in some creatures melanin forms a capsule around invading pathogens, protecting them against disease. "My PhD was on insect immunology, and everyone knows that melanin is an important antimicrobial in insects," he says. "But it seems no one has ever suggested it would play the same role in vertebrates." In mammals, melanin is contained inside vesicles called melanosomes. Larger, more numerous melanosomes make for darker skin. Mackintosh suggests melanosomes might act like lysosomes in the immune system, which engulf invading microorganisms and use enzymes to kill them. In laboratory studies, melanosomes from human skin can inhibit microoganisms, says Mackintosh. "Melanin is a sticky molecule. The bacteria and fungi get all tangled up, and it stops them from proliferating." Also, a protein called attractin is known to regulate both melanisation and immunity in humans, suggesting a link between the two. He also points out that darker-skinned people are less likely than people with fair skin to develop serious skin diseases. During the Vietnam war, for example, American soldiers from a variety of racial backgrounds were sent into the Mekong Delta. White soldiers were three times as likely to contract "jungle sores", a skin disease caused by Streptococcus pyogenes, compared with their black comrades. Mackintosh's hypothesis is "a very good bet", says Anders Møller, an evolutionary ecologist from the CNRS, France's centre for scientific research in Paris. "It solves a lot of problems with these other theories." It also explains why we don't all have black skin. Melanin is made from the amino acid tyrosine, which is also needed to build proteins. In prehistoric days when food was scarce in cold, dry areas, tyrosine was probably conserved to make essential proteins, Mackintosh says. It was only worthwhile converting it into extra melanin in the warm, damp tropics where food was abundant and pathogens were rampant. More
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