An Australian researcher suggests dark skin could be better than white at fending off fungi and bacteria. A paper by Sydney-based biologist James Mackintosh says it could explain why dark skin evolved in humans and animals living in tropical environments.
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.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.
"Melanin is a sticky molecule. The bacteria and fungi get 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.
It also explains why all do not 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.