BLACK skin could be better than white skin at protecting against disease,
according to an Australian researcher. This enhanced immunity could explain why
dark skin evolved in humans and animals living in tropical climes.
No one knows why darker skin prevailed in some areas and lighter skin in
others. A popular theory is that the extra melanin in darker skin protects
against cancer and sunburn from ultraviolet radiation. But 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鈥檚
more, melanin has been shown to be a poor sunscreen that doesn鈥檛 protect well
against UVB radiation.
James Mackintosh, an independent biologist based in Sydney, realised that in
some creatures melanin forms a capsule around invading pathogens, protecting
them against disease. 鈥淢y PhD was on insect immunology, and everyone knows that
melanin is an important antimicrobial in insects,鈥� he says. 鈥淏ut it seems no one
has ever suggested it would play the same role in vertebrates.鈥�
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In mammals, melanin is contained inside vesicles called melanosomes. Larger,
more numerous melanosomes make for darker skin. Now Mackintosh suggests in a
paper to appear in the Journal of Theoretical Biology that
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. 鈥淢elanin is a sticky molecule. The bacteria and fungi get all
tangled up, and it stops them from proliferating.鈥� 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 鈥渏ungle
sores鈥�, a skin disease caused by Streptococcus pyogenes, compared with
their black comrades.
Mackintosh鈥檚 hypothesis is 鈥渁 very good bet鈥�, says Anders M酶ller, an
evolutionary ecologist from the CNRS, France鈥檚 centre for scientific research in
Paris. 鈥淚t solves a lot of problems with these other theories.鈥�
It also explains why we don鈥檛 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
rampant.
If Mackintosh鈥檚 hypothesis is true, then evolving skin coloration should
correlate with past temperature and humidity rather than latitude or exposure to
sunlight. Those studies haven鈥檛 been done, says Mackintosh. He also hopes
researchers will examine melanin鈥檚 role in protecting skin against microbes.
M酶ller says it could have a significant effect, 鈥渟o doctors will take notice鈥�.