Darker skin healthier?
Many people think that dark skinís advantage in tropical climates is purely because extra melanin protects against cancer and sunburn. But melanin does not protect well against the most damaging short-wave ultraviolet (UVB) radiation.
In insects, melanin is known to protect against attack from disease, by engulfing and killing invading micro-organisms. Researchers now suggest that melanin might have the same function in humans. This would help explain why the throat and nasal passages, mostly hidden from direct sunlight, are packed with melanin cells. And it might also explain why fair-skinned soldiers in humid jungle environments are three times more likely to contract serious skin diseases than their darker-skinned comrades.
On the other hand, melanin is made from the amino acid tyrosine, so in cold areas with fewer skin diseases and scarcer food, it could be an advantage to have less melanin.
New Scientist, 28 April 2001, p. 7.
Note that the same principles as in the book One Blood still apply, but the strongest selective factors may be temperature and humidity rather than sunlight strength, although the latter may still be important in pale-skinned peopleís avoidance of rickets in high-latitude areas.