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ANSWERS: 2
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Vitamin D3 is produced from 7-dehydrocholesterol being broken down using the UV energy of sunlight. Vitamin D deficiency carries serious health risks including weakened bones (rickets). Melanin, the pigment that gives darkness to skin, is quite an effective barrier to sunlight. In very sunny climates melanin levels were high because it offered protection against the harmful effects of UV: i.e. DNA damage potentially leading to cancer. However the sunlight is intense enough in these areas that enough UV gets through to permit the production vitamin D3. In less sunny areas too much melanin meant that you couldn't produce enough vitamin D3 because the intensity of the sunlight was much lower so the melanin blocked too much UV energy. At the same time, having a reduced level of melanin carried less risk of lethal DNA damage in these areas precisely because the UV intensity was less. So it became selectively favourable to lose melanin as less was required for protection and having too much led to problems.
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What nucleotideboy said. Whites are built for colder climates. Loads of the first generation of black children in the UK had congenital rickets (a bone weakness) because their mothers' skin was too well protected to be able to create vitamin D under a weaker sun.
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