eli5: If more melanin is advantageous in warm climates, why is less of it advantageous in colder climates? Wouldn’t darker skin still be most advantageous in cold climates where it might occasionally be hot?
In: 1641
Think of melanin as a “shield”.
Melanin protects against UV radiation from the sun, and also determines how much vitamin D we produce, as a result of the sun that does reach us.
On the equator, this is good – there is so much sun so often that we need the melanin to shield us from the UV, and we get just enough sun to make the vitamin D we need.
But in the North, having that level of melanin blocks out all possibility of vitamin D reaching us – it shields too well, to the point we produce no vitamin D.
Therefore, when we live in the North, we reduce our shield against UV, adjusting it according the UV levels we encounter – less UV, less shield needed. That way, we can still make the right amount of vitamin D.
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