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?
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More like ELI12 but I hope this helps:
So melanin production or lack thereof isn’t really a matter of climate per se. It’s more to do with intensity of UV radiation due to differences of Sun angle in the sky at different latitudes, which happens to correlate to different climates. However, melanin production has nothing to do with temperature per se.
One thing to understand is the concept of how and why UV radiation is stronger in the tropics. Since the Sun shines directly straight down relative to its angle with the tropics, it only passes through a vertical comumn of air with less distance for that air to absorb said UV radiation.
At higher latitudes, the Sun angle is more slanted, and a ray of Sunlight has to pass through a much larger distance of air due to this slatned angle rathar than straight down, so more UV rays are filtered by the atmosphere, resulting in weaker UV radiation.
UV radiation from Sunlight has the advantage of offering a source of Vitamin D synthesis, but also increases the risk of skin cancer because UV rays have enough energy to mutate DNA.
Melanin aborbs UV radiation from the Sun hitting the skin, which protects the skin from it.
Populations who live in areas with more intense UV radiation have more of an advantage with higher melanin levels, because with more intense UV radiation, it’s easy to get enough vitamin D. It’s more advantageous to block more UV rays with more melanin to reduce the risk of harmful carcinogenic DNA mutations.
In higher latitudes where Sun angles are too low to even be capable of generating vitamin D synthesis in the autumn and winter months, it’s more advantagenous to have less melanin so that you can get all the vitamin D you can during that spring and summer period when the Sun is high enough in the sky for vitamin D production.
It seems like the overall cloudiness of a climate is a factor. Compare how pale skinned ethnic Danes living at mid 50 latitudes in Europe with a more oceanic cloudy climate are compared to Indiegnous Siberians living at even higher latitudes, but with more contintental and much Sunnier climates, tend to be darker skinned than European from cloudy oceanic climates.
TL;DR It’s mostly about the trade-off between getting Vitamin D and skin cancer risk depedning on how much atmosphere UV rays needs to pass through at different Sun angles at different latitudes which affects their intensity.
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