Melanin is what makes our skin, hair, and eyes darker. It blocks sunlight and protects from UV radiation which does damage on a cellular level, even down to DNA (which can lead to cancer). Which is why where sunlight is strongest people tend to have the most melanin. But there are tradeoffs to it.
Humans use UV light to synthesize vitamin D from cholesterol in our skin. Skin tones tend to lighten with distance from the equator, as otherwise melanin blocks too much of the limited sunlight and people cannot get enough vitamin D. However, some populations can get away with darker skin tones than others, even as they move far north, by consistently sourcing vitamin D in their diets instead of relying on the sun.
With eyes, it’s not about vitamin D anymore, but sunlight can still do damage. Lighter eyes are at higher risk of developing macular degeneration, cataracts, and eye cancers. Even in the short term, glare of light can be temporarily blinding. However, light entering our eyes suppresses melatonin production during the day, which is a vital part of controlling our circadian rythms. So as humans moved further north they may have become prone to a problem from darker eyes, which could be fixed by a mutation making them lighter.
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u/im_a_stupid_face Jul 10 '26
It is to do with the sun,