Brighter skin = more Vitamin D from a given amount of sunlight
Darker skin = better protection against skin cancer
So in an area with less sunlight, evolution leads to people with brighter skin color and vice versa. In the modern world where people are mobile across continents, they need better information about technology that can help against both problems (like supplements and protection against skin cancer).
My best guess would be that the sun breaking down folate (Vitamin B9) would be more of a reproductive fitness disadvantage especially during pregnancy compared to the very few skin cancer cases late in life. I don’t have strong evidence either way though. Probably both effects are roughly within one order of magnitude? Also light skin could be faster selected for than dark skin since vitamin D is so important. As Carl Feynman notes, there could be other biochemistry stuff.
I may be wrong here, but IIRC the trade-off is:
Brighter skin = more Vitamin D from a given amount of sunlight
Darker skin = better protection against skin cancer
So in an area with less sunlight, evolution leads to people with brighter skin color and vice versa. In the modern world where people are mobile across continents, they need better information about technology that can help against both problems (like supplements and protection against skin cancer).
My best guess would be that the sun breaking down folate (Vitamin B9) would be more of a reproductive fitness disadvantage especially during pregnancy compared to the very few skin cancer cases late in life. I don’t have strong evidence either way though. Probably both effects are roughly within one order of magnitude? Also light skin could be faster selected for than dark skin since vitamin D is so important. As Carl Feynman notes, there could be other biochemistry stuff.