Also, it’s worth noting that our ancestors have been outside all day every day for millions of years.
It is also probably worth noting that skin color in indigenous populations is extremely correlated with average UV levels in the region, which to me indicates that if you are living in a location other than where your ancestors lived, you can’t really use “our ancestors dealt with this just fine” since it seems the ancestors that didn’t look like the people who evolved in your region failed to breed at much higher rates than the ones who did.
Of course that cuts both ways, since if it were a pure win to be protected from the sun, we would all be dark skinned.
Of course that cuts both ways, since if it were a pure win to be protected from the sun, we would all be dark skinned.
Not necessarily.
We know that humans need sunlight for optimal Vitamin D production, so this does happen to be true. But your logic here doesn’t hold up. Synthesizing melanin is metabolically costly. Cave creatures, which have no use for pigmentation but also incur no obvious direct disadvantages from it, often evolve albinism because of the benefits of repurposing melanin precursors for other needs.
Humans aren’t cavefish, but the principle applies. If you don’t need melanin, making it is a waste of energy, and benefits from sunlight aren’t necessary to explain the evolutionary pressure against pigmentation.
I admit to having missed that footnote, but I think I am intending to make a subtly different point here, which is that not only does a mismatch with ancestral environment undercut the “our ancestors didn’t have sunscreen” argument, but rather the strong correlation between UV intensity and skin color indicates that this is not a weak selection pressure and getting exactly the right amount of sun is probably kind of a big deal.
Especially if it is true that it’s just vitamin D that was imposing pressure towards lighter skin and we can get the same effect that through dietary supplementation.
Sure, but that could still be consistent with “sunburns bad, suntans fine” theory, I figure. Maybe even if our ancestors were outside all the time, they would still sometimes lose their tan during a cloudy week and get sunburned?
I’m definitely open to the possibility that e.g. people of Scandinavian descent living in Nairobi simply cannot accommodate to the UV exposure by tanning, i.e. even if they are as tanned as they can possibly be, they’ll still get burned, there’s just too much UV. If so, then sunscreen (+ clothes, shade, etc.) is their only option to avoid sunburns, and again everyone agrees that sunburns are bad, both immediately and long-term.
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.
It is also probably worth noting that skin color in indigenous populations is extremely correlated with average UV levels in the region, which to me indicates that if you are living in a location other than where your ancestors lived, you can’t really use “our ancestors dealt with this just fine” since it seems the ancestors that didn’t look like the people who evolved in your region failed to breed at much higher rates than the ones who did.
Of course that cuts both ways, since if it were a pure win to be protected from the sun, we would all be dark skinned.
Not necessarily.
We know that humans need sunlight for optimal Vitamin D production, so this does happen to be true. But your logic here doesn’t hold up. Synthesizing melanin is metabolically costly. Cave creatures, which have no use for pigmentation but also incur no obvious direct disadvantages from it, often evolve albinism because of the benefits of repurposing melanin precursors for other needs.
Humans aren’t cavefish, but the principle applies. If you don’t need melanin, making it is a waste of energy, and benefits from sunlight aren’t necessary to explain the evolutionary pressure against pigmentation.
Yup
Well I did note it, see footnote 4 :-)
I admit to having missed that footnote, but I think I am intending to make a subtly different point here, which is that not only does a mismatch with ancestral environment undercut the “our ancestors didn’t have sunscreen” argument, but rather the strong correlation between UV intensity and skin color indicates that this is not a weak selection pressure and getting exactly the right amount of sun is probably kind of a big deal.
Especially if it is true that it’s just vitamin D that was imposing pressure towards lighter skin and we can get the same effect that through dietary supplementation.
Sure, but that could still be consistent with “sunburns bad, suntans fine” theory, I figure. Maybe even if our ancestors were outside all the time, they would still sometimes lose their tan during a cloudy week and get sunburned?
I’m definitely open to the possibility that e.g. people of Scandinavian descent living in Nairobi simply cannot accommodate to the UV exposure by tanning, i.e. even if they are as tanned as they can possibly be, they’ll still get burned, there’s just too much UV. If so, then sunscreen (+ clothes, shade, etc.) is their only option to avoid sunburns, and again everyone agrees that sunburns are bad, both immediately and long-term.
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.