rationalskepticism.org

Papa Smurf wrote:

The_Piper wrote:My reply meant to point out that it's absorption of vitamin d, not skin cancer, which is the more important factor in skin color afaik (I may be half wrong).

Maybe the selective force depends on the direction you're going. I suppose I could use my Google Fu before theorizing but where's the fun in that . So let me theorize:

• Supposedly furry hominids (hominins?) started out having fair skin (like some but not all other mammals). Fur provided ample protection against UV/skin cancer so fair skin was fine. But how did we (and other furry animals) get our vitamin D with our fur supposedly blocking a lot of the UV we need for synthesizing that in our skin? Just from food intake?
• We lost our fur. Driving force: ? (use Google Fu for theories)
• Our skin got darker. I'm pretty sure the selective evolutionary selective force here was mainly much needed protection against the sun to prevent skin cancer, not to reduce the amount of vitamin D produced in our skin
• Some of us went north to colder regions with much less sun.
  Note that this group did not grow back fur, perhaps because we used clothing instead. This is similar to what I said about how the use of suncreen might prevent re-evolving dark skin if Europeans migrated back to Africa. So we now have two instances where our own 'intelligent' actions prevented 'natural' evolution.
  This group of humans developed lighter skin, driving evolutionary force the need to synthesize more vitamin D in this case
  If we had grown back hair up north, perhaps it would have been transparent like a polar bear's, to let through the sunlight to make vitamin D
• If this group went back to Africa and did not use sunscreen, protective clothing, medical science etc and lived outdoors I suspect the main evolutionary force would be the same as in the original case when we lost our fur: to protect against the sun, not to reduce the amount of vitamin D produced in our skin.

Sounds plausible to me. Skin cancer would have to regularly be contracted in the first 20 years of life for it to be the major driving force for dark skin, would be my guess. What if too much vitamin d is toxic? Google Fu time.

ETA - a few minutes reading articles suggests that the skin cancer idea is purported by some scientists who know a hell of a lot more than I do. If their only basis is a study of African albinos, then I say " ". Caucasians aren't albinos. Also one of the articles wondered why a certain gene associated with light skin was found in east Africans. To which I say " " again. This time I scratch my head because most articles point out that there is more than one gene associated with light skin, Caucasians are more closely related to east Africans than west or south Africans, and our ancestors had light skin before losing their hair. Oh well. These people know a hell of a lot more about genetics than I do. That's not hard to accomplish either, I know very little.