Somesupport for Honesty’s hypothesis.
Why those who migrated northwards apparently lost the more effective vitamin D receptor and lost the more adaptive response to vitamin D deficiency (the PTH response in this post’s linked article) along with the decrease in pigmentation in response to less light exposure is something that I do not get. My confusion remains; its just shifted a bit!
Blacks reflect UV light from the sun away, while, in comparison, whites absorb UV light (which is why you age faster). The photoconversion of cholesterol to pre-vitamin D occurs in the skin and requires UV light. Since blacks require more sun exposure to make Vitamin D (since they reflect UV light away), the body likely accumulates calcium in bones as a protective mechanism to prevent bone loss from Vitamin D deficiency.
Think about it.
Again though the evolutionary process went the other way. The common ancestors of modern Black and modern White populations were presumptively darkly pigmented and lived in areas with lots of light exposure and lots of skin exposed to that light. Vitamin D deficiency is rare in that circumstance.
As sub-populations migrated northwards to areas of less sun exposure and which required more clothing to stay warm vitamin D deficiciency increased. Pigmentation lessened presumptively as those who had less pigment developed vitamin D deficiency less often and were at a selective advantage in that environment.
So Blacks did not develop protective mechanisms - the result from the circumstance of more vitamin D deficiency occurring with northward migration was loss of pigment and aparently loss of what seem to be protective mechanisms.
There is something more to this that I am missing anyway.
Uh, Honesty-
I have always been taught that darker pigments absorb more light than light ones. Tanning results from UV exposure as a protective response. The melanin absorbs the UV, limiting its damage. People who live in high-UV areas have more melanin to absorb and protect them from UV, not reflect it.