> current day black Africans would have higher levels than the RDA- they wear shorts and tshirts, so they should still have >50% of prehistoric levels.

Shorts and t-shirts? Em, a rather large amount of modern urban African spends most of their day indoors and wears long-sleeved shirt/pants/long dresses/long robe/hijabs/etc.

I'm not sure why one would expect them to have >50% of prehistoric levels.

The paper you linked to (or really, the metaanalysis it cited) showed low D levels were associated with urbanization, but the lowest concentrations observed in northern African countries and in South Africa with seasons. The highest levels were among those practicing traditional lifestyles.