>>“... quite dark skin, that changed with time to a lighter skin, as an adaption to the changes in climate and diet of the farmers. Farmers consume much less vitamin D in their diet compared to hunter-gatherers.”
For anyone wondering, the connection with 'vitamin' D (really a hormone) is that low levels mean low fertility, so classic Darwinian selection pressure. D is normally made in a sublayer of the skin - that reaction needs sunlight - melanin interferes. Climate comes into this as people expose less skin as it gets colder.
I hadn't put together the dietary connection, but dairy (even unfortified) and meat have more vitamin D than plant based foods. Interesting that the agricultural revolution played a part in this...