I think our ancestors lived in the full sun of the African savannah a lot more than the forest, and even in northern Europe, hunter gatherers were killing meat-on-the-hoof (mammoth, wild horse, etc.) so they were running about in the open a lot.

However, the change in exposure is surely true. I'm just not at all convinced that complete shielding from UV is a better choice than occasional UV.

Until recently, children would run around outside a lot during the summer, even in urban environments, and especially in suburban or rural ones.