Modern humans evolved in Africa, but we've been living in Europe and North America for almost 50,000 years, very far from the equator. Native Americans living in higher latitudes were much darker than their European relatives, yet we can assume they weren't deficient in Vitamin D, otherwise selective evolution would have produced lighter skin tones. Some think that light skin came about only with the advent of agriculture (12,000 years ago), as food sources in the hunter-gatherer diet provided supplemental Vitamin D even in higher latitudes.
I think it's very safe to assume that people living in higher latitudes (and their descendents) are adapted to a seasonal rise and fall in serum D levels. There were no supplements in 50,000 B.C., and while hunter-gatherers' food sources would have provided much higher levels of vitamin D than a modern diet, it couldn't have been more than a few thousand IUs daily. Vitamin D is fat-soluble and hunter-gatherers could have been producing upwards of 20,000-40,000 IU from the sun daily for 5-6 months out of the year, so it would make sense that we can build up stores of vitamin D to access during the "vitamin D winter."
I'm lucky in that I'm able to get an hour or two of full-body (including torso) sun exposure year round (if I want to; I usually put a shirt on in December). I live around 39 degrees latitude so I might experience a vitamin D winter for 2-4 months, but I'm of European descent and I think the vitamin D I produce during the spring, summer and fall is enough to hold me over. I take FCLO during those months as well, but that's only a couple thousand IUs - again, I'm (hopefully) relying on vitamin D stores.
If you work indoors and aren't able to sunbathe on your lunch break, you're in a tough spot. Food supplements don't have very high levels of D, but supplemental D is also a bit iffy. Every day we find out that some vitamin's dangerous to supplement with (but fine if you get it naturally or in food). Maybe move to Africa?