You hit the nail on the head!
Another way to look at it is that as humans migrated north into Europe they got a lot less sunlight (less intense sun, plus the blocking of sunlight from wearing animal hides for warmth). We need sunlight to produce Vitamin D, and we need vitamin D to absorb calcium through the intesinal wall.
Just like you said, as humans went into Europe, they suffered Vitamin D and therefore calcium deficiencies-- so it's no surprise that our corresponding fossil records in Europe are full of paleopathologies for rickets (which causes bones to bend). The people who happened to have lighter skin (less melanocyte pigment) had a selective advantage, since they got a bit more sunlight than those with darker skin. So with progressive generations this particular genetic makeup was favored.
One interesting thing is that Vitamin D (along with Omega 3's etc) is also found in fish oils. You mentioned or alluded to that.
(It's also found in milk).
So when we examine archeological remains across European sites, we see that the rickets significantly drops at precisely the same time that fishing implements are found.
Eventually, following the last ice-age when domestication of wild goats allowed milk to be available, we see rickets almost disapperar completely.
So if you're doing the night shift, make sure you either drink milk or eat fish!
and thanks again, Mike.