We are told we all came out of Africa. Seven sister of Eve and all that.
So those who wandered into the northern regions have lost their darker skin colouring. No need obviously in a land where the sun is bye and large weak.
But why? What evolutionary driver would push the lighter skin option? Sure there is no need to 'waste' the effort of producing a dark skin but what about men's nipples? No NEED for them either. Nor for a beard, or hair on the head that just grows and grows.
There can be no 'advantage' in having a lighter skin which would make those pre-disposed to a light skin more successful in the breeding stakes.