I could see that the darkest babies in a seasonally low sunlight, cloudy latitude and climate not getting enough Vitamin D and therefore not thriving or even dying.
So lighter skin would be a life long advantage in such places. The lighter skin individuals would at least maximize their Vitamin D production over the darker skinned ones and pass their genes on by producing children—or more children over a longer life. How many generations would that take? I would think less than 100.
So there’s my take on skin color. Still, Eskimos are far darker than Laplanders. Why is that? Both are at high latitudes and similar climates. Maybe Eskimos get far more Vitamin D from fish, seals, whales and polar bears in their diet?
There have been a number of fortuitous new genes in Europe. The Neanderthal are now suspected of being very fair and possibly blue-eyed redheads. My husband of Scots/Viking ancestry had that coloring (ginger gene) and massive, muscular physique. This enabled them to survive the poor sunlight of the north and ice ages. The “white” gene gained by Africans moving north enhanced survival. Eskimos get Vitamin D from heavy fish diet. Laplanders use land mammals more frequently so need lighter skin. The gene that enabled enzymes to digest milk in adulthood greatly enhanced surviveability in Europe and led animal rearing and increased population.
Has any work been done on the mysterious Basque population? Are they M or N? I know their language is unique and no one knows where they came from. What % of Neanderthal do they have in their genes?