Reply #113: How's this then, if 'novelty' isn't sufficient cause for the propagation

[Igel]

of the mutation.

Humans like to form tribes. Specifically against neighboring tribes. When population density is low, the gene would be transmitted: mates would be in short supply. It wouldn't have been an overly glaring difference so as to be stigmatized; but I'm not sure there's a greater chance of lighter skin being stigmatized than there is of its being postively valued. But the mutation would easily have spread within a clan or tribal area.

If the slight variation in color was taken as a distinctive feature of a tribe, it would have been favored, and therefore accentuated.

Even a trivially slight difference in reproductive success can account for population shift quite nicely over time, whether that difference results from survival or selection by prospective mates. (Consider the wonderful variety in plumage/coloration and mating rituals, some seemingly bizarre, among animals.)