It was substantially driven by the arrival of new populations carrying different pigmentation genetics.

The farmers who began arriving in Europe from Anatolia (the region that is now Turkey) approximately 8,000 to 10,000 years ago — the Neolithic transition — carried genes for lighter pigmentation that were then distributed across Europe as the farming population expanded.

And the steppe pastoralists who arrived later, around 5,000 years ago, from the Pontic-Caspian steppe north of the Black Sea, brought additional genetic variants.

The light skin of modern Europeans is largely the result of these population turnovers and mixtures, not of selection acting on a single continuous European population.