The ancient DNA evidence provided the answer: the light skin variants that are common in modern Europeans were not present at high frequency for most of the time that modern humans have been in Europe.

They were brought to Europe, or driven to high frequency, primarily by the Neolithic and Bronze Age population movements.

Selection had been acting on variants that were rare or absent in the earlier Paleolithic and Mesolithic populations; it was only when those variants arrived in significant frequencies — through migration — that selection could drive them to current high frequencies.

This resolves the apparent paradox.

Light skin in Europeans is not the result of slow, steady selection acting on the original Paleolithic European population over forty thousand years.