The strength and extent of this sweep can be used to estimate roughly when the selection began.

These analyses consistently suggested that light skin variants in Europeans had been under strong positive selection for a few thousand to roughly ten thousand years — not tens of thousands of years, as might have been expected if selection had begun with the initial entry of modern humans into Europe forty-five thousand years ago.

This result was puzzling at the time.

Why would selection for lighter skin have been weak for thirty to forty thousand years after modern humans entered Europe, and then suddenly strong in the past five to ten thousand years?