I am not surprised at this. I remember reading somewhere that the original inhabitants of the British isles were darker skinned. Then the celts moved in and mixed with them.
The genetics for skin color of all kinds is in every human. Its just a matter of generational breeding that switches the genes on or off in favor of one or the other over time. There is environmental pressure too, for example darker skin affords a better resistance to dry, arid, and sunny climates. Lighter skin affords better survivability in generally colder climates.