I have speculated as much for years now, so this finding is not much of a surprise. After Homo sapiens spread out of Africa to colonize the far reaches of the planet, it is not surprising the geographical barriers allowed cultural and selective isolation, even though such isolation was never complete. Trade and warfare always dampened complete isolation. However, the world is no longer spreading apart as it did in the recent past, but rather, drawing together these days in intensified trade, direct population blending, and cultural intermingling. The earlier means of rapid evolutionary selection leading to diversity, as noted by the authors, I would expect is now over.