[wahern]

> Among their results the team found that from the iron age, southern European genes entered Denmark and then spread north, while – to a lesser extent – genes from Asia entered Sweden.

A friend of Scandinavian extraction found through 23&Me that his Y chromosome comes from China. His variant traveled from SE Asia north to Siberia, then west to Scandinavia, presumably carried by progenitors of the Sámi.

Circumpolar peoples certainly got around, but I guess it's also much easier to travel around the world at those latitudes.