For years, archaeologists have wondered why the Norse settled on Greenland’s inhospitable, ice-bound edges at the end of the first millennium A.D. The living certainly wasn’t easy—so why did they stay, and how did they survive?

The Norse communities farmed and fished. But a new study suggests that they had another valuable source of support: trading valuable walrus ivory with an avid European market.

The study relies on the fact that walruses from Iceland and Scandinavia are slightly gentically different than walruses from Greenland and Canada. Scientists tracked down walrus tusks, bones, and objects fashioned from their ivory from museum collections around Europe, and analyzed their ancient DNA to pinpoint the original sources of the treasured material.

What they found was striking.