More than a millennium ago, people in the Viking Age city of Haithabu dined on a dish of freeze-dried cod and tossed the bones aside. It was a relatively unremarkable meal, except for one thing. The DNA in those bones was preserved into the present day, and scientists in Norway have just sequenced it. What they found has confirmed the truth of stories from the Icelandic sagas about Vikings sailing exceptionally long routes to trade with other groups.

Today, the coastal city of Haithabu is an archaeological site in Germany on the Baltic Sea. But the people who munched on that dried cod roughly 1,000 years ago were living under Danish rule in a cosmopolitan port city. Haithabu was a key stop on a lively sea trade route that brought tasty treats and trinkets like walrus tusks from distant lands. Though there is ample evidence of this kind of trade 800 years ago, University of Oslo environmental biologist Bastiaan Star and his colleagues have pushed that date back at least 200 years, and possibly 400, just by sequencing cod DNA. This dramatically changes our understanding of long-distance trade in Northern Europe during the Viking Age.

In Proceedings of the National Academy of Sciences, Star and his team describe how they used DNA analysis to trace the origins of 15 different cod eaten centuries ago in Germany, Norway, and the UK. The group has been studying ancient cod DNA to better understand the way humans have affected the migration routes and populations of this staple fish over time. This new discovery, however, has shed light on international trade. By comparing DNA sequences from ancient cod with modern ones, the researchers found that certain populations of fish have stuck to the same breeding grounds and migration routes for at least 1,200 years. Small mutations in the cod genome reveal which population the individual comes from, and that in turn reveals where they spawned.

University of Cambridge archaeologist James Barrett, a co-author on the paper, said in a release:

By extracting and sequencing DNA from the leftover fish bones of ancient cargoes at Haithabu, we have been able to trace the source of their food right the way back to the cod populations that inhabit the Barents Sea, but come to spawn off Norway's Lofoten coast every winter.

Those people who dined in Haithabu one ordinary day were enjoying freeze-dried cod that was born off the north coast of Norway. Norwegians from that region have been drying fish in the cold winter air for centuries, creating a unique flavor and preserving the meat for up to 10 years. Dubbed "stockfish," it was a popular food with Viking sailors on long ocean voyages and a valued trade item in more southerly regions.

It’s not clear from the remains at Haithabu whether the stockfish dinner was eaten by sailors stopping off at port, or if it was traded to locals eager for a change from their usual cuisine. The researchers say they studied five separate cod bones from meals in Haithabu eaten between 1,200 to 1,000 years ago. In their paper, they note that these dates corroborate evidence from an account written in the 800s of “the voyage of a Viking chieftain and trader from Arctic Norway to Haithabu.” Though that account doesn't mention stockfish, it does describe the chieftain sailing all the way from Norway to England on a trade mission. Haithabu would have been on the way.

During their work, the researchers also learned something surprising about DNA preservation. Though ancient mammal DNA seems to be preserved best in extremely dense bones, that doesn’t seem to be the case with fish. Cod have very porous bones, and yet their DNA was extremely well-preserved, allowing the researchers to match small mutations in ancient sequences to modern ones.

Star and his colleagues’ study is a standout example of why we analyze ancient DNA from different nodes in our ecosystems. No life form exists in a vacuum. There is a rich tradition of fishing in Northern Europe, and cod became the basis for a trade network that joined people across oceans and nations. By exploring the history of humanity’s interaction with these animals, scientists shed light on our relationships with each other.

PNAS, 2017. DOI: 10.1073/pnas.1710186114