Our pet dogs are descendants of domesticated wolves, who allied with ancient humans at some point in our prehistory and stuck with us for millennia, shaped by our selective breeding. Researchers have long debated whether these ancestor wolves just started hanging out with humans (self-taming) or whether there was a more deliberate attempt to domesticate them.
A new study published in Proceedings of the National Academy of Sciences offers up evidence for the latter, that humans intentionally cultivated relationships with wolves. And the findings reveal a previously unappreciated episode in the human-wolf saga that had an ancient population ferrying wolves to an island that the canids would have otherwise not been able to access.
Researchers from Sweden and the United Kingdom analyzed two sets of dog-like bone remains found in a cave (Stora Förvar) on a remote island (Stora Karlsö) in the Baltic Sea. These remains were discovered in the late 19th century and housed in museum collections since then. The tiny island, totaling just one square mile, was inhabited by people who subsisted on fishing and hunting seals during the Neolithic and Bronze Ages. Due to its size and remoteness—with no land connection to mainland Scandinavia—Stora Karlsö lacked native land mammals altogether.
So, the bone remains must have been from animals brought over by boat.
A genomic analysis of the 3,000- to 5,000-year-old animal bones showed them to be from wolves (Canis lupus), not the domesticated dogs (Canis lupus familiaris) you’d expect under the circumstances.
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“It was a complete surprise to see that it was a wolf and not a dog,” said Pontus Skoglund of the Ancient Genomics Laboratory at the Francis Crick Institute and co-author in a statement.
Furthermore, the individuals studied showed signs of domestication. Compared to mainland wolves, the Stora Karlsö wolves were smaller-bodied, and, based on isotope analysis, the animals lived on a diet of seals and fish similar to that of humans who inhabited the island. It’s difficult for wolves to capture such marine organisms, suggesting they were dependent on their human companions for food.
Equally telling was the Stora Karlsö wolves’ lower genetic diversity relative to other ancient wolves. “This is similar to what you see in isolated or bottlenecked populations, or in domesticated organisms,” explained co-author and University of East Anglia evolutionary biologist Anders Bergström in a statement.
The findings suggest that humans ferried wolves to the island and shared food with them, perhaps raising them as domestic companion animals. One of the wolves had a leg injury that would have probably made independent survival impossible, suggesting a further dependence on humans.
Although scientists estimate the domestication of wolves—and thus the origin of dogs—to have occurred some 15,000 years ago, the Stora Karlsö wolves suggest this was a long, iterative process. It appears that humans maintained relationships with these large predators long after the sun set on the Stone Age. 
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Lead image: Stéfan / Wikimedia Commons
