Ancient humans made dogs their best friend not once but twice, by domesticating two separate populations of wolves thousands of miles apart in Europe and Asia.
That is the conclusion of scientists who said on Thursday they had used modern genetics to unravel canine evolutionary history, revealing a deep internal split between dogs from opposite ends of the Eurasian continent.
People and hounds go way back - they were living together at least 15,000 years ago, or 5,000 years before cows, goats and pigs arrived - but how, why, when and where the two species got friendly has been a mystery.
It was widely believed dogs were tamed just once, with some experts claiming this happened in Europe and others favoring central Asia or China.
But a new story emerged when researchers used the inner ear bone from a 4,800-year-old dog unearthed in Ireland to sequence its full genome, and then compared it to both modern animals and DNA traces from 59 ancient dogs.
"Our data suggests that dogs were domesticated twice, on both sides of the Old World," said Laurent Frantz, a geneticist at the University of Oxford, whose work was published in the journal Science.
"This suggests that at least two group of humans independently came to the same conclusion: dogs can be domesticated. It also suggests that the process of domestication, while mostly rare, may be replicated more often than we think," he said.
After constructing a family tree for dogs based on the genetic data, the scientists concluded there were very old domesticated animals in both the east and west of Eurasia but not in the middle.
At some point in prehistory, they believe the eastern dogs dispersed with human migrants and replaced most of the western ones, so Asian ancestry is now dominant in modern dogs.
Although it is possible there was only one domestication event in Asia, followed by early transportation to Europe, the research team argues the lack of archaeological evidence for dogs in the middle of the continent makes this very unlikely.
Other scientists not involved in the work believe more samples from ancient dogs and wolves will be needed to prove the point conclusively.
Wolf, Afghan, Yorkie
What remains unclear is how grey wolves started down the long road that has ended up with today's kaleidoscope of dog breeds from Afghan hounds to Yorkshire terriers.
The idea that it began with a hunter-gatherer picking up a wolf pup and breeding tamer and tamer offspring is probably too simple, according to Greger Larson, a genetics expert in Oxford's archaeology department.
"It's likely to have been co-evolution. At first a pack of wolves got close to humans, then humans got used to the wolves and, finally, there would have been something more intentional on the part of people," he said.