Dogs may have been domesticated more than once

For years, scientists have debated where dogs came from. Did wolves first forge their special relationship with humans in Europe, or in Asia? The answer, according to a new study, is yes. This week in Science, researchers report that genetic analysis of hundreds of canines reveals that dogs may have been domesticated twice, once in Asia and once in Europe or the Near East, although European ancestry has mostly vanished from today’s dogs. The findings could resolve a rift that has roiled the canine origins community—but the case isn’t 
closed yet.

“These are fantastic data that are going to be extremely valuable for the field,” says Peter Savolainen, a geneticist at the Royal Institute of Technology in Stockholm and the leading proponent of Asian dog origins. But 
Robert Wayne, an evolutionary biologist at the University of California, Los Angeles, whose work has shown that dogs arose in Europe, says the results—although plausible—are too preliminary to settle the question. “The story is still a bit of a muddle.”

The study includes a unique specimen: the inner ear bone of a nearly 5000-year-old dog unearthed from Newgrange, a football field–sized mound of dirt and stone on the east coast of Ireland, built around the time of Stonehenge. Researchers led by Laurent Frantz, an evolutionary geneticist at the University of Oxford in the United Kingdom, sequenced this specimen’s entire nuclear genome—the first complete genome from an ancient dog to be published—and compared it to the nuclear DNA of 605 modern dogs from around the world. The team then created a family tree for the animals, which revealed a deep divide between European dogs (like the Newgrange canine and the golden retriever) and Asian dogs (like the shar pei and free-ranging village dogs from Tibet and Vietnam). “I was like, ‘Holy shit!’” says project leader Greger Larson, an evolutionary biologist at Oxford. “We never saw this split before because we didn’t have enough samples.”

To figure out when this divide occurred, the Newgrange specimen was critical. Researchers used it, in conjunction with the complete genomes of several modern dogs and wolves, to calculate a genetic mutation rate for canines. This rate suggests that the East-West split happened sometime between 6400 and 14,000 years ago. The analysis also revealed a “genetic bottleneck” in Western dogs—a reduction in genetic diversity typically tied to a sharp decline in a population’s numbers, as can occur when a small band of individuals splits off from the main group. (A similar pattern is seen with the original human migration out of Africa.)

Taken together, the data suggest that humans domesticated dogs in Asia more than 14,000 years ago, and that a small subset of these animals eventually migrated west through Eurasia, probably with people. This implies that all modern dogs, as well as the Newgrange canine, can trace their ancestry back to Asia.

Science

But here’s the twist: Archaeologists previously had found the remains of dogs in Germany that may be more than 16,000 years old, suggesting that dogs had already been domesticated in Europe by the time the Asian canines got there. Some of today’s dogs may carry genetic traces of that early domestication—but it’s hard to find, in part because scientists are still trying to recover DNA from those ancient German dogs. “We don’t know if the dogs that evolved [early] in Europe were an evolutionary dead end,” Frantz says, “but we can safely say that their genetic legacy has mostly been erased from 
today’s dogs.”

To Savolainen, the story makes sense. “If people in one place got these fantastic dogs, of course everyone wanted to have them,” he says. “Over the course of a few hundred or a thousand years, you could have dogs spread throughout all of Eurasia.” Still, he’s not completely sold on the idea of two domestications, arguing that if the team’s mutation rate is just a bit off, it could allow for all dogs, even those ancient European ones, to have Asian roots. Wayne adds that interbreeding between dogs and wolves could have muddied the picture. Both say that many more samples, especially of ancient dogs and wolves, are needed.

That could happen soon. Although neither Wayne nor Savolainen were involved in the current study, both joined Larson in 2013 as part of an international collaboration to solve the mystery of dog domestication once and for all. Dozens of scientists have been pooling resources and gathering thousands of new samples from around the globe. “The new model is provocative and exciting, but the full collaboration is going to be essential to untangling this complicated story,” says John Novembre, a population geneticist at the University of Chicago in Illinois who is not involved with the collaboration or the new work.

For now, a dual origin for dogs remains an intriguing possibility, especially because research has also suggested multiple domestications for cats and pigs. Does that mean these animals were bound to be domesticated? “If it only happened in one place, it was probably a very hard thing to do,” Savolainen says. “But if it happened twice, maybe it wasn’t as hard as we thought.”

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