Dogs have lived alongside humans for thousands of years. In fact, dogs became "man's best friend" before humans even began settling down and farming, making them, as far as anyone knows, the first domesticated animal. Although the story of the first Fido is a key piece of our own history, anthropologists know little about how that human-dog bond developed.
Scientists have been working doggedly to determine where canines were first domesticated, but they haven't all agreed on a location. Different research teams have pointed to Europe, central Asia, or southeast Asia among potential birthplaces for our beloved pooches.
But perhaps, in this dogfight, nobody is wrong.
"It appears that dogs may have been domesticated twice independently, from two different wolf ancestors, one in eastern Eurasia and one in western Eurasia," says Greger Larson, principal investigator at the Wellcome Trust Palaeogenomics & Bio-Archaeology Research Network at Oxford University.
After using ancient and modern dog DNA to build Fido's family tree, Dr. Larson and an international team of collaborators propose this new model in a paper published Thursday in the journal Science.
If they're right, here's what would have happened: Sometime before about 15,000 years ago, a group of friendly wolves in Europe and another group in Asia cozied up to hunter-gatherer populations in these disparate regions. Over time, the two species developed a bond that had enough of an influence on the canines that they transformed into domestic dogs.
These twin events happened thousands of miles apart but with the same result: humanity's first pet.
Larson and his colleagues proposed this dual origin story as a way to explain something strange they found in the doggie DNA.
The team built a family tree using DNA sequences from ancient and modern dogs alike and identified a distinct split in the trunk of the tree. They found that dogs from eastern Eurasia, breeds like Shar Peis and Tibetan mastiffs, and those from western Eurasia, like Golden Retrievers and Labradors, sit on these two distinctive branches, having diverged from each other between 14,000 and 6,400 years ago.
Along the west Eurasian branch the scientists spotted a genetic bottleneck, a point associated with a significant population decrease. Likening that bottleneck to one found in human genomes associated with the migration out of Africa, the scientists speculate that this too could represent a migration.
That would suggest that the scientists who said dogs were domesticated just once, in Asia, were right and a group of these dogs migrated to Europe where they became a distinctive group. But something didn't fit.
Dog fossils have been found in Europe from about 15,000 years ago (and in Asia from about 12,500 years ago). That would put domestic dogs in Europe before the bottleneck. Furthermore, dog fossils don't appear in the archaeological record in the middle of Eurasia until thousands of years later.
But there's more. As part of the new study, Larson and his colleagues sequenced the complete genome of a 4,800-year-old dog bone unearthed at Newgrange, an prehistoric monument in Ireland.
"Whenever you have an ancient dog, that gives you a snapshot of how things were at a particular point in time and at a particular place," Daniel Bradley, who leads the Molecular Population Genetics laboratory at Trinity College Dublin that sequenced the genome of this ancient dog, tells The Christian Science Monitor.
As expected, the Newgrange dog's DNA was most similar to that of modern European canines. But, Dr. Bradley says, there appeared to be something else in that dog's DNA, a sort of relic ancestry.
"We're guessing this is from the original European population that was there before the east Asian dogs arrived," Larson says. So, if this scenario is right, the Asian dogs may have nearly replaced the European ones.
The researchers still have to figure out how readily this relic DNA appears in modern dogs. "We need to get the genome of that mesolithic population before we can say what proportion of that genome continues to exist in modern breeds in both east and west and is there anything left at all," Larson tells the Monitor. "It's similar to trying to get what percentage of your genome is Neanderthal without having the Neanderthal genome."
But not all scientists are ready to discard the idea of a single domestication event just yet. "It's an intriguing hypothesis. But I don't think one genome proves it," Adam Boyko, who's own research suggests the first dogs were domesticated in central Asia, tells the Monitor. The genetic distinctiveness between the dog populations could still be representative of a mutt migration from Asia to Europe.
And perhaps that relic signature comes from wolves, not ancient western dogs, Peter Savolainen from the KTH Royal Institute of Technology in Stockholm, whose research points to southern China for the origin of dogs, told the Atlantic.
"This paper does not yet have the smoking gun," Larson says. "This paper is a hypothesis based upon a series of bits of evidence that we've strung together to make what we think is a really plausible argument. We think it's more likely there were two rather than one domestication episode. But, what's nice about this is it gives a framework to interpret all future data."
"I think with more ancient genomes, from dogs and wolves, we could more easily tease this apart," Dr. Boyko agrees. Both scientists add that multiple research teams are currently sequencing more ancient DNA that can be used to test this new hypothesis.