Humans have a tighter bond with dogs than perhaps any other animal. But figuring out exactly how that bond developed has been a huge, tantalizing challenge for researchers. Now, an ambitious study of canine genetics suggests that dogs’ history is even more complex than we thought — and that the animals we know now actually come from two separate groups of ancient domesticated wolves in Europe and Asia.
The study, published today in Science, is part of a major attempt by the University of Oxford to pin down the origin of the dog. Comparing the DNA of ancient and modern dogs, the researchers found a distinct genetic split between dogs from East Asia and Western Eurasia, an area comprising modern Europe and the Middle East. This split doesn’t seem to be the result of one population diverging over time, the study authors say. Instead, it looks like East Asian dogs came from a completely different source and moved west over time, eventually mixing with and partially replacing a group of existing European dogs. In other words, the East Asian dogs and the European dogs were domesticated separately, in two different occasions, the study authors say.
While geneticists haven’t previously ruled out this possibility, the most common theory so far is that dogs were domesticated once. As far as we currently understand, our furry companions likely descended from one group of wolves that began following or scavenging from humans, becoming more gregarious and less dangerous over generations. But no one can agree on where or when that happened. Greger Larson, an Oxford biologist and co-author of this study, has previously called modern dog genetics a confusing "tomato soup" whose parts are almost impossible to separate. The oldest identifiable dog remains are dated to 15,000 years ago in Europe, although some scientists argue that dogs have been around for about 30,000 years. Different studies have put their origin in Europe, Central Asia, and East Asia. If this latest theory is borne out, it would mean that "everyone has been a little bit right" about where dogs came from, says Larson.
The key to this study was recently sequenced DNA from a 4,800-year-old dog excavated from a tomb in Newgrange, Ireland. Because the researchers knew how long ago that particular dog lived, they could better calculate the probable age of other dog DNA samples, based on how frequently mutations should accumulate with each new generation of dog. This allowed them to date when the genetic split between East Asian and Western Eurasian dogs may have occurred, putting it between 6,400 and 14,000 years ago, after they had already started appearing on both sides of the continent. "The split between Western and Eastern dogs that we find nowadays is too young to explain dogs in Europe," says Laurent Frantz, the study’s lead author.
The work has been well-received by other experts in the field. "The data is extensive and detailed and their multifaceted analysis is impressive," says Urs Giger, who specializes in genetics at the University of Pennsylvania’s School of Veterinary Medicine and was not involved in the study. "It supports a dual to multiple sites of dog origin and domestication."
Frantz says that the theory is also backed up by the archaeological record, which has found very old dog remains in Europe and East Asia, but only newer ones in the space between. "For right now, I can't think of another way of actually explaining all this information altogether, besides suggesting that there were two domestications," he says. "If we find some very old dogs that are older than the one in Europe and older than the one in Asia in the middle of Eurasia, that would be sort of the blow in the theory — but right now, we do not have these samples."
In fact, our spotty collection of ancient canine DNA is this study’s greatest weakness, according to some. "It's an intriguing hypothesis about two domestication origins," says Adam Boyko, who researches the evolution and genetics of dogs at Cornell University’s College of Veterinary Medicine and was not involved in the study. "I think we're going to need to look at more ancient genomes to actually see if we can fill it in and say with more certainty." Frantz says his team is looking at very old European dog remains to see if they could yield DNA to further test the theory.
Given how closely the dog’s history is entwined with our own, knowing more about how they evolved also helps fill in the details of how humans lived during those periods. And if they were domesticated in two separate locations, it could cast the relationship in a new light. "It sort of suggests that whatever humans did at the time, wolves were sort of primed to become domesticated, and it was sort of an easy process," says Boyko. "So easy, in fact, that it happened multiple times."