Man’s best friend was the first of many animals that humans domesticated. But there was no clear before-and-after moment when dogs suddenly became a distinct population of wolves. While some ancient skeletons are clearly dogs, there are many earlier ambiguous skeletons. It’s possible to get a sense of what happened using the genomes of modern and ancient dogs. But that analysis depends heavily on what you think the wolf populations the dogs come from are like.
Now researchers have created a much clearer picture of the last 100,000 years of wolf evolution. The picture it paints is of a population that remained a single entity despite being spread across continents in the Arctic, with the population being sporadically refreshed from a nucleus in Siberia. Many dog breeds appear to have evolved from a population of East Asian wolves. But others also appear to have received significant contributions from a Middle Eastern population — though it’s unclear whether that population were wolves or dogs.
wolves in the north
The ability to sequence ancient DNA was critical to this new work, which aimed to obtain DNA from 66 wolf skeletons that together span about 100,000 years of evolution, including most of the last Ice Age. Wolves are found in the Northern Hemisphere, and the skeletons used here tend to be closer to the Arctic (probably in part because DNA survives better in cooler climes). But they are widespread, with Europe, Asia, and North America represented. The researchers also included five ancient wolf genomes that others had analyzed, as well as some genomes from modern wolves.
Typically you would expect to find regional populations that don’t often mix with their more distant relatives. Usually, when you map the most closely related genomes, you find them piling up. That is not the case here; Instead, ancient wolf genomes clustered together in time. That is, a given wolf was most likely closely related to other wolves that lived at about the same time, regardless of where on the planet those wolves lived.
Studies of modern wolves showed that local populations developed after the last peak of the last ice age. But all of these populations are more similar to each other than wolves were before the peak of the Ice Age.
How did these animals maintain their genetic continuity across the vast distances that separated them? Apparently through repeated population expansions in Siberia. Somewhere 100,000 years ago there was a distinct European wolf population. But sustained arrivals from Siberia gradually reduced the presence of Europeans’ ancestors to between 10 and 40 percent, depending on the animal. In contrast, in North America, all present-day wolves are primarily from Siberia, with the remainder being a contribution from interbreeding with coyotes.
A consequence of a global population is that favorable mutations spread rapidly worldwide. The researchers found 24 regions of the genome that appear to carry useful adaptations, and all of these useful stretches of DNA appear in all wolf populations studied.
Gone to the dogs
So what can we say about dogs? They also look like the Siberian wolves that lived just before the last peak of the Ice Age. But when every wolf older than this point was tested for a close relationship with dogs, the association was not robust. This suggests that if dogs come from a particular wolf population, we don’t have DNA from that population.
But the researchers found that if you had a population made up mostly of Siberian wolves, there was a good match, with a fraction of their DNA (between 10 and 20 percent) coming from another canid, the dhole, who also did occurs in Asia. Some dog breeds in East Asia appear to have retained this ancestry to this day.
But other breeds in Europe and Africa appear to have a large contribution from a wolf population most closely related to a modern day wolf from Syria. The researchers estimate that a Middle Eastern dog around 7,500 years ago had about half its genome from this local source and the other half from Siberian ancestors. Many dogs in Africa and Europe have between 20 and 60 percent of their genomes from this additional ancestor.
Overall, their data support a model in which dogs were first domesticated in East Asia, where most extant breeds are descended solely from Siberian ancestors. But as our best friend spread with us across Asia, he came into contact with another population, probably near the Middle East. This population could have been wolves, could have been a dog population that had been domesticated separately, or it could have been somewhere in between the two – there’s no way to tell from genetic data.
In any case, the wolf data provide some context for why canine ancestry has been so difficult to clarify: wolves are genetically unusual in that they have a global population that is regularly agitated in ways that disrupt stable, long-term regional populations. One implication of this is that looking for a wolf population that dogs are closely related to doesn’t make much sense to determine where dogs were domesticated. Even if this wolf population existed at the time, it would likely mix with other populations shortly thereafter.
Nature2022. DOI: 10.1038/s41586-022-04824-9 (About DOIs).