Man’s best friend was the first of many animals that humans domesticated. But there was no clear before-and-after moment when dogs were suddenly a distinct population of wolves. While some ancient skeletons are clearly dogs, rather there are many ambiguous skeletons. It is possible to get an idea of what happened using the genome of modern and ancient dogs. But this analysis depends a lot on what you think the wolf populations from which dogs are derived look like.
Now researchers have a much clearer picture of the last 100,000 years of wolf evolution. The picture it paints is a population that remained a single unit despite spreading across continents in the Arctic, with the population being sporadically refreshed from a core in Siberia. Many dog breeds appear to have been derived from a population of East Asian wolves. But others also appear to have received significant input from a Middle Eastern population, though it’s unclear whether that population was wolves or dogs.
Wolves around the north
The ability to sequence ancient DNA was essential to this new work, which involved obtaining 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 climates). But they are widespread, with representation in Europe, Asia and North America. The researchers also included five ancient wolf genomes that others had analyzed, along with some genomes from modern wolves.
Normally, you would expect regional populations that don’t often mix with their distant relatives. If you map the most closely related genomes, you will see that they tend to clump together. That is not the case here; instead, the ancient wolf genomes clustered over time. That is, a particular wolf was most likely closely related to other wolves living around the same time, regardless of where those wolves lived on the planet.
Studies of modern wolves indicated that local populations developed after the last peak of the last ice age. But all these populations are more alike than the wolves before the height of the Ice Age.
How did these animals maintain genetic continuity over the vast distances that separated them? Apparently due to repeated population expansions in Siberia. There was a distinct European wolf population sometime before 100,000 years ago. But continued arrivals from Siberia gradually reduced the ancestral European presence to somewhere between 10 and 40 percent, depending on the animal. In contrast, in North America, all current wolves are mainly from Siberia, with the rest being a contribution from crosses with coyotes.
A consequence of having a world population is that beneficial mutations spread quickly around the world. The researchers found 24 regions of the genome that appear to contain useful adaptations, and all of these useful pieces of DNA are found in all wolf populations studied.
Went to the dogs
So, what can we say about dogs? They also resemble the Siberian wolves that lived just before the last peak of the Ice Age. But when every wolf older than that point was tested for a close relationship with dogs, the connection was not robust. That suggests that if dogs are derived from a specific wolf population, we don’t have DNA from that population.
But the researchers found that there’s a good match if you had a population consisting mostly of Siberian wolves with a fraction of its DNA (between 10 and 20 percent) coming from another dog, the dhole, which is also found in Asia. Some dog breeds in East Asia seem to have retained these ancestors to this day.
But other breeds in Europe and Africa seem to be major contributors of a wolf population most closely related to a modern-day wolf from Syria. The researchers estimate that a Middle Eastern dog from about 7,500 years ago had about half of its genome from this local source and half from Siberian ancestors. Many dogs in Africa and Europe have anywhere from 20 to 60 percent of their genomes from this additional ancestor.
In general, their data favors a model in which dogs were first domesticated in East Asia, where most of the breeds present are derived exclusively from Siberian ancestry. But as our best friend spread across Asia with us, he came into contact with another population, probably near the Middle East. That population could have been wolves, could have been a dog population that had been domesticated separately, or it could have been somewhere between the two — there’s no telling from genetic data.
In any case, the wolf data provides some context as to why dogs’ ancestors are so difficult to trace: genetically, wolves are unusual in having a global population that is regularly churned up in a way that disrupts stable, long-lasting regional populations. As a result, it doesn’t make much sense to look for a wolf population that dogs are closely related to as a way of identifying where dogs were domesticated. Even if that wolf population existed then, it would likely mix with other populations shortly after.
Nature2022. DOI: 10.1038/s41586-022-04824-9 (About DOIs).
