One of the most confusing questions in science for years – next to “What is dark matter?” and “Why do we sleep?” — has been one that many pet owners may have casually thought of: Where did dogs come from?
Scientists generally agree that mankind’s best friend descended from gray wolves and burst into our lives at least 15,000 years ago. Pretty much everything else is up for debate.
“When and where did this happen and with whom – with what human group?” said Pontus Skoglund, a paleogeneticist at the Francis Crick Institute in London. “It’s really a mystery.”
Studies have produced widely varying answers, concluding that dogs were first domesticated somewhere in Asia or Europe or the Middle East or perhaps in multiple locations.
Now, a new analysis of 72 ancient wolf genomes over the past 100,000 years suggested a possible explanation for some of the seemingly conflicting results: Two different ancient wolf populations, one in Asia and another in the Middle East or surrounding area, contributed DNA to modern dogs.
How exactly that happened remains unclear. It is possible that dogs were domesticated twice, in two different locations, and the populations then mixed. An alternative scenario is that dogs originated only once, somewhere in Asia, and later bred with a more western wolf population, picking up additional wolf DNA.
“We can’t tell the scenarios apart,” said Anders Bergstrom, an expert in evolutionary genomics at the Francis Crick Institute and author of the study, published Wednesday in Nature. “But we can say there were at least two source populations of wolves.”
The study also provides insight into the evolutionary history of gray wolves and hints at how the species managed to escape the fate of other long-extinct Ice Age mammals.
“Ancient DNA is the only way to unravel this history,” said Elisabetta Cilli, a paleogeneticist at the University of Bologna in Italy, who was not involved in the new research. “This study raises the bar for our understanding of dog domestication and wolf population dynamics.”
In the new study, an international team of researchers wanted to learn more about the origins of dogs by painting a detailed picture of the gray wolf’s past.
The goal was to “build a big map of the ancestors of wolves, wolf genetics, around the time of domestication,” says Dr. Skoglund, the study’s senior author. “And then we take the dog puzzle piece and put it in the big wolf puzzle and see where it fits.”
Archaeologists from 38 institutions in 16 countries donated bones and teeth from ancient wolf specimens previously unearthed in Siberia, Europe and North America. Scientists from nine old DNA labs then got to work. They drilled small holes in the samples, collected samples of bone powder, and extracted and sequenced the DNA. Next, the scientists compared the wolf genomes they collected with genomes from both ancient and modern dogs.
The researchers found that, in general, dogs were more closely related to ancient wolves in Asia than those in Europe.
“This reinforces the idea that Central Asia would likely be the center of origin for dogs,” said Adam Boyko, a canine geneticist at Cornell University College of Veterinary Medicine, who was not involved in the study. “Of course we want more data to better record that. That’s a big area.”
But there were differences between different dog specimens. Early dogs in Siberia, eastern Asia, the Americas and northeastern Europe seemed to derive almost all of their genetic material from an ancient wolf population in Asia.
But early dogs from the Middle East and Africa also had significant ancestry from a second, unidentified wolf population. This genetic material was most closely related to DNA from modern Middle Eastern wolves, the scientists found.
Modern village dogs in Africa and the Middle East — as well as breeds that originated in those regions, such as the Basenji — still have significant ancestry from this second wolf population, the researchers found. But this genetic inheritance persists in almost all modern dogs; worldwide, most dog breeds today can trace between 5 percent and 30 percent of their ancestry to this second wolf population, said Dr. Bergstrom.
“It looks like there’s a smoking gun of a second population,” said Greger Larson, a paleogenomicist at the University of Oxford and author of the new study.
dr. Larson had previously suggested that dogs may have been domesticated twice, before changing his mind when later evidence suggested a single origin. Now he said he was “on the fence” as to whether dogs really showed up twice or simply bred with wolves after their migration. “We’re back to square one,” he said.
dr. Boyko said the simplest explanation, and the scenario he preferred, was that dogs were domesticated just once, in Central Asia, and later mated by another. population of wolves. “It just seems more frugal to believe — because we already know that dogs and wolves exchange genes and have done it at other times — that’s what happened, and not a secondary domestication event.”
None of the ancient wolf specimens the scientists analyzed matched well enough to be part of the actual population that gave rise to our canine companions, suggesting that dogs’ ancestors may have come from locations not covered by the study.
To pinpoint the specific ancestral populations and determine how the second lineage got into dogs, more ancient wolf and early dog specimens are needed, especially from more southern regions not included in this study, scientists say.
“We only have the northern half of the map,” said Dr. Skoglund.
Wolves in the wild
The researchers also found that ancient wolves living in remote locations were genetically very similar — more closely related than today’s wolf populations — indicating extensive movement and interbreeding between wolves around the world.
This liquid, global population could explain how the species survived the end of the last ice age, the scientists say.
“Cave lions, hyenas and bears and other mammals became extinct, but wolves survived,” said Dr. Bergstrom. “And maybe it was this great connectivity and mobility of wolves that allowed the population to thrive.”
The researchers also monitored natural selection in wolf populations over time and identified multiple genetic variants that spread rapidly through ancient wolves, likely because they conferred a significant survival advantage.
For example, about 40,000 years ago, new mutations arose in a gene called IFT88, which appeared to be involved in the development of the skull and face, the researchers found. Over the next 10,000 years, these mutations spread to all wolves — and are still present in both wolves and dogs.
“It’s a very dramatic episode of natural selection,” said Dr. Bergstrom.
What drove it is unknown, but the mutations could have helped ancient wolves adapt to changing diets after some of their favored prey species became extinct, the researchers speculated.
The scientists also found that certain genetic variants that affect wolves’ olfactory receptor genes were much more common in wolf populations over time, suggesting that the animals’ sense of smell also benefited from natural selection.
While many questions remain, the new data will provide a valuable starting point for scientists still trying to solve the many mysteries of wolf evolution and dog domestication, scientists said.
“Having so many old wolf genomes is a huge advance in the field,” said Dr. boyko. “I’m sure other researchers will love getting their hands on it and exploring some of their own pet theories.”
So to speak.