DNA from dog remains traces a relationship across time and space
Sometime toward the end of the last ice age, a gray wolf gingerly approached a human encampment. Those first tentative steps set his species on the path to a dramatic transformation: By at least 15,000 years ago, those wolves had become dogs, and neither they nor their human companions would ever be the same. But just how this relationship evolved over the ensuing millennia has been a mystery. Now, in the most comprehensive comparison yet of ancient dog and human DNA, scientists are starting to fill in some of the blanks, revealing where dogs and humans traveled together—and where they may have parted ways.
“Its a really cool study,” says Wolfgang Haak, an archaeogeneticist at the Max Planck Institute for the Science of Human History. “Were finally starting to see how the dog story and the human story match up.”
Dogs are one of the biggest enigmas of domestication. Despite decades of study, scientists still havent figured out when or where they arose, much less how or why it happened. A 2016 study concluded that dogs may have been domesticated twice, once in Asia and once in Europe or the Near East, but critics said there wasnt enough evidence to be sure. A few years later, researchers reported signs of dogs in the Americas as early as 10,000 years ago, yet those canines appear to have vanished without a genetic trace. Other studies have found evidence of ancient dogs in Siberia and elsewhere, but scientists dont know how they got there or how theyre related.
To fill in some of the blanks, two big names in dog and human genetics teamed up: Greger Larson, an evolutionary biologist at the University of Oxford, and Pontus Skoglund, a paleogenomicist at the Francis Crick Institute. Larsen, Skoglund, and colleagues sifted through more than 2000 sets of ancient dog remains dating back nearly 11,000 years from Europe, Siberia, and the Near East. In the process, they added 27 ancient dog genomes to the five already on record. They then compared those with the genomes of 17 humans living in the same places and times as the dogs.
The dog DNA alone revealed some surprises. As early as 11,000 years ago, there were already five distinct dog lineages; these gave rise to canines in the Near East, northern Europe, Siberia, New Guinea, and the Americas, the team reports today in Science. Because dogs had already diversified so much by that time, “domestication had to occur long before then,” Skoglund says. That fits with archaeological evidence: The oldest definitive dog remains come from Germany about 15,000 to 16,000 years ago.
Remarkably, pieces of these ancient lineages are still present in todays pooches. Chihuahuas can trace some of their ancestry to early American dogs, for example, whereas Huskies sport genetic signatures of ancient Siberian dogs, the team found. “If you see a bunch of different dogs in a dog park,” Skoglund says, “they may all have different ancestries that trace all the way back 11,000 years” (see figure below).
When the researchers compared their dog DNA with modern and ancient wolf DNA, they got another surprise. Most domesticated animals pick up genetic material from their wild relatives—even after domestication—because the two species often live in close proximity and can still mate (think pigs and wild boars). But dogs show no such “gene flow” from wolves. Instead, the wolves gained new DNA from the dogs—a one-way street.
Larson chalks this up to the intimate relationship between dogs and humans. If your pig or chicken becomes a bit wilder thanks to an infusion of feral DNA, it doesnt matter, because youre going to eat them anyway, he explains. But dogs that go native make bad guards, hunting companions, and friends. “If youre a dog and you have a bit of wolf in you, thats terrible,” Larson says. People will “get rid of the dog.”
The wolf-dog analysis also suggests dogs evolved only once, from a now-extinct wolf population. Still, Larson, who led the 2016 study on multiple domestication events, says more data are needed to seal the deal.
Then the scientists brought humans into the mix. They selected human DNA samples from the same places and eras for which they had ancient canine DNA, and traced the genetic history of each. “Its like you have an ancient text in two different languages, and youre looking to see how both languages have changed over time,” Skoglund says.
In many places, the team found a strong overlap between human and dog genomes. For example, farmers and their pups in Sweden about 5000 years ago both trace their ancestry to the Near East. This suggests early farmers took their dogs with them as agriculture spread throughout the continent. “Writ large, as humans moved, they moved with their dogs,” Larson says.
But sometimes the stories didnt match up. Farmers in Germany about 7000 years ago also came from the Near East and also lived with dogs. But those animals seem more similar to hunter-gatherer pups, which came from Siberia and Europe.
That suggests many early migrants adopted local dogs that were better adapted to their new environment, Haak says. The benefits were many, adds Peter Savolainen, a geneticist at the Royal Institute of Technology and an expert on dog origins. “They were cute. You could use them. You could even eat them.”
Savolainen calls the study “very thorough,” and adds its “fantastic” that the researchers were able to bring together so many data. But he has long argued that dogs arose in Southeast Asia and says the work is incomplete without samples from that corner of the globe. “Without those, you could be missing an important part of the picture.”
For now, Larson says his team is analyzing “a ton” of wolf and dog genomes. He and his colleagues have also begun to look at ancient skull shape and genetic markers that could give clues to what early dogs looked like. Whatever he finds, hes counting on being surprised. “We have to expect the unexpected,” he says, “because thats all ancient DNA ever gives us.”
David Grimm is the Online News Editor of Science.
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The scientists involved in the study sequenced the genomes of four gray wolves from Russia and China, three Chinese street dogs, and three domesticated breeds—including a German shepherd, a Belgian malinois, and a Tibetan mastiff.
When Zhai and colleagues took their canine sequences and compared them with the human genome, the team found that sequences for things such as the transport of neurotransmitters like serotonin, cholesterol processing, and cancer have been selected for in both humans and dogs.
Southeastern Asia street dogs, including the Chinese street dogs in the study, may be an evolutionary bridge between wolves and purebred dogs due to their greater genetic diversity when compared with other street dogs from around the world, Zhai explained. This would make the Chinese street dogs a kind of “missing link” among canines.
This parallel evolution was likely driven by the shared environments of humans and dogs, wrote the authors in a study published May 14 in the journalNature Communications.
Furthermore, Wayne said, without further comparisons between humans and other domestic animals like goats or horses, its hard to know whether the parallel evolution in the genomes of humans and dogs is unique or not.
Our relationship with pups spans millennia, and new DNA analysis shows just how much people influenced canine evolution
The team also mapped how ancient DNA can be traced in todays pups. For example, Siberian huskies carry DNA from the ancient lineage originating in Siberia, and chihuahuas have genetic roots in Mexico.
Their analysis also revealed that by the end of the last ice age—around 11,000 years ago—at least five distinct lineages of dogs existed in New Guinea, the Americas, northern Europe, the Near East and Siberia. To achieve this genetic diversity, dogs had to have been domesticated much earlier, supporting the archaeological evidence that the origin of canine domestication goes back to 15,000 years ago, reports Science.
To piece together this timeline, an international team of researchers examined more than 2,000 remains of ancient dogs, reports David Grimm for Science magazine. Of those specimens, they sequenced the DNA of 27 dogs from across Europe, the Near East and Siberia that lived between 800 and 11,000 years ago.
Then, they compared those samples to ancient human DNA from similar places and time periods as the dogs to trace their evolutionary lineages.