“This study has answered some of the big questions about the arrival of dogs in the Americas,” says Sarah Brown from the Washington Department of Fish and Wildlife. “It’s an excellent contribution that has been much needed.”

Dogs are domesticated descendants of gray wolves, which first arose in Eurasia on perhaps more than one occasion. There have long been wolves in the Americas too, but genetic studies show that those wild animals were not the ancestors of the continent’s indigenous dogs. Instead, the dogs arose from already-tame ancestors that lived in Asia around 14,600 years ago. Those ancestors were most closely related to prehistoric sled-pulling dogs from Siberia’s Zhokhov Island, whose residents were among the first people to deliberately breed dogs for a purpose.

Humans likely introduced dogs into the Americas—but not immediately. The oldest dog bones in the continent are 4,500 years younger than the oldest human ones, and genetic evidence supports a similar time lag. But once dogs entered the continent, they spread widely, and “evolved along a trajectory completely of their own,” says Larson. “If you look at their bone structure, they are very clearly dogs. They’re not from outer space with a third ear or a fifth leg. But they were also slightly different.”

But when Europeans arrived with dogs of their own, the native dogs started disappearing. Perhaps they were directly persecuted. Perhaps they were killed by introduced diseases, much like Native Americans themselves were. Today, they are almost entirely gone.

When the team analyzed the DNA of 5,000 modern American dogs, they found only tiny traces of indigenous DNA in modern Arctic breeds, like Alaskan huskies, Alaskan malamutes, and Greenland dogs. And they found no traces at all in most other groups, including street dogs, which are the least likely to be affected by introduced European breeds, and in supposedly “native” breeds like Catahoulas and Xolos. “Some people have said that these represent ancient lineages that haven’t been diluted by the introduction of European dogs,” says Larson, “but that’s all bullshit.”

Amazingly, the organism that is most closely related to the vanished American dogs isn’t even a dog. It is, however, of a dog.

Canine transmissible venereal tumor, or CTVT, is a type of contagious cancer. It arose in the genitals of a single dog some 8,000 years ago, and somehow evolved into an independent parasite that can spread sexually from one individual to another. Through millions of doggy-style liaisons, CTVT has traveled all over the world—an immortal pandemic tumor that afflicts dogs today, long after its original owner’s death. And it still retains that progenitor’s genes.

By analyzing those genes, Elizabeth Murchison, from the University of Cambridge, showed that CTVT is the closest living relative of the indigenous American dogs. It didn’t arise from one of them, but from a very close cousin. “It felt too amazing to be true,” she says. “There are many issues when you’re working with both ancient DNA and CTVT. But every way we cleaned up the data, they pointed to the same conclusion.”