Before Europeans began to colonize the Americas about 500 years ago, the land, north and south, was populated with people who had been here for thousands of years. And their dogs.
The devastation visited on the native human inhabitants of North and South America is well known. Whether their dogs survived in some form, perhaps only as a portion of the DNA of some modern dogs, has been a matter of dispute. The available evidence indicated that only traces were present in current breeds and mixed breed dogs, but questions remained.
An international team of researchers who conducted the most detailed and thorough study yet of ancient and modern dog DNAreported Thursday in the journal Science that new evidence overwhelmingly suggests that the so-called pre-contact dogs have disappeared to an extent similar to the Neanderthals. The study found no more than 4 percent of pre-contact dog DNA in any sample, and those results could be interpreted as zero. By comparison, some modern humans may have a bit more than two percent Neanderthal DNA.
In a macabre scientific twist, the new study found that the closest living DNA match to the pre-contact dogs is a strange, but well known cancer, a tumor in which the cancerous cells themselves spread from dog to dog during sex, like rogue tissue transplants. Called canine transmissible venereal tumor,it originated thousands of years ago in one dog, probably from East Asia. The cancer is now present worldwide, still carrying the genome, much mutated but still identifiable, of that original host dog.
Greger Larson at the University of Oxford, an author of the paper, and the leader of an international effort to investigate the evolution and domestication of dogs, said the study emphasizes how inseparable are the fates of humans and their animals.
“The Europeans come through. They knock out the humans. They knock out the dogs,” he said. Given the necessary caveats that a pocket of dogs with substantial ancient American ancestry could turn up somewhere, Dr. Larson said that he was convinced by the evidence so far that, “It’s a complete disappearance.”
Laurent Frantz, an ancient DNA expert at Queen Mary University of London, who led the research, said that until now, there had not been enough evidence to know “the story of these dogs and what happened to them after the Europeans arrived.” Now, he said, it is clear that the pre-contact dogs were an identifiable group, separate from any other, and that some combination of disease and European persecution of native dogs led to their disappearance.
Elaine Ostrander, a comparative geneticist at the National Institutes of Health, who studies dogs as a model for human cancers, and who has reported traces of ancient American dog DNA in some modern breeds, said the study was “unique, really well done.” But she was not giving up hope for the possibility of uncovering dogs somewhere with a substantial portion of DNA from ancient American dogs.
Fifty researchers collaborated on the study, which included both biological and archaeological evidence. They derived DNA from the remains of 71 ancient dogs from the Americas and Siberia and compared them to genomes of modern dogs.
The story they draw from their analysis is familiar, but backed up with more data than ever before. Dogs, which were domesticated at least 15,000 years ago, came over to North America with humans from Siberia, but perhaps not with the first wave of migration.
The earliest archaeological evidence for dogs in North America is around 10,000 years ago, and attempts to date dog movements using changes in DNA are not accurate enough to say how much earlier they arrived. Humans first arrived at least 14,000years ago, so the first wave may have come without dogs.
The ancient American dogs, derived from East Asian ancestors, evolved into their own group, distinct from their ancestors, and from modern dogs. They are related to but distinct from Arctic breeds like huskies and malamutes, which appear to have arrived with later migrations into North America.
The American dogs spread with humans through both continents and remained undisturbed for more than 9,000 years until the arrival of European colonists. And then they were gone.Until further notice, that is. There are odd bits of data, like a Carolina dog with 30 percent of its genome that is either pre-contact dog — highly unlikely, or, more probably, genetic material from Arctic breeds. Genomic science is never free of anomalies.
As to that strange tumor, Elizabeth Murchison at the University of Cambridge, one of the main authors of the new research paper, sequenced the tumor genome in 2014 and joined with Dr. Frantz and Dr. Larson in the recent work to learn more about its origins.
She said the new work provides “stunning” insights into the founder dog, the one animal who first had the tumor, and passed it along to all dogs since. Dr. Murchison said it now seems to have first appeared 6,000 to 8,000 years ago.
Cell lines are usually kept growing in laboratories like the famous HeLa cells, whose story was told in the book and movie, “The Immortal Life of Henrietta Lacks.” The cervical cancer cells from Ms. Lacks are the oldest human cell line and have been growing since they were taken from her in 1951.
Transmissible tumors are rare natural cancer cell lines that live not in petri dishes but in the bodies of animals they infect. Only a few are known other than the canine tumor: two in Tasmanian devils and others recently discovered in mussels, cockles and clams.
Geneticists characterize different groups of dogs, or any organisms, on the basis of differences in their DNA, showing how closely they are related. Among modern, living dogs, the closest relatives to the pre-contact dogs are the Arctic breeds, because they shared a common ancestor. But the founder dog is even closer. It falls between the Arctic breeds and the pre-contact dogs.
The DNA of the tumor has become jumbled over the course of time. “If you look at the chromosomes it doesn’t look like a dog at all,” said Dr. Murchison. And it’s not possible to tell whether it was a male or a female because of the changes. But there are a few things known, she said. It wasn’t small, it didn’t have an unusually colored coat like, say, a Bernese Mountain dog, it didn’t have short hair.
It was, she said, “A bit wolfy.”
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