A vast population of indigenous domestic dogs once roamed the Americas, concludes one of the largest studies yet of ancient dog DNA, published in Science on 5 July1. Today, almost nothing remains of this dog family, apart from a bizarre transmissible cancer.
The oldest known domestic-dog remains in the Americas are approximately 9,900-year-old skeletons from a site in Illinois; they were deliberately buried, implying that the animals were important to their owners2. But exactly when those dogs arrived in the Americas or how they relate to domestic dogs elsewhere has been unclear, says Angela Perri, an archaeologist at Durham University, UK.
To find out, Perri and her colleagues analysed DNA from 71 ancient dogs that lived across North America and Siberia over the past 10,000 years. On the basis of the animals’ mitochondrial genomes — which are inherited maternally — the researchers found that all of the ancient American dogs belonged to the same population, distinct from modern and ancient Eurasian dogs. Analysis of the nuclear genomes of seven of the canines confirmed this.
From the genome data, the researchers estimate that the last common ancestor of the ancient American dogs lived about 14,600 years ago — and that it separated from Siberian dogs roughly 1,000 years before that. Humans first crossed into Alaska from Asia around 20,000 years ago, and the dogs may have been imported by later waves of hunter-gatherers.
To study the legacy of the first American dogs, the researchers examined the DNA of more than 5,000 modern dogs from across North and South America. The team concluded that these animals traced only 2-4% of their ancestry to indigenous American dogs.
The researchers speculate that when Europeans arrived in the New World in the 15th century, they favoured their own dogs and prevented them from breeding with indigenous ones, and so the indigenous dogs died out. That would make sense, says Elaine Ostrander, a geneticist at the National Human Genome Research Institute in Bethesda, Maryland. “You’re going to believe what you bring with you is better than what’s already there,” she says.
Elinor Karlsson, a geneticist at the Broad Institute in Cambridge, Massachusetts, isn’t persuaded. “It seems ridiculous to me, given the scale of loss of the dogs, to argue this came down to human preference,” she says. In an essay3 accompanying the research paper, she suggests that a contagious cancer contributed to the indigenous dogs’ demise.
Canine transmissible venereal tumour (CTVT) is one of a handful of known contagious cancers — more famous are the two forms that threaten Tasmanian devils with extinction. CTVT is a parasitic clone of a tumour that emerged in a single dog and has since gone global, largely owing to contact between dogs during mating. It creates large tumours on the genitals of males and females.
Karlsson’s idea emerges from the paper’s discovery that CTVT originated as early as 8,225 years ago, in a dog that was more closely related to indigenous American dogs than to modern Eurasian dogs.
Despite its close genetic ties with indigenous American dogs, the researchers think the tumour emerged in Asia in a relative of the dog population that had entered the Americas several millennia earlier. Other evidence suggests that the tumour diversified in Asian dogs, before spreading to Europe and Africa in the past 2,000 years4. It probably reached the Americas only 500 years ago, with the arrival of Europeans and their dogs.
Karlsson speculates that the close genetic relationship between the tumour and indigenous American dogs might explain the dogs’ disappearance. CTVT isn’t fatal in most dogs, because their immune system recognizes the tumour cells as foreign and limits the damage they cause. Perhaps, Karlsson says, the immune systems of indigenous American dogs overlooked the tumour cells because the cells’ DNA was so similar to their own. The tumours might, then, have grown more aggressively in indigenous dogs, eventually killing them or stopping them from mating.
Elizabeth Murchison, a geneticist at the University of Cambridge, UK, who co-led the latest study, finds that to be a plausible explanation for the disappearance of indigenous American dogs. “The last remaining vestige of this dog’s group might have contributed to its downfall,” she says.