After that domestication event, some things do seem to have stayed constant. According to the team’s results, after dogs split off from wolves over 11,000 years ago, wolves never made a major reentry into dog populations (until, perhaps, the contemporary craze for wolfdogs). Given that dogs and wolves belong to the same species and produce perfectly healthy offspring, this discovery came as a surprise to the authors. They inferred this result from the observation that some wolves are equally related to all ancient and modern dogs, which indicates that all dogs have the same amount of wolf ancestry. The logical explanation is that wolves didn’t contribute substantially to the dog gene pool after domestication. If, instead, wolves had continued interbreeding with dogs, the team would have expected to observe that all wolves were more closely related to some dogs—which had wolves in their family trees post-domestication—than others, which only had dog ancestors.
But, for some reason, the opposite happened when it comes to the wolf genome: Dogs are universally more related to some wolves than they are to others, which indicates that dogs did in fact contribute genetic material to wolf populations. This asymmetry between dogs and wolves may have a simple explanation: humans. “It shows us,” Lindblad-Toh says, “that probably people held onto their dogs and took good care of them and made sure that they didn’t let wolves in.” The wolves had no such guardians.
But Liisa Loog, a postdoctoral researcher in the Genetics Department at the University of Cambridge who was not involved in the study, believes that it is important to keep this result in perspective. She notes that the authors’ argument depends on some specific assumptions about how ancient wolves relate to modern wolves, assumptions that are impossible to confirm without studying ancient wolves directly. “The authors here rely on the assumption that this happened on a now-extinct wolf population that hasn’t been sampled, and that is equally related to all modern-day wolf populations,” she says. “This may be the case, but it also may not be the case.”
This assumption, and the assumptions about geographic and climatic consistency that undergird Bergström and Frantz’s trade hypothesis, do mean that their results and theories can’t be confirmed without additional research, like similar studies of ancient wolf DNA. But, ultimately, 27 dog genomes are a narrow window onto the past: When working with such a small amount of data, assumptions become necessary. “The DNA itself is just DNA,” Bergström says. “It needs that wider context of interpretation.”
The scarcity of evidence, coupled with the difficulty of extracting high-quality DNA from such old bones, might make ancient DNA research seem like a foolhardy endeavor—why not just obtain genetic samples from modern dogs and figure out the family tree from there? But ancient DNA also has some distinct advantages over modern DNA, especially when it comes to dogs. Many contemporary dogs owe their genetic profiles to the Victorian dog breeding craze, so the signatures of their more distant past may be difficult to discern. Looking for evidence about ancient dogs in the genomes of modern ones is like “searching for a needle in a haystack,” Loog says. So it can help to go directly to the source. “Ancient DNA,” Loog says, “literally gives us this time-stamped genetic picture of the past.”
So, while it may be difficult to learn about prehistoric dogs by studying their modern descendants, the special insights afforded by ancient DNA can provide invaluable context for understanding how humans relate to dogs today. “Dogs are kind of unique in that they are a predator, a carnivore. And they were domesticated by hunter-gatherers, way before agriculture, and they were also able to spread so quickly to most groups,” Bergström says. “It’s somehow a surprisingly good fit for the human species to take on this animal as a companion—even though, a priori, it seems like an unlikely candidate for domestication.” If Bergström and his colleagues are right, the human tradition of living with, breeding, and protecting dogs, and of treating canines not just as useful tools but as sources of social connection and emotional support, could have an 11,000-year history. Even before they figured out how to cultivate crops, humans may very well have known how to take care of, and be taken care of by, their animals.
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