The Mother of All Dogs

Long before history began, our ancestors began an unwitting scientific experiment. Somehow humans managed to domesticate the wolf, and the two-legged masters began breeding their four-legged companions in a primitive form of genetic engineering that would, thousands of years later, result in Lassie, pooper scoopers and the Taco Bell chihuahua.

That it happened more or less this way is not in doubt. Where, when and why it happened are other matters. Until last week the theories had been wide ranging and contradictory. Depending on whom you asked, wolves became dogs either independently in several places or in a single location. The transformation took place either 15,000 years ago or maybe 135,000. Most important, nobody has had a good explanation for how dogs learned to get along so well with us in the first place.

Three studies presented in the journal Science last week help resolve all these questions. The first paper addresses where the wolf-to-dog transformation took place. Biologist Jennifer Leonard, at the Smithsonian's National Museum of Natural History in Washington, and her co-authors collected the remains of dogs buried in North, Central and South America before Columbus showed up (and thus before interbreeding with European dogs could have taken place) and sampled their mitochondrial DNA, or MTDNA, which is passed on only from the mother. If these ancient American dogs had arisen locally, their MTDNA should have been similar to that of American wolves. Instead, it was closest to that of modern Eurasian dogs. Conclusion: the transition from wolf to dog took place exclusively on the Eurasian landmass. When the first humans crossed to North America from eastern Asia, they brought their newly domesticated canines with them.

That also jibes with the work of Peter Savolainen of Stockholm's Royal Institute of Technology. Savolainen and his colleagues used MTDNA in their study too--in this case, to see how much the genetic material varied within dog populations. Animals from East Asia turned out to have the most variation, suggesting that this group's MTDNA had been around the longest and had had more time to branch into different subtypes. Presumably, then, the dogs arose there.

By comparing the dog MTDNA with the MTDNA of wolves, Savolainen and his colleagues were also able to estimate when the change happened. Beginning with approximations of how frequently MTDNA mutations take place and then counting the number present in their samples, they concluded that the genetic branching began 15,000 years ago, if three families of wolves were involved, or 40,000, if the entire process began with a single family.