Domesticated dogs first appeared in East Asia, spread across Asia and Europe, and then accompanied their two-legged companions into the New World 12,000-14,000 years ago. This scenario is suggested by two reports in the journal Science, published by the American Association for the Advancement of Science.
Two research teams, one studying dog origins in the New World, the other in the Old, found that Eurasian wolves were dogs' likely forebears. Breeding over the last 500 years--not different genetic origins--is responsible for the dramatic size and shape differences among modern dogs, according to author Peter Savolainen of the Royal Institute of Technology, in Stockholm.
Domestication may have happened as recently as 15,000 years ago, according to Savolainen's study on Old World dogs. The results from New World dogs suggest an older origin, according to author Carles Vilà of Uppsala University in Uppsala, Sweden. The oldest known dog remains are 14,000 years old, but Vilà thinks dogs and humans were living together well before that.
"We found that dogs originating in the Old World arrived to the New World with immigrating humans. Thus, even before the development of trade as we know it now, humans had to be exchanging dogs," Vilà said.
Researchers don't yet know just how or why humans domesticated dogs, but the speed at which dogs seem to have multiplied and diversified indicates that they did something important for humans, according to Vilà.
"I can imagine that if dogs were, for example, improving the quality of hunting, that would be a very great advantage for humans. It could even have made the colonization of the New World easier," Vilà said. "There must have been something advantageous about those dogs that made them extremely successful and allowed them to spread all over the world."
Savolainen and his colleagues analyzed DNA samples taken from dogs in Asia, Europe, Africa, and arctic America. They found that, while most dogs shared a common gene pool, genetic diversity was highest in East Asia, suggesting that dogs had been domesticated there the longest. Previously, researchers had generally looked to the Middle East as the setting for domestication of plants and animals, according to Savolainen.
"Most earlier guesses have focused on the Middle East as the place of origin for dogs, based on the few known facts--a small amount of archeological evidence from the region, and the fact that several other animals were domesticated there," Savolainen said.
The same East Asian human population seems to have domesticated dogs from several different wolves, suggesting that "this was not a chance event," Savolainen said.
Vilà and his colleagues set out to investigate whether dogs in the New World were domesticated from wolves there, independently from Old World dogs, or whether the two groups were related.
The researchers compared DNA sequences of New and Old World dogs, including some Latin America and Alaskan dogs that pre-dated the first European explorers in the Americas. The similarities among the sequences indicated that all the dogs shared a common ancestor.
A certain cluster of sequences from ancient Latin American dogs didn't match any from modern dogs. Thus, the researchers concluded, European colonists probably did not use Native American dogs to create the breeds that we know today.
"These dogs were left behind by modern breeding programs," Vilà said.
Both research teams studied genetic sequences that came from the dogs' mitochondrial DNA, which, unlike the DNA in the cell nucleus, is inherited directly from the mother. The scientists looked specifically at the "control region," a stretch of DNA known to accumulate mutations relatively quickly, making it a good place to look for differences between dogs and wolves.
Computer programs grouped certain sequences inherited together, or "haplotypes," according to their similarities. The haplotypes formed four main "clades" that were roughly similar in both studies, plus two additional smaller clades in Savolainen's study.
95 percent of the dogs in that study belonged to three of the major clades, "A," "B," and "C," at similar rates in all regions. Thus, the major present-day dog populations at some point had a common origin from a single gene pool containing the three clades, the authors suggested
Further analysis of the relatively large number of Clade A haplotypes showed that the sequences clustered into several subgroups. By assuming that mutations in the control region have occurred at a constant rate, the researchers could estimate how long it had taken the subgroups to evolve.
If the subgroups were the result of multiple wolf introductions, domestication should have begun around 15,000 years ago, the researchers found. If there had only been one wolf introduction, domestication could have begun as early as 40 thousand years ago. Because the oldest archeological evidence for dogs is 14,000 years old, the 15,000 year date was more likely, Savolainen and his team decided.
The Native American dog sequences in Vilà's study also clustered in several subgroups, whose haplotypes were similar or identical to those of Old World dogs. These subgroups likely represented the multiple lineages that crossed the Bering Strait along with the first humans to do so, approximately 12,000-14,000 years ago, Vilà and his colleagues concluded.
Savolainen's co-authors are Ya-ping Zhang and Jing Luo at the Chinese Academy of Sciences, in Kunming, China; Joakim Lundeberg at the Royal Institute of Technology, in Stockholm, Sweden; and Thomas Leitner at the Swedish Institute for Infectious Disease Control, in Solna, Sweden. The study was supported by the Swedish Research Council, the Swedish Kennel Club, the State Key Basic Research and Development Plan of China, and the Natural Sciences Foundation of China.
Vilà's co-authors are Jennifer Leonard and Robert K. Wayne at the University of California, Los Angeles, in Los Angeles, CA; Jane Wheeler at CONOPA in Lima, Peru; Raúl Valadez at Universidad Nacional Autónoma de México, in Mexico D. F., Mexico; and Sonia Guillén at the Bioanthropology Foundation Peru, in Lima, Peru. The study was supported by UC-MEXUS and the National Science Foundation.
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