The latest genetic study to trace the origins of dogs confirms the view that they were domesticated by hunter-gatherers at least 9,000 years ago - but the results raise almost as many questions as they answer.
Exactly what kind of wolf gave rise to "man's best friend"? Did domestication take advantage of a rare genetic quirk, or did early humans merely take advantages of wolfish traits?
The study published on Thursday in the journal PLOS Genetics doesn't resolve those questions. But one of its senior authors, University of Chicago geneticist John Novembre, says researchers are working on ways to get the answers.
"Ancient DNA studies are going to be very exciting for this field in the near future," he told NBC News in an email.
Novembre said Thursday's study complements earlier work that analyzed mitochondrial DNA from modern-day wolves as well as ancient wolf remains. It suggests that dogs descended from a wolf strain that has since gone extinct. The genetic tale is complicated, however, due to interbreeding between dogs and wolves in recent times.
The PLOS Genetics study relies on detailed analyses of the genomes for two dog breeds, a dingo from Australia and a Basenji whose lineage goes back to Africa. The genomes for three wolves living in different parts of the world - Croatia, Israel and China - were also analyzed. To round out the picture, the researchers sequenced the genome of a golden jackal and included the previously published genome of a boxer from Europe.
The wolves were selected to represent the three regions of the world that have been identified as the potential points of origin for domesticated dogs: Europe, the Middle East and Asia. "These are the highest-quality wolf genome sequences published to date, and among the very first," Novembre said. "Without high-quality wolf genomes, it's difficult to learn about the origins of dogs."
He and his colleagues expected to find that the dog breeds had closer genetic connections to one of the wolves over the others. That would have provided a new clue in the detective story, but that's not what happened. Instead, the results suggested that the Basenji and the dingo both descended from an older, wolflike ancestor.
"Perhaps the closest lineage of wolves to dogs went extinct and is not represented well by modern wolves," Novembre said. "The ancient mitochondrial DNA paper suggests that the ancient lineage of wolves may have existed in Europe."
Hunting vs. farming
The earlier research estimated that domestication occurred sometime between 18,000 and 32,000 years ago. This week's paper cites a similarly wide range of dates, from 9,000 to 34,000 years. Both time frames predate the rise of agriculture, which occurred several thousand years ago. Those findings favor the view that dogs started out as hunting companions rather than village scavengers.
Novembre and his colleagues also followed up on earlier findings that dogs adapted to digest starch, perhaps due to their proximity to human agricultural settlements. They determined that most dogs do indeed have high numbers of the amylase genes that promote starch digestion. However, that's not the case for dogs that lack a close connection to agrarian societies, such as Siberian huskies and dingoes.
The researchers found evidence for amylase genes in wolves, too. Taken together, the findings lend further support to the hunting-dog scenario: Dogs probably started out mostly as meat-eaters, but gradually adapted to a starchy diet when they had to.
The genetic record suggests that dogs went through a narrow population "bottleneck" after they diverged from wolves. Wolves went through a similar bottleneck, and that may have been when the breed of wolf that gave rise to dogs died out. Why did the bottleneck occur? That's another question yet to be answered.
"Presumably, changes in habitat and prey availability as humans expanded and altered landscape is part of the story," Novembre said.
In addition to Novembre, the authors of "Genome Sequencing Highlights the Dynamic Early History of Dogs" include Adam H. Freedman, Ilan Gronau, Rena M. Schweizer, Diego Ortega-Del Vecchyo, Eunjung Han, Pedro M. Silva, Marco Galaverni, Zhenxin Fan, Peter Marx, Belen Lorente-Galdos, Holly Beale, Oscar Ramirez, Farhad Hormozdiari, Can Alkan, Carles Vilà, Kevin Squire, Eli Geffen, Josip Kusak, Adam R. Boyko, Heidi G. Parker, Clarence Lee, Vasisht Tadigotla, Adam Siepel, Carlos D. Bustamante, Timothy T. Harkins, Stanley F. Nelson, Elaine A. Ostrander, Tomas Marques-Bonet and Robert Wayne.