The dogs of ancient Europe probably looked a lot like the mutts roaming Europe today, new DNA discoveries from dog fossils suggest. In the ongoing debate over how many times dogs were domesticated from wolves, this new study suggests it happened just once.

Dogs are the very first species that humans tamed, but the details surrounding dogs’ origins are a little fuzzy. Now, ancient DNA extracted from two 7,000-year-old and 4,700-year-old dog fossils discovered in Germany offer scientists a glimpse at dog evolution. Modern dogs probably descended from just one population that lived continuously in Europe for millennia, according to the research led by Krishna Veeramah at Stony Brook University.

Two prehistoric dog fossils offer scientists a glimpse at dog evolution

Our furry friends likely evolved from a population of wolves domesticated sometime between 20,000 and 40,000 years ago. Exactly who domesticated these wolves, when, and how many times, is still a mystery, and scientists don’t agree on the answer. Dogs were probably domesticated by accident, when wolves began trailing ancient hunter-gatherers to snack on their garbage. Docile wolves may have been slipped extra food scraps, the theory goes, so they survived better, and passed on their genes. Eventually, these friendly wolves evolved into dogs. “People want a story that someone picked up a wolf cub and made a dog — but it’s been a much more complex process than that,” Veeramah says.

Last year, researchers led by Oxford’s Greger Larson argued that DNA from a 5,000-year-old Irish dog fossil showed signs that this complex evolution happened not once, but twice: once in Europe, and once in Asia. The dogs domesticated in Asia later replaced some of the early European dog population, they reported.

Today’s study disputes those findings, however, arguing instead that a single group of dogs were probably first domesticated between 20,000 and 40,000 years ago. (They don’t say where.) These ancestral dogs then split into Eastern and Western populations. The dogs that stayed in Europe are likely the distant ancestors of modern European mutts and many of today’s breeds, the study reports today in Nature Communications.

A single group of dogs were probably first domesticated between 20,000 and 40,000 years ago

It’s a solid paper, says Adam Boyko, a dog geneticist at Cornell University who wasn’t involved in the research. And most of the field would probably agree with its conclusions: that dogs were probably domesticated just once, and within the 20,000-year window Veeramah proposes. “Certainly dog geneticists can be a contentious group,” Boyko says. “I don’t think anyone’s overly invested in their own theory. It’s just that these are complicated questions, and everyone’s trying the best that they can to get the right answer.”

In fact, Gregor Larson’s team at Oxford — whose study last year supported the two domestications hypothesis — shared their data with Veeramah’s team. Their analysis of a 5,000-year-old Irish dog fossil revealed genetic traces of what might have been an extinct, European dog lineage, which they concluded could have resulted from a separate, earlier domestication event. But when Veeramah’s group reanalyzed the data, they couldn’t replicate the signal. “There wasn’t any evidence that this dog had anything special about it,” he says. Instead, he says they discovered a technical glitch behind the findings supporting two domestications, which they reported in their study today.

“The paper brings us back to the idea that there’s a single event.”

Veeramah’s team also extracted DNA from two more dog fossils discovered in Germany over the last 20 years. They re-created a canid family tree by comparing chunks of DNA from these ancient dogs and today’s purebreds, mutts, and wolves. By counting the genetic differences, and estimating how long it would take for those differences to show up, they could roughly date when each of these groups split apart. For wolves and dogs, that was roughly 20,000 to 40,000 years ago. For Eastern and Western dog populations, it was probably between 17,000 and 24,000 years ago.

The two ancient German canines turned out to be genetically related to one another, and to the dogs of today despite living thousands of years apart. There was a key difference though: today’s dogs are much more able to digest starches than these ancient dogs, thanks to a digestive enzyme. More copies of the gene for this enzyme help dogs digest starches better, and modern dogs have a lot of copies. These ancient dogs didn’t have nearly as many, however, so this adaptation to domestic life may have emerged later, possibly when agriculture and grain became more widespread.

“The paper brings us back to the idea that there’s a single event,” Boyko says. And it highlights how important ancient DNA will be for piecing together dogs’ contentious origin stories.