Dogs were some of the first animals that humans domesticated. These furry pals were living with people for thousands of years before we invented agriculture and started keeping other animals like goats and pigs. Though we have archaeological evidence of dog bones within human communities dating back 15,000 years, scientists still aren't sure where humans began the process of converting wild wolves into snuggly companions. Now, a new study suggests that dogs were domesticated twice—once in Europe and once in Asia, probably around the same time.

A large group of researchers with expertise in everything from archaeology to paleogenetics has collaborated on a paper in Science explaining how it reached this conclusion. The group began by sequencing DNA from ancient and modern dogs to measure genetic drift. The linchpin of the study was a well-preserved bone from a dog that lived 4,800 years ago in Ireland, roughly around the time that Stonehenge was being constructed. By comparing this dog's DNA with that of more than 600 modern dogs and snippets of DNA from other ancient dogs, the team could determine that this Western dog belonged to a genetic group that diverged from Asian dogs between 14,000 and 6,400 years ago.

Evolutionary biologist Greger Larson told Science's David Grimm, "I was like, ‘Holy shit!’ We never saw this split before because we didn’t have enough samples."

If that were the whole story, we'd have strong evidence that dogs were domesticated in Asia and spread west into Europe, but there is one pesky problem. There's evidence of domesticated dogs in Europe 15,000 years ago, at least 1,000 years before the Irish dog's ancestors diverged from Asian stock. In fact, the researchers note, we have evidence of domestic dogs during this late paleolithic period on both sides of the Eurasian continent—but nothing from the center of the continent. If Asian domestic dogs had spread to Europe, you'd expect to see signs of them thousands of years ago in central Asia. Yet, that evidence doesn't exist. Based on what archaeologists have found in paleolithic sites, it seems that human-domesticated dogs emerged about 15,000 years ago in Europe and eastern Asia. It was only later that Asian dogs crossed the continent with humans and supplanted ancient European dogs.

To be absolutely certain that this account is accurate, more research needs to be done. We need to sequence more dog genomes and get a fuller picture of ancient dog population genetics. It's also possible that we may find archaeological evidence of ancient dogs in central Asia, which would mean that humans might have brought dogs with them from Asia to Europe before 15,000 years ago. For now, at least one thing is certain: at some point between 14,000 and 6,400 years ago, dogs from Asia did make it to Europe, interbreeding with local dogs and mostly replacing them. Nearly all dogs today are descended from dogs that were domesticated in Asia.

What's fascinating is that we now have indication that there was no single domestication event when it comes to dogs. Millennia ago, humans throughout Eurasia came to very similar conclusions about wolves. Somehow, vastly different groups of humans looked at those wolves and saw potential friends and allies. And that spark of recognition changed the course of human and canine life forever.

Science, 2016. DOI: 10.1126/science.aaf3161