Greenland sled dogs, a fluffy, curly-tailed canine native to the harsh Arctic tundra, could be the oldest dog breed, according to the first study to take a deep dive into the animals’ genetic history. The sled dog branch of the family tree, which includes various types of huskies and malamutes, broke off from the rest of the dogs around 9,500 years ago, versus something like a labradoodle, which only became a breed in 1989.Scientists know that dogs likely evolved from Eurasian wolves, but exactly when or where that transformation took place is a matter of great mystery. To better understand the genetics of sled dogs and their place in the world, scientists sequenced the genome of a dog from Siberia’s Zhokhov archaeological site, dating to around 9,500 years ago.“I was actually anticipating that we would find some sort of precursor of domestic dogs,” says lead author Mikkel-Holger Sinding, a paleogeneticist and Ph.D. student at the University of Copenhagen in Denmark.Instead, he and his colleagues found today’s sled dogs and the Zhokhov dog descended from the same branch. “It means that all dogs must have diversified earlier than this,” he says. (Read more about how sled dogs have been pulling humans around for millennia.)That’s a huge finding, Sinding says, because it provides the “first firm date for diversification in dogs”—in itself an important clue in the mystery of dog domestication.Built for the coldThe analysis, which compared genes between ancient and modern dog sled dogs with those of other breeds, also revealed all sorts of fascinating and unique adaptations to Arctic life, such as the ability to thrive on a high-fat diet. (Learn how centuries of breeding have reshaped dog brains.)“One of the biggest differences between a brown bear and a polar bear is that the polar bear has a specific genetic adaptation for eating lots of blubber. And we see almost precisely the same solution in [sled] dogs,” Sinding says.This makes logical sense, as the Inuit and Thule peoples of the Arctic and their working dogs have survived for thousands of years by hunting blubber-rich marine mammals, like seals and whales.The scientists also compared the Zhokhov dog’s DNA with an even more ancient canid—a Siberian Pleistocene wolf that lived about 33,000 years ago. Together with genomes from modern wolves and domesticated dogs, the team revealed that, remarkably, sled dogs haven’t interbred with gray wolves in the past 9,500 years, unlike other breeds. This is especially strange, given that indigenous peoples have documented dog-wolf pairings. The fact that traces of wolf genetics don’t show up in the Greenland sled dogs’ genome suggests that either hybrids didn’t survive well, or that there was some reason humans did not breed them.The research also showed that sled dog genomes contain mutations related to their cold environments, such as running and pulling sleds in low-oxygen conditions.“So, being able to still exercise even if you can’t catch a breath,” says Elaine Ostrander, a geneticist at the National Institutes of Health who studies canine genomes but was not part of the study.Another mutation allows sled dogs to highly regulate body temperatures, says Ostrander—necessary not only to survive the cold, but to cool down after a period of exertion.