According to a new hypothesis, it turns out that in the process of domesticating dogs, we might have actually been affecting some of their stem cells.In a dog embryo, there’s a group of stem cells called the neural crest. And these cells are responsible for forming a specific set of physical features–like the dog’s coat, and the structure of its face, and its adrenal glands. The earliest dogs may have been less aggressive because they had smaller adrenal glands. So when early humans bred for tameness, the dogs probably also ended up with changes to other traits that are controlled by the neural crest–like floppy ears, and the faces with more juvenile features, such as smaller jaws. So basically, by domesticating dogs, we may have ended up selecting for mutations in their stem cells that made them less like wolves and more like the animal that’s probably sleeping in your living room right now.