Many different versions of the dog origin story exist (and more than one may be true), but the gist tends to go like this. Some tens of thousands of years ago, wolves and humans started spending a lot more time together, and began to coevolve. It’s not clear who made the first move—maybe it was the canines, lured into encampments by their noses; maybe the two species just found themselves thrown together, and bonded over a mutual love for meat. In any case, the chillest and chummiest canines of the bunch kept coming back. At first perhaps lopsided, the relationship soon became more mutually beneficial: People realized that dogs could enhance humans’ ability to feed and protect their families, and eventually, corral their sheep and cattle; the animals would do this in exchange for calories, shelter, and maybe some well-earned belly rubs.

The first chapter of the dog-human relationship, then, was about function. People noticed behaviors they liked in the animals, and started to favor them, “maybe giving them extra food, giving them a chance to breed,” says Kathryn Lord, a dog-behavior-and-evolution expert at UMass Chan and the Broad Institute, where she’s working with Morrill. Slowly, a wolfish lineage shed some of its fear of people, and some of its grump; it lost the sharpness of its lupine features, and its apex-predator edge. Even the animals’ tightly tuned predatory sequence—search, stalk, chase, grab, kill—fractured, yielding groups of dogs that specialized in, for instance, stalking and sprinting (herders), pursuing and catching (retrievers), all of the above (terriers), or none of the above (livestock guardians). Under the pressures of employment, dogs diversified.

Then, in the 1800s, dog-rearing underwent a massive pivot. “Victorians changed the way we think about dogs,” says Michael Worboys, a historian of science at the University of Manchester, in England, and an author of The Invention of the Modern Dog: Breed and Blood in Victorian Britain. It was in this period—the era of “fancy everything,” as Lord puts it—that the modern concept of breed was born. Suddenly, people prized dogs more for their looks than the tasks they could perform. Puppeteering the sex lives of dogs became ultra-purposeful, ultra-fashionable; the idea of breed became so valuable that it needed to be policed by stringent criteria and formal clubs. And as the goalposts shifted to achieving purity of blood and physical ideals, canine evolution bent fast. “Once you start selecting on form—your coat color, your shape,” Lord told me, “it’s so much more powerful than selecting on what it behaves like.” The number of distinct breeds ballooned, and the dogs within them grew more and more alike.

Nowadays, that uniformity seems a scientific dream: Purebred genomes have been stripped of much of diversity’s noisiness, making patterns within groups easier to spot; with the dog genome sequenced, it should be easy to go in and figure out how human meddling has, at various points, cemented both physical and behavioral propensities into DNA. But behavior is extremely complicated—sometimes involving many, many genes that may each have only a small influence—and it’s been repeatedly wrung through humans’ changing ideas about what makes for a very, very good boy.