It was only by the tiniest bit of genetic chance that our cross-species union was forged at all. Dogs and wolves share 99.9% of their mitochondrial DNA—the DNA that’s passed down by the mother alone—which makes the two species nearly indistinguishable. But elsewhere in the genome, there are a few genetic scraps that make a powerful difference. On chromosome six in particular, investigators have found three genes that code for hyper-sociability—and they are in the same spot as similar genes linked to similar sweetness in humans.

Our ancestors didn’t know what genes were many millennia ago, but they did know that every now and then, one or two of the midsize scavengers with the long muzzles that came nosing around their campfires would gaze at them with a certain attentiveness, a certain loving neediness, and that it was awfully hard to resist them. So they welcomed those few in from the cold and eventually came to call them dogs, while the animals’ close kin that didn’t pull the good genes—the ones we would come to call wolves or jackals or coyotes or dingoes—would be left to make their way in the state of nature in which they were born.