By evolutionary standards, the time since this split has been remarkably short for two new facial muscles to have developed. For a species to change that quickly, a pretty powerful force must be acting on it. And that’s where humans come in. We connect profoundly with animals capable of exaggerating the size and width of their eyes, which makes them look like our own human babies and “hijacks” our nurturing instincts. Research has already demonstrated that humans prefer pets with more infantlike facial features, and two years ago, the authors of this latest study showed that dogs who made the facial movement enabled by the RAOL and LAOM muscles—an expression we read as distinctly humanlike—were more likely to be selected for adoption from a shelter than those who didn’t. We might not have bred dogs for this trait knowingly, but they gained so much from having it that it became a widespread facial feature. “These muscles evolved during domestication, but almost certainly due to an advantage they gave dogs during interactions with humans that we humans have been all but unaware of,” Hare explained.

“It’s such a classically human system that we have, the ways we interact with our own infants,” says Angie Johnston, an assistant professor at Boston College who studies canine cognition and was not involved with the study. “A big theme that’s come out again and again in canine cognition and looking at the domestication of dogs is that it seems like they really just kind of dove right into our society in the role of being an infant or a small child in a lot of ways. They’re co-opting existing systems we have.”

Read: Why do humans talk to animals if they can’t understand?

The same humanlike facial gestures could also be a dog’s way of simply securing attention in the first place. Eyebrow raising is one of the most well-understood examples of what researchers call ostensive cues, a family of nonverbal signals (often facial movements and expressions) humans send one another to convey their intention to directly communicate. Dogs’ uncanny ability to mimic this human expression likely leads us to project certain human emotions onto them in ways we don’t with other animals, regardless of what they might actually be feeling.

The movement of the RAOL and LAOM muscles is particularly open to interpretation. “In different contexts we’ll call that something different,” says Alexandra Horowitz, a senior research fellow at the Barnard College Dog Cognition Lab. “In one case, I might say it’s sad, but in another case I’ll say, He’s really paying attention. It can look wry, like a questioning or unbelieving look.” According to Horowitz, dogs are the only animals aside from our primate cousins that are expressive in this eerily familiar way. Horses alone share the ability to twist their eyes into the same doleful shape, but their overall expressions don’t strike us as humanlike in the same way that dogs’ do. With dogs, Horowitz points out, we’re so driven to connect that we often search for “smiles” in the shapes of dogs’ mouths. The new research, she says, “makes me think it’s more about being able to move the face in a way that humans move the face. We don’t like unexpressive faces.”