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Dogs, more so than almost any other domesticated species, are desperate for human eye contact. When raised around people, they begin fighting for our attention when they’re as young as four weeks old. It’s hard for most people to resist a petulant flash of puppy-dog eyes—and according to a new study, that pull on the heartstrings might be exactly why dogs can give us those looks at all.

A paper published today in the Proceedings of the National Academy of Sciencesfound that dogs’ faces are structured for complex expression in a way that wolves’ aren’t, thanks to a special pair of muscles framing their eyes. These muscles are responsible for that “adopt me” look that dogs can pull by raising their inner eyebrows. It’s the first biological evidence scientists have found that domesticated dogs might have evolved a specialized ability used expressly to communicate better with humans.

For the study, a team at the University of Portsmouth’s Dog Cognition Centre looked at two muscles that work together to widen and open a dog’s eyes, causing them to appear bigger, droopier, and objectively cuter. The retractor anguli oculi lateralis muscle and the levator anguli oculi medialis muscle (mercifully known as RAOL and LAOM) form two short, straight lines, which connect the ring of muscle around a dog’s eye to either end of the brow above.

These researchers have long been interested in the ways dogs make eye contact with humans and, in particular, how they move their eyebrows. In 2017, Juliane Kaminski, the lead author of the new paper, found that dogs moved their eyebrows more often while a human paid attention to them, and less often when they were ignored or given food (which, sorry to say, is a more exciting stimulus for them than human love). That suggested the movement is to some degree voluntary. On our side of these longing glances, research has also shown that when dogs work these muscles, humans respond more positively. And both man and mutt benefit from a jolt of oxytocin when locked in on each other.

This isn’t simply a fortuitous love story, in which the eyes of two species just so happen to meet across a crowded planet. Like all the best partnerships, this one is more likely the result of years of evolution and growth. If dogs developed their skill for eyebrow manipulation because of their connection to humans, one way to tell would be to look for the same capacity in wolves. Because dogs split off from their wolf relatives—specifically, gray wolves—as many as 33,000 years ago, studying the two animals is a bit like cracking open a four-legged time capsule. Divergence between the two species marked the start of dogs’ domestication, a long evolutionary process influenced—and often directly driven—by humans. Today, researchers can identify and study differences between the species to gain an understanding of exactly how dogs have changed over time.

In this case, those eyebrow-raising muscles do appear to be an addition to dogs’ anatomy. In the four gray wolves the researchers looked at, neither muscle was present. (They did find bundles of fibers that could be the precursors to the RAOL and LAOM.) In five of the six breeds of dogs the researchers looked at, both muscles were fully formed and strong; in the Siberian husky, the wolflike, oldest breed of the group, the researchers were unable to locate a RAOL.

Sometimes, the origins of changes like these aren’t immediately apparent. Certain physical dog traits—including floppy ears and short snouts—likely originate from the same set of developmental cells that code for tameness, a preferable trait in household pets, for instance. In the case of this new research, though, the connection between the physical trait and the related behavior is a bit more direct. “Previous work—and much of it by these same authors—had shown that these muscles were responsible for enhancing positive responses in humans,” Brian Hare, the director of Duke University’s Canine Cognition Center and the editor of the paper, told The Atlantic via email, “but the current suggests the origin of these facial expressions is after dogs split from wolves.”

Read: A new origin story for dogs

“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.”

Both Horowitz and Johnston suggested that similar studies looking at populations of dingoes (which Johnston researches) and Siberian foxes could provide yet another time capsule of sorts for understanding eyebrow movements and other evolutionary traits. Both species live near humans and are some of the closest living relatives to the earliest dogs. Why did they stay wild while dogs drifted into domestication? “Anything to do with getting to the bottom of why we as a species picked out this one animal can carry a huge amount of information,” Horowitz says. “In some ways, it’s discovering something about ourselves.”

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HALEY WEISS is an editorial fellow at The Atlantic. Twitter