There’s a reason we call them “puppy dog eyes”—those soulful, innocent expressions can sway even the most hardened human.

That’s no accident, a new study says. Centuries of domestication have radically reshaped a dog’s eyebrow anatomy, making their faces—and emotions—easily readable to people.

When meeting a person’s gaze, dogs often raise their inner eyebrow muscle to make their eyes look larger and more appealing. (See dog-evolution pictures.)

“There’s no evidence that dogs move this [eyebrow] muscle intentionally, but it creates an exaggerated movement that for us means ‘dog,’” says study leader Juliane Kaminski, a psychologist at the University of Portsmouth in the U.K.

Eyebrow movement plays a major role in human communication, Kaminiski says: “I’m doing it now when I’m speaking to you, even though I know you can’t see me."

The study is the latest example of how 20,000 years of cohabitation has made our pets finely tuned interpreters of human emotion—possibly more so than any other species. (See 19 ways dogs tell us what they want.)

Eyebrow raising

In her past research, Kaminiski has found dogs are uniquely skilled at understanding gestures, outperforming even non-human primates such as chimps.

Several years ago, Kaminski began investigating the flip side of this relationship, looking at how people decipher dog behavior. In one experiment, published in 2013, she filmed shelter dogs to see if any of their behaviors were linked to how quickly the animal was adopted.

Of all the factors Kaminski examined, only one stood out as significant: the movement of the dog’s eyebrows upward and inward. (See National Geographic's portraits of dogs.)

Dogs Change Facial Expression When Humans Pay Attention

Initially, “it was a very surprising result. We didn’t expect something as small as eyebrow movement to have a big effect,” Kaminski says.

But a question remained: Whether this eyebrow movement was unique to dogs, or if it could be found in their ancestor, the gray wolf.

The new study, published this week in Proceedings of the National Academy of Sciences, Kaminski and colleagues dissected and analyzed the facial muscles of six dogs—including a mongrel, a Labrador retriever, a bloodhound, a Siberian husky, a Chihuahua, and a German shepherd—as well as four wild gray wolves. The animals had all died natural deaths, and their bodies were donated to science.

The team discovered the levator anguli oculi medialis, a large and prominent muscle, in all six dog specimens—but it was almost completely absent in wolves.

Kaminski and colleagues also found that the retractor anguli oculi lateralis muscle was smaller and more variable in size and presence in wolves than it was in dogs, with the exception of the Siberian husky, a more ancient dog breed that's closely related to the wolf.

This muscle, which runs along the eye’s outer edge, exposes more of the white of the eye—making the dogs appear human-like.

“Small shifts can have perceivable impacts when it comes to changes in anatomy," says Molly Selba, a Ph.D. student who studies evolution and dog domestication at the University of Florida. "The muscles of facial expression are relatively tiny muscles, but they can make a big impact on the way we perceive a face," says Selba, who wasn't involved in the research.

An evolutionary leg up

Brian Hare, an evolutionary anthropologist at Duke University, notes that dogs are wolves are very similar anatomically—with the exception of these eye muscles.

“This new study shows that these morphological changes evolved as dogs and humans have interacted over the past 20,000 years,” Hare, who also wasn’t part of the new study, says by email. (See "Dog and human genomes evolved together.")

“They almost certainly did not evolve due to intentional selection, but instead gave dogs an advantage in their interactions with humans.” (Read why dogs are so friendly.)

Next, Kaminski hopes to examine a wider variety of dog breeds, including more ancient breeds and street dogs, to understand precisely how these muscular changes evolved.