Dog lovers are well aware of the ability of a dog to communicate with its eyes. Now, scientific research suggests this ability developed over time as dogs learned to live with humans.

Just as with humans, the eyes can be one of the most ________ things about dogs, the animal known in many cultures as “man’s best friend.”

But a new study found evidence that dogs developed in physical ways to present “puppy dog eyes” as a way to help ________humans. Researchers in Britain and the United States did the study. The study compared the ________muscles of dogs and wolves, which share ancestral history. Dog ________ from wolves after being domesticated about 33,000 years ago. During that time, dog changed physically and behaviorally to ________ life with humans.

The researchers ________ the heads of six dogs and two wolves for comparison. They found the facial structure of both animals was mostly very similar. But one major________ was found above the eyes.

The dogs were found to have two well-formed muscles around the eye that were not________in the wolves. These small muscles permit dogs to ________ raise their inner eyebrow, the study found.

Comparative psychologist Juliane Kaminski led the research. The team suggests this eyebrow-raising movement causes a nurturing feeling in humans because it makes the dogs’ eyes appear larger.

This ________also makes the dog look more like a human baby. The eye movement is similar to that which humans make when they are sad, the statement said. “The evidence is________ that dogs developed a muscle to raise the inner eyebrow after they were domesticated from wolves, Kaminski said.

In a separate part of the study, the researchers observed how 27 dogs and nine wolves ________ with a human. “We also studied dogs' and wolves' behavior. And when ________to a human for two minutes, dogs raised their inner eyebrows more and at higher intensities than wolves,” Kaminski said.

The researchers suggest that the eye movements developed over time as a way for dogs to get humans to do things for them. This could ________ humans giving them food, care or attention.

The only dog species in the study that did not have the muscles was the Siberian husky, which is an ancient kind. The husky could be the best living example of what the ________ between dogs and wolves looked like.