The 33,000-year journey from wolf to dog changed our canine companions in many ways, making them more suited to the hearth than the forest. A key aspect has been growing ever more adorable in our eyes. New research reveals how the spread of a single muscle may have played on our love for their puppy eyes.

There's no question most humans are suckers for puppy-eyed dogs. Two years ago, Dr Juliane Kaminski of the University of Portsmouth demonstrated that our so-called best friends have identified our weakness and use their facial expressions to pull at our heart strings. Those big eyes, showing of tongues, and vocalizations all happen more when humans are around than when dogs are being filmed with hidden cameras. They might want you to think they just can't hide how they're feeling, but dogs may be master manipulators, putting it on to win our love or food.

Kaminski has since turned her attention to how this trait evolved. In Proceedings of the National Academy of Sciences, she reports on the occurrence of the levator anguli oculi medialis (LAOM), the muscle that raises the inner eyebrow as part of a facial movement dogs have perfected.

“The movement makes the eyes appear bigger, hence more infant-like and potentially more appealing to humans,” the paper notes. “This inner brow raise also resembles a facial movement humans produce when they are sad, potentially eliciting a nurturing response from humans.”

Although wolves' and dogs' facial muscles are similar in other ways, Kaminski found the (LAOM) is “routinely present in dogs”. Wolves, on the other hand, usually have scant muscle fibers at this location, surrounded by connective tissue. A sample of dogs in shelters found they most frequently used their LAOM to make their eyes look bigger when approached by unfamiliar people. Wolves in wildlife parks sometimes show similar facial expressions, but less often and with much lower intensity.

As Kaminski points out, human-dog facial interaction has played a major part in the development of this important bond. Dogs have been shown to make eye contact with humans when they hit a problem they can't solve. Staring into each other's eyes produces an oxytocin feedback loop between humans and dogs, similar to the one between mothers and newborn children, which doesn't occur with wolves.

Although much of this shift can be attributed to behavioral changes, Kaminski argues that dogs with the capacity to raise their eyebrows and make their eyes look larger hijacked humans' caregiving response to our own young. Dogs that raise their eyebrows frequently are more likely to be rehomed from shelters than those that do not. This has probably represented an evolutionary advantage for thousands of years, and led those with the capacity to dominate the symbiosis-with-humans evolutionary niche, at least until cats provided competition.