You know how when your dog wants something, she makes that face? You know the one - all beseeching, with eyes that seem to positively quiver with longing? You'd give her anything, right?

It turns out that our response to canine looks of longing or love may be the very reason dogs can make them. New research has found that the facial muscles involved in making these expressions can only be found in dogs, not wolves - suggesting our furry best friends evolved the ability specifically to communicate with humans.

"The findings suggest that expressive eyebrows in dogs may be a result of human unconscious preferences that influenced selection during domestication," said behavioural psychologist Juliane Kaminski of the University of Portsmouth.

"When dogs make the movement, it seems to elicit a strong desire in humans to look after them. This would give dogs that move their eyebrows more, a selection advantage over others and reinforce the 'puppy dog eyes' trait for future generations."

Previously, Kaminski and her team have demonstrated that dogs do actually make facial expressions as a means of communicating with humans, by studying their behaviour when a human was facing towards them, compared to facing away. The team found that dogs used facial expressions far more when the human was looking at them.

For this research, the team did something different: they studied dog (Canis familiaris) behaviour as compared to wolves (C. lupus), and performed a comparative analysis of the facial anatomy of both species.

Because dogs and wolves are thought to have evolved from a common ancestor, such comparisons can potentially reveal the ways in which 33,000 years of domestication have changed the dog.

The researchers conducted their facial anatomy comparison on four wolves and six dogs.

"To determine whether this eyebrow movement is a result of evolution, we compared the facial anatomy and behaviour of these two species and found the muscle that allows for the eyebrow raise in dogs was, in wolves, a scant, irregular cluster of fibres," said anatomist Anne Burrows of Duquesne University.

"The raised inner eyebrow movement in dogs is driven by a muscle which doesn't consistently exist in their closest living relative, the wolf."

And with two wolves and another six dogs, the team observed how they used their faces around humans.

"We also studied dogs' and wolves' behaviour, and when exposed to a human for two minutes, dogs raised their inner eyebrows more and at higher intensities than wolves," Kaminski said.

Muscular anatomy typically evolves very slowly, so this seems to be a pretty huge change to have taken place in just a few ten thousand years, the team said. The reason, they believe, is because of the positive effects of canine facial expressions on dogs' interactions with humans.

"This movement makes a dogs' eyes appear larger, giving them a childlike appearance. It could also mimic the facial movement humans make when they're sad," said evolutionary psychologist Bridget Waller of the University of Portsmouth.

"Our findings show how important faces can be in capturing our attention, and how powerful facial expression can be in social interaction."

The research has been published in PNAS.