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