A recent study published in the journal Emotion, finds that chimpanzees are capable of making some nuanced distinctions between facial expressions. The study is notable not only for what it reveals about chimps’ social intelligence—it’s pretty sophisticated, it turns out—but for what it suggests about the evolution of human emotion.

In the study, led by Lisa Parr of Emory University, researchers showed chimpanzees a series of computer-generated faces. The chimps had the chance to match each face either to an identical image or to an image of a similar but different emotional expression, characterized by slightly different facial muscle movements. If they chose correctly, the chimps were rewarded with food; if their choice was wrong, they got nothing.

Some scientists would have been skeptical that chimps could make these kinds of distinctions. They assume humans’ emotional categories are really just products of the language we use to describe emotions—human constructions that don’t really represent the true nature of our emotional world.

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However, the chimps in this study proved adept at recognizing the subtle distinctions between facial expressions; for the most part, their selections were accurate enough to indicate that they weren’t just getting lucky. This suggests chimps don’t only make broad, simplistic distinctions between positive and negative displays of emotion. Instead, they seem to interpret facial muscle movements with enough accuracy to differentiate between similar emotional states, like how we distinguish between different colors, foods, or scents.

The fact that chimps make these emotional distinctions suggests these distinctions are probably deeply rooted in human nature, stretching back to at least seven million years ago, when humans and chimps split off from one another on the evolutionary line.

Consider one kind of distinction the chimps were asked to make: between a laugh-like “play” face and a bared-teeth smile. These two expressions are represented in the images above (two of the actual computerized images the researchers used in their study). The bared-teeth expression, on the left, is a predecessor to the human smile. The zygomatic major, the risorius, and the buccinator muscles around the upper and lower lip all contract to reveal the teeth. Chimps usually make this expression as a sign that they’re submitting or cowering before a dominant other.

The other image depicts a different expression that also resembles a smile; the researchers call it a play face. Chimps, especially adolescent chimps, make this expression in different contexts from the bared-teeth expression—when they’re playing, when they’re tickling, when they’re feeding, or just goofing off. This expression is more relaxed: their jaw drops and their mouth is open, so there’s no tension in the muscles that reveal the teeth.

We can see similar differences in the accompanying human faces, which are from a set of photos I’ve taken in my lab. The first photo is very close to the bare teeth display of the chimp—it’s more of a deferential smile—and the second shows a jaw drop and open mouth, roughly equivalent to a chimp’s play face.

It’s one thing to observe that humans and chimps share certain differences between these kinds of positive emotional displays. But it’s even more profound to recognize that these differences have deep significance in the mind of the chimpanzee. It suggests that primates, including humans, are intrinsically attuned to these nuances in emotional expression. Part of our evolutionary legacy, it seems, is our capacity to express and interpret a variety of cooperative, positive signals.

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