Humans Retain ‘Ancestral’ Understanding of Ape Gestures, Study Says

A game played by more than 5,500 people revealed that humans can often interpret the gestures of chimpanzees and bonobos.
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Humans are surprisingly good at interpreting the meanings of gestures made by chimpanzees and bonobos, our closest living relatives, reports a first-of-its-kind study that quizzed more than 5,500 human participants about ape movements. 

The results hint that humans may retain some level of fluency in a physical repertoire of gestures shared by many apes, suggesting that our own ancestors may have used similar motions to communicate before the emergence of our modern complex language. 

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Chimpanzees and bonobos, members of the Pan family of great apes, often communicate with physical movements that translate to messages such as “groom me,” “get on my back,” or “let’s be friendly.” Kirsty Graham and Catherine Hobaiter, who are both primatologists at the University of St Andrews, have carefully cataloged these motions in their research, which got them thinking about the implications of this gestural vocabulary in unraveling the origins of human language.

To probe this question, the pair enlisted 5,656 people to complete an online game that tested whether participants could interpret gestures produced by nonhuman apes in a series of videos, using a multiple-choice format for answers. The short quiz, which is still available at this link, offers “the first test of the hypothesis that language-competent adult humans still share access to ‘family-typical’ great ape gesture,” according to a study published on Tuesday in the journal PLoS Biology.

“Cat [Hobaiter] and I both study wild chimpanzees and bonobos and have spent a long time trying to understand their gestures by looking at what they achieve,” said Kirsty Graham in an email to Motherboard. “Nonhuman great apes share most of their gesture repertoires (vocabularies) and so it’s possible that this might be something shared by our last common ancestors.” 

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“But modern humans use gestures in so many diverse ways that it would be impractical to try to pick out great ape gestures just by observing people,” she added. “So we thought, what if we flip it around and see whether people understand the gestures used by other great apes.”

To assess our human ability to understand ape gestures, the team compiled videos of common ape gestures such as the “Big Loud Scratch,” which means “groom me,” or the “Object Shake,” which is a concise way to say “let’s have sex.” The participants were able to correctly identify the meaning of the gestures more than 50 percent of the time, a result that is substantially above chance. 

“We kind of expected that people would be able to understand these gestures, but it’s striking just how good they are without any other information,” Graham said. “When the apes use these gestures, they have a lot of extra contextual knowledge (what’s their relationship? What have their recent interactions been like? What were they just doing?) that participants didn’t have access to in the experiment.” 

“And when we told participants a bit about what the apes were doing before, it did improve their understanding significantly but only by about 5 percent,” she noted. “So it seems like the gestures themselves are really meaningful to people.”

The outcome implies that humans still retain some basic understanding of the gestural system shared by our last common ancestor with chimpanzees and bonobos, some 6 to 7 million years ago. However, the “underlying mechanism that makes gestural communication comprehensible across great ape species, now including humans, remains unresolved,” according to the study. 

To that end, the team hopes to build on their findings with new experiments that further isolate the gestures from context, to explore just how universal some of these motions are among great apes. The answers to these questions will not only help us understand how our closest relatives communicate, it could shed light on the origins of our own complex language, which has set us apart from every other species on Earth.

“Between knowing that human infants use these gestures and now knowing that human adults understand these gestures, we can have more confidence that this kind of gestural communication would have been in use by our human ancestors, and may have scaffolded the evolution of human gesture and language,” Graham concluded.