In the forest near Wamba, a village in the Democratic Republic of the Congo, some of the last remaining bonobos breed, feed and lounge in the trees. Like other great apes, these animals have a rich social life, using about 80 different gestures to communicate with their peers. Primatologist Kirsty Graham of the University of St Andrews in Scotland spent hundreds of hours inside this endangered herd to decipher the non-verbal interactions of its members. The study confirmed, for example, that repeatedly swiping the black fluff on her chest by one of her animals is asking for grooming. And when you put your hand under someone else’s chin, it’s asking for food.
Graham’s colleague at St. Andrews University, Catherine Hoveter, has developed a similar body language dictionary by observing East African chimpanzees in the Budongo Central Forest Reserve, Uganda. Her two closest relatives to humans, both species’ gestures are more complex and varied than vocalizations that primarily reflect urgent needs such as finding food or finding predators.
In contrast, ape gestures serve as a means of deliberately communicating specific everyday goals, and some scientists believe these signals are precursors to human language. “There is a theory that human language may have evolved from this gestural basis because they use gestures in a more linguistic way,” says Graham.
In a paper published today at PLOS biology, Graham and Hobiter provide startling evidence that this ancestral ability may have persisted in modern humans. They show that our species can guess the meaning of chimpanzee and bonobo gestures fairly well. This is another hint that language may have evolved from an elaborate system of hand-body signals.
The study found that when thousands of people watched online videos of wild apes raising their arms, scratching, and posing in various poses, animal specializations were much more frequent than one might expect by chance. I got the gist of the terminology. “Even without training and without looking at the results or the behavior of their surroundings, humans can understand what the gestures of chimpanzees and bonobos mean,” Graham says. .
This finding suggests that humans still have some grasp of this ancestral vocabulary. “Perhaps this is something that was shared with our last common ancestor, and indeed has retained this ability to understand and use ape gestures,” says Graham.
This work fills a hole in the shared linguistic lineage case. Scientists have found that the vocabulary of great apes overlaps extensively. About 95% of bonobo gestures are the same as those used by chimpanzees. Additionally, in a 2019 pilot study, Hobaiter and her colleagues found that most of the gestures used by young children under the age of 2 were identical to those of chimpanzees.
However, when wild apes and humans reach adulthood, it has been difficult to find such overlaps in gestures. Adult human communication is dominated by spoken or signed language and a wide variety of gestures, many of which are culturally specific. “It’s very difficult to tell if apes’ gestural communication is still accessible just by observing people,” says Graham.
A strategy for addressing this problem came to Graham and Hobaiter several years ago when they were collecting and analyzing videos of apes. Researchers deciphered the meaning of each ape gesture by looking at what happened next. For example, if an ape bends its back knee and raises its leg, and the child jumps on its back, we might conclude that raising the leg means “I’ll give you a ride.” The deciphering process took years and required sifting through thousands of examples of such behavior. “We’ve spent all our time trying to understand them,” says Graham.
What the researchers didn’t know was whether ordinary people who spend little time with apes have the same intuition. In 2017 Graham and Hobaiter decided to investigate. They designed what they envisioned to be a small pilot study in which the public would attempt to identify chimpanzee gestures from videos in an online quiz. were shown 20 short clips of , and asked to choose one of four possible answers explaining the meaning of each gesture. In one of his clips, a bonobo shoves a buddy. In this case, the animal was saying, “Get on my back.” But when a chimpanzee did the same thing, it was telling its mate to move to a new location.
After the media covered the team’s work, over 17,000 people logged in to watch the video. The researchers excluded participants who had not seen all the clips, those who had seen them more than once, or those who said they had experience with primates, leaving 5,656 points. “This is a very impressive work of citizen science,” said Erica Cartmill, an anthropologist at the University of California, Los Angeles. “The number of people watching the ape gestures is amazing,” she said.
If you didn’t know what the ape was doing, you would choose 1 of the 4 answers correctly and have a 25% chance of getting it right by chance. However, the average score was just over 50%, a statistically strong result given the size of the study. “What’s interesting is what people seem to be able to do. [this task], and somehow their speculation, though not perfect, is definitely beyond the possibilities,” says Federico Rossano, a comparative psychologist at the University of California, San Diego. The human capacity to understand is more limited, he says. “Some people are good at interpreting their pets, and many people aren’t,” says Rossano. “Sure, they might bite you.”
Study participants understood ambiguous ape gestures that have multiple meanings in ape society, with one exception: when chimpanzees shake an object such as a branch, it often means “let’s have sex.” Means “away from me”, depending on the context. People have chosen the correct meaning of this gesture. Researchers call this “object shaking,” and nothing beats chance. And in general, the accuracy rate in this study varied considerably by gesture type. “This shows that we don’t understand something very specific to chimpanzee language. [understand]says Thibaut Gruber, a primatologist at the University of Geneva, who was not involved in the study.
However, test takers’ achievements should not be underestimated. Living together, apes gain important context for interpreting ambiguous gestures. “Considering that people don’t get it at all in these videos, it’s really amazing that they can still understand the gestures,” says Graham.
It is still not clear why humans can understand ape gestures. Human and chimpanzee lineages are separated by up to 6 million years. One possibility is that all great apes, including humans, inherit a common set of gestures. Second, humans and other apes share the ability to use body movements as a communication tool. Graham calls this “embodied communication”.
A third explanation is that the similarities in body shape between humans and other great apes, combined with human cognitive abilities, make it easier to infer meaning from ape movements. These possibilities remain open in the study, he says, Cartmill. She calls the study a “beautiful first step” to show that humans can recognize the intent and meaning of another species’ communication. But she wonders, “Is this because we’re good at reasoning, or is it because there’s a common underlying gestural system?”
“I think gestures played a big role in the evolution of language,” continues Cartmill. “I think this paper contributes to that story and helps open up new possibilities that there is a deep-seated set of gestures, or deep-seated sensitivities for seeing, perceiving, and understanding gestures.”
But Rossano isn’t convinced that humans and other apes share an innate gestural repertoire. “Humans can perceive the barking of dogs and the roaring of lions as threats, but this does not mean barking or roaring as a communication tool,” he said.
Even if Rossano is right, the research itself has value as an educational tool. Involving so many people in this research has benefits beyond science, says Gruber. “What really excels is the methodology and how people are involved in it,” he says. “That way they can understand, ‘Oh, we’re very close to our closest relatives.’ That’s a win for me. A win for conservation. Remember them, protect them, save them.” It’s a victory that shows how important things are.”
