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In the 1960s, Jane Goodall started spending weeks at a time in Gombe Stream National Park in Tanzania watching chimpanzees. One of her most important discoveries was that the apes regularly made gestures to one another. Male chimpanzees tipped their heads up as a threat, for example, while mothers motioned to their young to climb on their backs for a ride.
Generations of primatologists have followed up on Dr. Goodall’s work, discovering over 80 meaningful gestures made by not only chimpanzees, but also bonobos, gorillas and orangutans.
Now researchers are using these gestures to peer into the minds of apes. Some even think they offer clues about how our own species evolved full-blown language. “Certainly, gestures played a big role,” said Richard Moore, a philosopher of language at the University of Warwick.
In the 1980s, Michael Tomasello, then a young comparative psychologist, pioneered the first theory about ape gestures based on observations of infant chimpanzees in captivity as they grew into adults.
He noticed that the baby apes made gestures to their mothers and, as they matured, developed new gestures directed at other chimpanzees.
Based on his observations, Dr. Tomasello argued that gestures develop among apes as simple habits. If a baby repeatedly tries to grab food from its mother’s mouth, for example, the mother may eventually start to give it food while the baby is still stretching out its arm. The baby, in turn, may stop bothering with the full action.
According to Dr. Tomasello’s ritualization idea, apes don’t use gestures to communicate the way we do. When we point to a cannoli in a pastry shop, we know the gesture will make the baker understand that’s the one we want to buy. But according to the theory, an ape doesn’t get inside the head of other apes when it makes a gesture. The animal simply learned that the gesture delivered what it wanted.
By the 2010s, however, some primatologists saw some serious problems with that theory. It predicted that there would be a lot of variety in the gestures that emerged from one-on-one interactions between apes. But large-scale surveys of chimpanzees showed that they all made the same gestures. Some of those movements were even shared across different species.
The critics developed a new theory rejecting the idea that apes spontaneously developed gestures. Instead, they proposed that gestures were encoded in the genes, much as a courtship dance is encoded in a bird’s DNA. The inherited gestures that helped apes reproduce were favored by natural selection.
“Gestures are innate,” said Richard Byrne, a primatologist at the University of St. Andrews in Scotland, who helped develop this theory.
But Kirsty Graham, a former student of Dr. Byrne’s, grew dissatisfied with the innate theory as well. “We’re saying it makes more sense that 80 gesture types are all genetically encoded?” recalled Dr. Graham, who now teaches at Hunter College in New York. “Is this really the simpler explanation?”
Dr. Graham discovered that Dr. Moore shared her skepticism, as did Federico Rossano, a comparative psychologist at the University of California, San Diego, who got his Ph.D. with Dr. Tomasello. The trio unveiled a third theory last month in the journal Biological Reviews. “It’s a story that we hope will reconcile existing data and divergent views,” Dr. Moore said.
Apes, they argue, do not inherit specific gestures, but they do inherit the sense that they can use gestures to communicate with other chimpanzees. The animals create new gestures by borrowing — or “recruiting,” in the scientists’ lingo — movements that apes commonly make.
An ape can reach out an arm, for example, as a way to ask for food, like a piece of a freshly caught monkey.
“I have the monkey, and you’re sitting very close to me, looking very closely at the monkey,” Dr. Graham said. “I already know that you want to eat the monkey, and the gesture makes it an explicit request.”
The theory explains some observations that did not make sense otherwise, she said. Primatologists have noticed, for example, that when an ape fails to get something it wants through a gesture, it may move so that the other ape can see it more clearly. If the gesture were just a learned, habitual motion, the animal wouldn’t be able to display it so flexibly, Dr. Graham said.
“They’re being intentionally produced, in a goal-directed way,” she said.
The recruitment view offers an explanation for how apes can share so many gestures even if they are not innately programmed. Apes end up making the same gestures because they have similar bodies that move in similar ways. As a result, it’s easy for them to interpret the meaning of gestures simply by thinking about what they do with their own bodies.
Dr. Graham argues that the recruitment view offers new ideas about why apes struggle to recognize gestures that are easy for us to understand, such as finger-pointing. Those movements have no clear connection to what the apes do with their bodies in their everyday life.
That doesn’t mean that apes can’t learn any new gestures, however. Dr. Graham and her colleagues predict that the animals can do so as long as the gestures have a purpose. The researchers are developing experiments to observe if that can happen.
Dr. Byrne found the recruitment view, compared with his own innate theory, “quite a complicated system,” he said. It’s clear that other animals can inherit their displays, and so he questioned why apes would need a different explanation.
“The question, which will take a long time and lots of discussion to resolve, is whether all the extra theoretical apparatus is worth it for a better description of reality,” he said.
But Dr. Tomasello, who now teaches at Duke University, praised the recruitment view as an improved version of his ritualization theory.
When he first developed his ideas about gestures, Dr. Tomasello said, scientists did not yet appreciate how much apes can understand about one another. The recruitment view “gives more credit to the apes cognitively than did the original view,” he said. “It is an important advance.”
Some scientists have speculated that ape gestures directly gave rise to human language. Early humans converted gestures into sign language, complete with rules of grammar, which they then converted to spoken words.
But Dr. Rossano and his colleagues see a more distant connection from gestures to language. Our ape ancestors evolved an unprecedented flexibility in how they learned gestures and how they used them in their social lives. That’s a vital element of human language, too.
“Where does that flexibility in use and learning come from?” Dr. Rossano said. “It probably comes from the flexibility in gestures.”
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