Human communication is a complex weave of words and gestures — a mix of vocal and visual signals.
"People use vocal and visual communication in a very rich and combined way," says Joseph Mine, a biologist at the University of Rennes in France.
But how and when this capacity originated is somewhat mysterious. It's not like the emergence of human communication is visible in the fossil record — there are no gestures enrobed in amber, no syllables imprinted upon prehistoric rock.
"So there's this big open question of how did human language evolve," says Mine. "How did humans or hominins communicate hundreds of thousands of years ago, or even millions of years ago?"
Mine is trying to answer that question by looking to one of our closest living relatives — chimpanzees. The idea is that whatever traits humans share with chimps today could date back to when the ancestors of the two species split off from one another six million to eight million years ago.
In a study published in the journal PLOS Biology, Mine and his colleagues conclude that young chimps learn vocal and visual patterns of communication primarily from their mothers and maternal relatives. They think that this ability, which mirrors the way that young humans learn from their primary caretakers, may date back to at least that ancient moment.
Vocal gestural pairings
Mine did his field work with the Kanyawara community of roughly 60 animals in Kibale National Park in western Uganda — "a beautiful tropical rainforest where these chimpanzees live in the wild and therefore we can observe their natural behavior," says Mine.
The individuals divvy up into subgroups that are forever splitting off and reforming, though they tend to include at least one mom and her offspring. "Anywhere between two and maybe eight or nine individuals who are all related through their mother," he says.
Over many months, Mine and his colleagues followed the animals at a safe distance to film and record audio of them. Back in the lab, while working on his PhD at the University of Zurich, he pored over hundreds of hours of footage.
Mine searched for any vocal and non-vocal behaviors (such as facial expressions, gestures, gaze orientation, and body postures and movements) that were produced in tandem more often than expected by chance.
"They might combine a pant-hoot vocalization with running but also with slapping the ground or grabbing a branch," he says by way of example. "Just like they might combine a soft hoo vocalization with a sitting posture and maybe an arm reach."
In work published last fall, Mine identified a repertoire of 108 such combinations. And it was during that analysis when he noticed something curious.
"All the individuals from a certain family seem to converge around a similar number of combinations," he says.
So he decided to dig deeper.
A lot like mom
When Mine later examined chimps ten years and older — meaning subadults and adults — he found that individuals related through their mother produced similar amounts of these vocal and visual combinations. "There is a kind of signature within the group composed of you and your mother and your maternal siblings," says Mine.
On the paternal side, though, there was no such pattern. That is, "if your mother tends to gesticulate a lot while vocalizing, then you're also likely to do so and your maternal siblings are also likely to do so," he says. "But if that's the case for your father, then you won't necessarily show this kind of resemblance."
A chimp spends most of its early years with its mom, not its dad. So Mine concludes that these vocal-visual combos are most likely learned. That's because if genes were involved, the chimps would resemble both their parents.
"They don't really have the exposure that would allow them to learn from their fathers," says Mine. The mother, he adds, becomes a young chimp's "social template."
And once these behaviors are learned, they appear to stay put.
Young humans also learn to communicate from those they hang out with the most, meaning this ability may well date back at least to our last common ancestor with chimps.
"This fact that we acquire parts of our communication socially seems to be potentially a very ancient trait — a feature of our lineage for several million years," says Mine.
Cat Hobaiter, a primatologist at the University of St. Andrews in Scotland who wasn't involved in the research, was pleased to hear of the results. She says they provide the latest example of the way that the social world shapes primate communication.
"We're seeing such similarities across the ape species," she says. "My bet is that we would see something similar in gorillas and orangutans. Then we're talking about something that might be 16, 17 million years old — so long before humans were human, apes were learning socially from each other."
Hobaiter says future work could try to decode these combinations to figure out what they might mean — and whether these behaviors are specific to this group of animals or if they're perhaps shared by all chimpanzees.
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