Was the evolution of spoken language more of a song or a dance? The responses of chimpanzees to a stuffed snake suggest it might have been more like a stage show, with vocal and gestural modes of communication both being fundamental building blocks of language.
Darwin suggested we sang our way to modern language, gradually moving from vocal flourishes designed to attract mates to primitive vocal communication that eventually developed into speech.
But evidence from other apes has suggested that our ancestors' vocal cries were involuntary and strongly linked to emotions, whereas their hand gestures were the more intentional means of communication. That seemed to suggest language might have blossomed from gestural communication.
Not satisfied with the idea that chimps' vocal signals are involuntary, Katie Slocombe from the University of York, UK, and her colleagues stuffed the skin of a dead snake and placed it in a forest in Uganda. Using a fishing line, they wiggled the snake when chimpanzees came near and observed and categorised the ensuing alarm calls.
Goal orientated
They found that the signs that had led other researchers to think apes' gestures were intentional were also true of the alarm calls chimpanzees made when they noticed the snake.
These signs included the calls being directed to a recipient, that they changed depending on the recipient's apparent comprehension and that the caller was sensitive to whether the recipient was paying attention.
For example, the loud alarm and even louder were more likely to be produced in the presence of a friend rather than a non-friend. And when the team tracked the gaze of the alarm-calling chimp, they found the animal watched the other chimps around them as they reacted.
Crucially, they found that the alarm huus and waa barks continued until all other chimps were a safe distance from the snake, suggesting the cries were goal-directed, and not merely an automated fear-response.
"Chimpanzees seem to produce these calls tactically and target important individuals who are valuable to them," the team write in the paper.
Slocombe says the work undermines one of the main reasons for thinking language grew out of gestures rather than vocalisations.
"Until now it was assumed that primate calls were unintentional and more read-outs of emotion," she says. "We have challenged this view and shown an important commonality between human and chimp vocal communication."
Research heroes
"This is heroic work," says Tecumseh Fitch, a cognitive evolutionary biologist at the University of Vienna in Austria. "Four researchers dragging a large stuffed python and equipment through the rainforest, sometimes waiting days for an opportunity to do a single playback trial. Very impressive."
"This research, using the same criteria adopted by gestural researchers, calls the sharp distinction [between apparently involuntary vocalisations and voluntary gestures] into question," Fitch says. "This does erode support for gestural theories of the origin of language."
But Michael Corballis of the University of Auckland in New Zealand, a proponent of the gesture theory, says the argument is weak.
"It's an alarm call, not a conversation," he says. "It's a long way from language. But the way chimpanzees communicate with gestures is much closer to language – it's more communicative." He points out that when grooming, apes can tell another ape exactly where they want to be groomed with gestures called "directive scratches".
Slocombe expects the correct answer will be a combination of the two ideas. "It would likely combine elements of the competing vocal and gestural theories of language evolution," she says.
Journal reference: PLoS One DOI: 10.1371/journal.pone.0076674
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