You don't need to be Dr. Doolittle to know what a chimpanzee means when she reaches out with her hand, palm-side up. It's the begging sign, a gesture that speaks volumes even without any accompanying sound. Indeed, so evocative are the manual gestures of chimpanzees and bonobos that a team of researchers has rekindled an old hypothesis: that human language evolved from gesturing, rather than from vocal calls.
Chimps and bonobos use a variety of calls and gestures in their everyday lives. But many researchers assumed that both forms of communication were little more than reflexes, as when a chimp screams when it sees a snake. "It used to be thought that the communication of chimpanzees and bonobos was strictly emotional and tied to specific contexts," explains Frans de Waal, a primatologist at Yerkes National Primate Center in Atlanta, Georgia. But de Waal suspected there was something more to ape gestures. So he and colleague Amy Pollick videotaped the vocal, facial, and manual signals of two captive groups of bonobos and two groups chimpanzees. Of the 375 communicative signals the bonobos produced, nearly 79% were hand gestures, while 14% were facial and/or vocal signals; the chimpanzees made 383 signals, of which 56% were hand gestures, and 22% were facial and/or vocal signals. In some instances, both manual and facial and/or vocal signs were combined.
The researchers then identified the specific social contexts of the various types of communication. They predicted that the facial and vocal signs would be relatively inflexible--that is, these would be more closely tied to specific contexts than would the manual gestures, and that both species would largely use the same facial and vocal signs. And indeed, bonobos and chimpanzees scream when alarmed, threatened, or intimidated; and both species use a silent pout-face to express an interest in food. Hand gestures, on the other hand, were almost as varied as two different human languages, with signs differing not only between the two species but also between groups of the same species. Bonobos showed an especially diverse range of hand gestures, which suggests they may be better models for the study of language evolution, report the authors online this week in Proceedings of the National Academy of Sciences.
The greater variety of hand gestures "supports the idea that language evolved from manual gestures rather than animal calls," says Michael Corballis, a cognitive neuroscientist at the University of Auckland in New Zealand. Further, he says, the study reinforces evidence of the importance of gesturing: Human infants, for instance, make gestures long before they speak. The study also fits with brain research in monkeys, says Bill Hopkins, a psychobiologist at Yerkes. "Vocal calls are controlled by the more ancient limbic region of the brain, while gestures are tied to the frontal lobes." And that's why if you hear a chimpanzee make a pant-hoot, you can be sure she's excited about something. But if you see a male bonobo raise his arms overhead and clasp his fingers, you need an interpreter. (He's asking for sex.)