Reposted from:
http://www.newscientist.com/article/dn11756?DCMP=NLC-nletter&nsref=dn11756

Human spoken language may have evolved from a currency of hand and arm gestures, not simply through improvements in the basic vocalisations made by primates.

This "gesture theory" of language evolution has been given weight by new findings showing that the meaning of a primate's gesture depends on the context in which it is used, and on what other signals are being given at the same time. Gesture is used more flexibly than vocalised communication in nonhuman primates, the researchers found.



A proto-language using a combination of gesture and vocalisation is therefore more likely to have given rise to human language, than simply an improvement in the often involuntary vocalisations that primates make, they say.

Amy Pollick and Frans de Waal at the Yerkes National Primate Research Center in Atlanta, Georgia, US, tested the idea by looking at how strongly gesture and vocal signals are tied to context in our closest primate relatives — chimpanzees and bonobos.

Versatile gesture

By observing captive groups of bonobos and separate groups of captive chimps, Pollick and de Waal identified 31 gestures and 18 facial or vocal signals made by the apes, and recorded the context in which they were used. It turns out that the facial and vocal signals were practically the same in both species, but the same gesture was used in different contexts both between and within species.

For example, the vocal signal "bared-teeth scream" signals fear in chimps and bonobos, but the signal "reach out up" — where an animal stretches out an arm, palm upwards — has different meanings. It may translate as begging for food or as begging for support from a friend, says de Waal. "The open hand gesture is also used after fights between two individuals to beg for approach and contact during a reconciliation. So the gesture is versatile, but the meaning depends on context."

The work offers strong support for the gesture hypothesis of language evolution, says psychologist Michael Corballis at the University of Auckland in New Zealand. "It implies that manual gestures are freer of context than vocal ones, and can therefore be adapted to language," he says.

"To put it another way, manual gestures can be controlled voluntarily, whereas in non-human primates, vocalisations are largely involuntary and limited to emotional situations."

Closer than chimps

Humans often gesture in combination with speech — likewise, chimps and bonobos will make a gesture and vocalise at the same time, which is known as using "multimodal" signals. Pollick and de Waal found that multimodal signals were more likely to elicit a response in bonobos than in chimps.

"It seems to fit other indicators that bonobos have a more complex integration of signals, so that gestures do not just emphasise the meaning of other signals, but perhaps transform them," says de Waal.



Bonobos and chimps both split from the line that led to Homo sapiens about 6 six million years ago, and they themselves parted about 2.5 million years ago. However, several lines of evidence suggest we are slightly closer to bonobos than we are to chimps. De Waal says that their work suggests bonobos are a more useful species for understanding the evolution of language. For one thing, they seem to have moved further from the common ancestor, at least in terms of the complexity of their communication, than have chimps.

Bonobos also engage in dialogue-like vocalisations. For example, during confrontations, males alternate screams at each other. And now Pollick and de Waal have found that gestures vary from group to group, and are more effective in combination with other signals, which is not seen in chimps.

Journal reference: (Proceedings of the National Academy of Sciences, (DOI: 10.1073/pnas.0702624104).