Human language may have evolved from the use of gestures by our ape ancestors, and not just from primitive vocalisations, according to a new study.

Only humans and apes use hand or limb gestures to communicate, but until now little was known about how these gestures combine with other forms of communication in apes or what kind of responses they generate.

To learn more, primatologists Frans de Waal and Amy Pollick of the Yerkes National Primate Research Centre in Atlanta, studied bonobos and chimpanzees, in captivity. They focused on the way that our closest relatives combine gestures, facial expressions and vocalisations to communicate.

As they reveal today in the U.S. journal Proceedings of the National Academy of Sciences (PNAS) the pair found that the use of gestures is highly flexible, complex, and difficult to link to specific contexts. The use of gestures even varied widely between different groups of chimps and bonobos.

"The way they use gestures is extremely variable, especially compared with other forms of communication," said de Waal. "This makes gesture a possible candidate for symbolic communication in our shared ancestor."

Facial expressions and vocalisations, on the other hand, are more stereotypical, and are though to be largely instinctive and reactionary in apes. Gesture appears to be more under conscious voluntary control much like human language, write the authors.

"Far more than facial expressions and vocalisations, gestures are subjected to modification, conventionalisation, and social transmission, this is especially so when compared to their vocalizations and facial expressions, which are more directly tied to emotional and social contexts."