Man's closest relatives cannot be taught to speak, but chimpanzees (and other apes) can be taught to sign. Like their ape relatives, human babies find it difficult to say what they want, but they can sign for it. And although capuchin monkeys will beg for food with a gesture, no animal other than apes has been shown to regularly rely on gesticulation for communication. Now new research from the Emory University's Yerkes National Primate Research Center reveals the hidden complexity of ape gestures and argues that it may provide the symbolic basis for human language.

Amy Pollick and Frans de Waal recorded the interactions of two groups of chimpanzees in Atlanta and two groups of bonobos in San Diego. After 670 hours of watching and filming these animals, more than 1,000 video sequences of social interactions were analyzed for behaviors before and after a sign as well as what the reaction was. For the research, a gesture was defined as moving a limb or appendage in a way that is directed at another individual.

The researchers captured everything from a begging gesture that would suffice in most human households in the world to an oddly comic raising of the arms (and exposing of the armpits) to signal sexual interest. More importantly, analysis revealed that the meaning of these gesticulations depended upon context. "Paw outstretched could be used in food begging or after a fight for reassurance," Pollick says. "This kind of flexibility is a stepping-stone, perhaps, to symbolic language."

Chimpanzees and bonobos split from the human line roughly six million years ago and from each other roughly 2.5 million years in the past, according to genetic analysis, suggesting that perhaps such begging gestures are millions of years old. "We do know by looking at human gesturing that it's deeply ingrained in human communication strategies," Pollick says. "It's not just augmenting speech, it actually helps us think better and solve problems."

Of course, gesture as the basis for language remains speculative, says Pollick, and it may prove that other animals, such as dolphins, employ a language of gestures humans simply have not been able to detect. But a hand, or paw, rested lightly on the shoulder is a signal for attention that humans, chimpanzees and, perhaps, our distant ancestors would recognize.