WASHINGTON (Reuters) - A male chimpanzee may beg for food from another chimpanzee by gesturing with an extended arm and open hand.
Under different circumstances, the same chimpanzee may use the same gesture to try to coax a female chimpanzee to have sex. And the same gesture may be used after two males fight as a signal of reconciliation.
In research published on Monday, scientists seeking clues to the origins of human language analyzed the way two types of apes genetically closely related to people — chimpanzees and bonobos — use such hand and limb gestures to communicate.
They found that the apes use such gestures much more flexibly — in different contexts with apparently different meanings — than they used facial expressions and vocalizations. The findings, they believe, lend support to the idea that human language started with such gestures rather than speech.
“We are a naturally gesturing species that may have first developed language in the gestural domain, and once the brain parts related to language were well developed, then started using speech,” primatologist Frans de Waal of Emory University and Yerkes National Primate Research Center in Atlanta said in a telephone interview.
De Waal conducted the study, published in the Proceedings of the National Academy of Sciences, with fellow Yerkes researcher Amy Pollick, who observed and videotaped 13 bonobos at the San Diego Zoo and 34 chimpanzees at Yerkes.
De Waal and Pollick tallied 31 hand and limb gestures and 18 facial expressions and vocalizations. Both types of apes used facial and vocal signals in similar and predictable ways. Screaming was used by both, for example, in fear and pain.
But a particular gesture appeared to communicate wholly different messages depending on the social context in which it was used — for example if food was involved or mating.
“Gestures are used across a wide range of contexts whereas most facial expressions and vocalizations are very narrowly used for one particular context,” De Waal said.
Although all primates use their voices and facial expressions to communicate, only people and the great apes — chimpanzees, bonobos, orangutan and gorillas — use these types of gestures as well.
De Waal noted that great apes first appeared about 15 to 20 million years old, meaning such gestures may have been around that long.
“A gesture that occurs in bonobos and chimpanzees as well as humans likely was present in the last common ancestor,” Pollick said in a statement. “A good example of a shared gesture is the open-hand begging gesture, used by both apes and humans.”
This last common ancestor may date to about 5 million to 6 million years ago.
The researchers cited differences between the bonobos, the gentler and more sex-crazed, and chimpanzees, the more violent. The bonobos employed various gestures more flexibly, combining them with vocalizations and facial expressions to communicate a message.
De Waal said there might have been advantages to developing language from gestures before the spoken word. For example, silent communication might have been better when hunting for big prey.
He added that when the apes gesture, they like to use their right hands, which is controlled by the left side of the brain — the same side where the language control center appears in the human brain.