Researchers analyzed video of a female chimpanzee, a female bonobo and a female human infant to compare gestures they used at comparable stages of development, and found remarkable similarities in a new study.
"This was the first indication of a distinctive human pathway to language," said co-author Patricia Greenfield, a professor of psychology at UCLA.
"Predominately communicative" gestures made by all three species included reaching, pointing with fingers or the head, and raising the arms to ask to be picked up.
To be considered communicative, a gesture had to include eye contact with the conversational partner, be accompanied by vocalization, or include a visible effort to elicit a response.
Panpanzee the chimpanzee, and Panbanisha the bonobo were raised together at the Language Research Center in Atlanta, which is co-directed by Sue Savage-Rumbaugh, a co-author of the study.
There, the apes learned to communicate using gestures, vocalizations and visual symbols called lexigrams.
The human baby was raised in her home with her parents and older brother. Where the apes' environmental symbols were visual, the girl's symbols took the form of spoken words.
Video analysis for the human child spanned ages 11 months to 18 months, and analysis for the apes ran from 12 months of age to 26 months.
During the first half of the study, gestural communication was dominant in all three species. During the second half, all three species increased their symbol production -- words for the child and lexigrams for the apes.
Most of the child's gestures were accompanied by vocalization, and the apes' gestures rarely were, suggesting that the ability to combine gesture and vocalization contributed to the evolution of language.
The cross-species analysis provides insight into the "gestural origins of human language," Greenfield said, and the development of our capacity for symbolic language.