June 6, 2013 at 6:16 PM ET
When it comes to communication, a human infant starts out with hand gestures, much as baby chimps and bonobos do, and all three of those species shift to using more symbols as they grow up. But the shift to symbols is more pronounced for the human and relies more on vocalization. That pattern may well suggest how human language evolved.
These are the lessons drawn from more than a year's worth of observations of a chimp, a bonobo and a young girl. The results were published Thursday in Frontiers in Psychology, an open-access journal.
"It's a new kind of evidence in favor of the gestural origins of language, and it's also a new kind of evidence in favor of the co-evolution of gesture and speech," said one of the study's authors, Patricia Greenfield, a psychology professor at the University of California at Los Angeles.
The similarities in the gestures used by baby chimps, bonobos and humans suggest that the common ancestor of those three primate species made use of the same repertoire around 6 million years ago, Greenfield told NBC News. "The story that gesture was present in our common ancestors because of the similarities we see — that just hits you," she said.
How the study was done
The research team tracked the gestures used by the girl, known only as GN in the study, at her parents' home from the age of 8.5 months to almost 2 years. The chimp and the bonobo, named Panpanzee and Panbanisha respectively, were looked in on regularly at Atlanta's Language Research Center for almost four years, beginning soon after they were born.
All three species spontaneously started pointing with their fingers or arms, or even their heads, to the things that they wanted to call attention to. If they wanted to go somewhere, they just pointed, to nothing in particular. They reached out for things that they wanted, even if those things weren't within reach. They raised their arms when they wanted to be picked up. In all these cases, the researchers counted the gestures as communication only if they were accompanied by eye contact or other behavioral cues demonstrating that there was truly an intent to communicate.
As time went on, GN began vocalizing more and more — first, with noises, and eventually with words. Meanwhile, the chimp and the bonobo learned to communicate by pointing at a list of abstract symbols, known as lexigrams.
"Lexigrams were learned, as human language is, during meaningful social interactions, not from behavioral training," Kristen Gillespie-Lynch, a psychologist at City University of New York who headed the research team, said in a news release.
The shift to symbols
As they aged, all three subjects used relatively fewer gestures and significantly more symbols. "The child transitioned more quickly than did the apes," Greenfield said. "The apes never transitioned to using symbols more than gestures."
The researchers saw that as a lesson in human evolution. "The fact that symbols became far more dominant speaks to what happened after the common ancestor, in the human line," Greenfield said.
She said GN's vocalization provided another lesson: "It really speaks to the idea that gestures and vocal language or speech evolved together."
This study is only the latest in a long series of experiments comparing the communication capabilities of humans and other species — and it's hard to make sweeping generalizations on the basis of data from just three individuals. But at least there's now evidence tracing the roots of language back to gestures that may have existed millions of years ago.
"We can call it an existence proof," Greenfield said.
More about cross-species communication:
In addition to Gillespie-Lynch and Greenfield, the authors of "A Cross-Species Study of Gesture and Its Role in Symbolic Development: Implications for the Gestural Theory of Language Evolution" include Yunping Feng, Sue Savage-Rumbaugh and Heidi Lyn.
Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the NBC News Science Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with NBCNews.com's stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.