Scientists have discovered that monkeys exhibit a type of "lip-smacking" that closely resembles human speech, and could be considered a clue into how speech evolved.
Geladas are large African monkeys who are close cousins of the baboon. They live in Ethiopia in remote mountains, and make sounds and movements with their mouths that resemble human voices.
As researchers analyzed the recordings of their sounds, they discovered there is a structural rhythm to it that matches our speech.
They also believe that the lip-smacking is their form of "small talk," used only for social situations.
"Our finding provides support for the lip-smacking origins of speech because it shows that this evolutionary pathway is plausible," lead scientist Prof. Thore Bergman said.
"It demonstrates that non-human primates can vocalize while lip-smacking to produce speech-like sounds."
Bergman said he was in awe of the geladas' vocalizations when he was observing them in 2006. It was then that he discovered their noises were eerily similar to human speech, as he often looked over his shoulder to see who was talking to him.
"I would find myself frequently looking over my shoulder to see who was talking to me, but it was just the geladas," he said. "It was unnerving to have primate vocalizations sound so much like human voices."
They use other vocalizations to express emotions and messages. They utilize complex facial movements as well.
It is their lip-smacking, however, that has researchers stunned.
They found that the rhythm of it mirrors the gaps between syllables in most human languages, as both have a rhythm that corresponds to the opening and closing of the mouth.
Other primates exhibit lip-smacking behavior, but theirs is not of a pattern. Most calls by other monkeys are just one or two syllables long and do not have fluctuations in pitch and volume.
"Many verbal exchanges appear to serve a function similar to lip-smacking," Bergman said. "Vocal lip-smacking might make sense as a precursor to language. Language is not just a great tool for exchanging information, it has a social function and many verbal exchanges appear to serve a function similar to lip-smacking."
Although the geladas "small talk" is not discernible, when a group of them gets together, it sounds as though a group of humans are talking.
"However, if you are with geladas, you sometimes get the impression that people are talking around you - something I hadn't noticed in years of working with other monkeys," he said.