Friendly lip-smacks made by a large African monkey show striking similarities with human speech, say scientists.
Geladas, close cousins of the baboon that only live in the remote mountains of Ethiopia, produce "unnerving" sounds that can easily be mistaken for human voices.
Researchers who analysed recordings of the vocalisations uncovered a structural rhythm that closes matched that of people speaking.
They believe the evidence points to lip-smacking - a friendly behaviour displayed by many primates - being an evolutionary step towards speech.
"Our finding provides support for the lip-smacking origins of speech because it shows that this evolutionary pathway is plausible," said lead scientist Prof Thore Bergman, from the University of Michigan in Ann Arbor, US. "It demonstrates that non-human primates can vocalise while lip-smacking to produce speech-like sounds."
Prof Bergman became fascinated by the geladas' sounds while observing the monkeys in 2006.
"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 vocalisations sound so much like human voices."
Geladas are a highly gregarious species with a large vocal repertoire, expressed using complex facial movements.
The new research, reported in the journal Current Biology, showed that the rhythm of gelada lip-smacking closely mirrored the gaps between syllables in many human languages.
In both, the rhythm corresponded to the opening and closing of parts of the mouth.
Lip-smacking displayed by other primates does not have the same speech-like quality. Calls of other monkeys and apes typically consist of just one or two syllables and lack the geladas' rapid fluctuations in pitch and volume.
Gelada lip-smacks are thought to serve a similar purpose to "small talk" between friends by helping to bond individuals together.
"Many verbal exchanges appear to serve a function similar to lip-smacking," said Prof Bergman.