Gelada baboons' bizarre lip-smacks share a surprising similarity to human speech, according to a new study.
University of Michigan researchers found that the primates use the speech-like method to conduct friendly interactions.
The lip calls could be similar to those made by our early ancestors before speech.
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Indeed, the sounds could be an early step towards speaking.
"Our finding provides support for the lip-smacking origins of speech because it shows that this evolutionary pathway is at least plausible," said Thore Bergman of the University of Michigan in Ann Arbor in a statement.
"It demonstrates that nonhuman primates can vocalize while lip-smacking to produce speech-like sounds."
The geladas of Ethiopia are the only nonhuman primate to communicate in such a complex way.
The findings were published in the journal Current Biology.