Chimpanzees produce lip-smacks at a speech-like rhythm of open-close mouth cycles, suggesting that speech-rhythm was built upon existing primate signal systems
In the paper ‘Chimpanzee lip-smacks confirm primate continuity for speech-rhythm evolution’, published in Biology Letters, a consortium of UK-based researchers have found that the rhythm of chimpanzee lip-smacks exhibit a speech-like signature, which may provide the elusive key to the evolution of speech.
Like every language in the world, monkey lip-smacks have previously shown a rhythm of about 5 cycles/second (i.e. 5Hz). This exact rhythm had been identified in other primate species, including gibbon song and orangutan consonant-like and vowel-like calls. However there was no evidence from African apes, such as gorillas, bonobos and chimpanzees – who are closer related to humans, meaning the plausibility of this theory remained on hold.
Now, the team of researchers using data from four chimpanzee populations have confirmed that they too produce mouth signals at a speech-like rhythm. The findings show there has been most likely a continuous path in the evolution of primate mouth signals with a 5Hz rhythm. Proving that evolution recycled primate mouth signals into the vocal system that one day was to become speech.
African great apes, the closest species to humans, had never been studied for the rhythm of their communication signals. Researchers investigated the rhythm of chimpanzee lip-smacks, produce by individuals while they groom another and found that chimpanzees produce lip-smacks at an average speech-like rhythm of 4.15 Hz.
Researchers used data across two captive and two wild populations, collecting video recordings at Edinburgh Zoo and Leipzig Zoo, and audio recordings of wild communities including the Kanyawara and the Waibira, both in Uganda.
Dr Adriano Lameira, from the Department of Psychology at the University of Warwick, which led the study, comments: “Our results prove that spoken language was pulled together within our ancestral lineage using “ingredients” that were already available and in use by other primates and hominids. This dispels much of the scientific enigma that language evolution has represented so far. We can also be reassured that our ignorance has been partly a consequence of our huge underestimation of the vocal and cognitive capacities of our great ape cousins.
“We found pronounced differences in rhythm between chimpanzee populations, suggesting that these are not the automatic and stereotypical signals so often attributed to our ape cousins. Instead, just like in humans, we should start seriously considering that individual differences, social conventions and environmental factors may play a role in how chimpanzees engage ‘in conversation’ with one another.
If we continue searching, new clues will certainly unveil themselves. Now it’s a matter of mastering the political and societal power to preserve these precious populations in the wild and continue enabling scientists to look further.”