Chimpanzee lip-smacks confirm primate continuity for speech-rhythm evolution
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Speech is a human hallmark, but its evolutionary origins continue to defy scientific explanation. Recently, the open–close mouth rhythm of 2–7 Hz (cycles/second) characteristic of all spoken languages has been identified in the orofacial signals of several nonhuman primate genera, including orangutans, but evidence from any of the African apes remained missing. Evolutionary continuity for the emergence of speech is, thus, still inconclusive. To address this empirical gap, we investigated the rhythm of chimpanzee lip-smacks across four populations (two captive and two wild). We found that lip-smacks exhibit a speech-like rhythm at approximately 4 Hz, closing a gap in the evidence for the evolution of speech-rhythm within the primate order. We observed sizeable rhythmic variation within and between chimpanzee populations, with differences of over 2 Hz at each level. This variation did not result, however, in systematic group differences within our sample. To further explore the phylogenetic and evolutionary perspective on this variability, inter-individual and inter-population analyses will be necessary across primate species producing mouth signals at speech-like rhythm. Our findings support the hypothesis that speech recruited ancient primate rhythmic signals and suggest that multi-site studies may still reveal new windows of understanding about these signals' use and production along the evolutionary timeline of speech.
Pereira , A S , Kavanagh , E , Hobaiter , C , Slocombe , K E & R. Lameira , A 2020 , ' Chimpanzee lip-smacks confirm primate continuity for speech-rhythm evolution ' , Biology Letters , vol. 16 , no. 5 , 20200232 . https://doi.org/10.1098/rsbl.2020.0232
Copyright © 2020 The Author(s). Published by the Royal Society. All rights reserved. This work has been made available online in accordance with publisher policies or with permission. Permission for further reuse of this content should be sought from the publisher or the rights holder. This is the author created accepted manuscript following peer review and may differ slightly from the final published version. The final published version of this work is available at https://doi.org/10.1098/rsbl.2020.0232
DescriptionThis research was supported by the Research Incentive Grant of The Carnegie Trust for the Universities of Scotland (grant no. RIG008132) attributed to A.R.L.
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