How did human language evolve from earlier forms of communication? One way to address this question is to compare prelinguistic human vocal behavior with nonhuman primate calls. An important finding has been that, prior to speech and from early on, human infant vocal behavior exhibits functional flexibility, or the capacity to produce sounds that are not tied to one specific function. This is reflected in human infants’ use of single categories of protophones (precursors of speech sounds) in various affective circumstances, such that a given call type can occur in and express positive, neutral, or negative affective states, depending on the occasion. Nonhuman primate vocal behavior, in contrast, is seen as comparably inflexible, with different call types tied to specific functions and sometimes to specific affective states (e.g. screams mostly occur in negative circumstances). We examined the vocal behavior of wild infant chimpanzees during their first year of life. We found that the most common vocal signal, grunts, occurred in a range of contexts that were deemed positive, neutral, and negative. We also found acoustic variants of grunts produced in the affective contexts, suggesting gradation within this vocal category. Our results suggest that the most common chimpanzee vocalization, the grunt is not affectively bound. Affective decoupling is a prerequisite for chimpanzee grunts to be deemed ‘functionally flexible’. If later confirmed to be a functionally flexible vocal type, this would indicate that the evolution of this foundational vocal capability occurred before the split between the Homo and Pan lineages.

Published in Journal of Language Evolution

Here is the link: academic.oup.com/jole/advance-article/doi/10.1093/jole/lzaa009/6017427

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