Phonetic potential of primate vocal tracts and the legacy of Philip Lieberman (1934–2022): Refuting purported challenges and reconciling recent findings

Abstract

The phonetic potential of nonhuman primate vocal tracts has been the subject of considerable contention in recent literature. Here, I critically assess the validity of this research. The work of Philip Lieberman (1934-2022) is considered at length, and two research papers – both purported challenges to Lieberman’s theoretical work – and a review of Lieberman’s scientific legacy are critically examined. I argue that various aspects of Lieberman’s work, based on Kenneth Stevens’ ‘quantal theory’ of speech production, have been consistently misinterpreted in the literature. A paper by Fitch et al. (2016) presents a vowel space based on non-articulatory data, and the data presented in reality supports Lieberman’s principal position – that nonhuman primates cannot articulate quantal vowels. The suggestion that no vocal anatomical evolution was necessary for the evolution of human speech is not supported by phonetic or anthropological data and cannot be reasonably supported by works that solely or primarily investigate capacities of vowel production. The second purported challenge, by Boë et al. (2017), seemingly misattribute their data to articulatory capacities based on audio data, and the authors’ claim of baboon ‘protovocalic’ properties likely result from mandibular, as opposed to lingual, maneuvers – also supporting Lieberman’s position. Finally, the review of Lieberman’s scientific legacy by Boë et al. (2019) attributes a view of speech evolution to Lieberman inconsistent with Lieberman’s writings. I attribute documented misconceptions in the literature to a lack of integration of phonetic sciences in the fields of bioacoustics and primatology, and corresponding ill motivated focus on vowel articulation, at the expense of other parameters crucial to human speech. I conclude that the ‘Lieberman account’ of primate vocal tract phonetic capacities remains supported by research: lingual articulation of quantal vowels remains a unique feature of human speech, reflecting species-unique anatomy.

Evolution of the human tongue and emergence of speech biomechanics

“…Thus, it is the relative position and shape the tongue, rather than position of the larynx per se , which are central for speech ( Lieberman, 1984 , 2012 ; Carre et al, 1995 ; de Boer and Fitch, 2010 ). No nonhuman mammal have ever been shown to attain the same configuration necessary for the extremities of human speech ( Lieberman, 1984 , 2012 ; de Boer and Fitch, 2010 ; Fitch et al, 2016 ; Ekström, 2023a ).…”