The phonetic potential of nonhuman primate vocal tracts has been the subject of considerable contention in recent literature. Here, 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 research have been consistently misinterpreted in the literature. A paper by Fitch et al. overestimates the would‐be “speech‐ready” capacities of a rhesus macaque, and the data presented nonetheless supports Lieberman's principal position—that nonhuman primates cannot articulate the full extent of human speech sounds. The suggestion that no vocal anatomical evolution was necessary for the evolution of human speech(as spoken by all normally developing humans) is not supported by phonetic or anatomical data. The second challenge, by Boë et al., attributes vowel‐like qualities of baboon calls to articulatory capacities based on audio data; I argue that such“protovocalic” properties likely result from disparate articulatory maneuvers compared to human speakers. A review of Lieberman's scientific legacy by Boë et al. ascribes a view of speech evolution (which the authors term “laryngeal descent theory”) to Lieberman, which contradicts his writings. The present article documents a pattern of incorrect interpretations of Lieberman's theoretical work in recent literature. Finally, the apparent trend of vowel‐like formant dispersions in great ape vocalization literature is discussed with regard to Lieberman's theoretical work. The review concludes that the “Lieberman account” of primate vocal tract phonetic capacities remains supported by research: the ready articulation of fully human speech reflects species‐unique anatomy.

 There is renewed interest in potential vowel production by nonhuman primates, but no agreedupon methodologies for its estimation from reallife vocalizations. Here, we present a set of supervised approaches for estimating primate vowel-like articulation, with reference to orangutan long call pulses (N=36). We summarize our approach as a cohesive framework, the Primate Quasi-Vowel (PREQUEL) protocol. We (1) estimated f0 from correlograms, (2) and vocal tract resonances (formants) from spectrograms, (3) the results of which were then compared against synthesized vowels for those frequency values; and (4) presented to uninformed listeners (N=16), who largely agreed on the categorization of vowel-like qualities for vocalizations (Cronbach’s alpha=.701). We also provide descriptions of methods that are seemingly inadequate for formant estimation in great ape calls. We argue that a combination of phonetic methods is required to develop a science of nonhuman primate articulation.

Understanding the production of vowel-like sounds by nonhuman primates requires bridging the gap between animal calls and human speech. In particular, it has been argued that call behaviors whose acoustic properties seemingly overlap with human vowels reflect disparate production mechanisms. However, there are no as of yet agreed-upon methods for the study of primate articulation. We explore the potential of video content analysis for illuminating the production of primate vowel‒like qualities and provide evidence bearing on primate articulation using an audiovisual corpus (N=29 videos) of gibbon (Hylobatidae) call behavior, sourced from online video sites. Videos were coded for apparent jaw height, lip rounding, visible tongue movements, and visible inflation of species’ throat sacs (for Symphalangus Syndactylus), and the audible vowel‒like or diphthong‒like quality of the call. Results illustrate that the vowel-like quality of gibbon calls is tightly connected to degrees of jaw height and lip rounding. For “diphthong‒like” vocalizations, articulator trajectories effectively constitute movements between the same two extremes: high jaw-rounded lips, and low jaw-flared oral cavity. Animals’ tongues were often visible but never observed moving to systematically affect the vowel-like quality of the call (though several instances of possible tongue retraction were noted). The study adds to available methods for the study of articulation by nonhuman primates.  

Great ape call production may inform research on the evolution of speech but remains poorly understood. The vowel-like qualities of long-distance vocalizations of nonhuman great apes are seemingly both acoustically and perceptually comparable to human close back vowel [u]. However, nonhuman great ape vocal tract morphology, including the species-typical tongue and lack of an expanded pharynx, preclude comparable articulation. Here, we explore possible vocal tract configurations underlying chimpanzee (Pan troglodytes) pant hoots, gorilla (Gorilla gorilla) hoots , and orangutan (Pongo abelii) long calls. We present the result of computer simulations of acoustic tube vocal tract models based on MRI great ape articulator data and behavior. Predicted first and second formant simulation data were compared against data collected from adult male chimpanzees, silverback male gorillas, and flanged male orangutans in the wild. We explored the explanatory value of four sets of models corresponding to (i) uniform tubes, (ii) narrowed lip passage, (iii) narrowed and extremely protruded lip passage, and (iv) a “retracted model”, with dorsal oral tract constriction achieved via tongue retraction, combined with a narrowed lip passage. Our results show that great ape hoot data are most consistent with an articulatory model assuming dorsal oral tract stricture through tongue retraction (the only model to achieve a fit for the second formant). Our work indicates articulatory configurations employed in great ape call production may exist in continuity with speech production, without being identical to those observed in modern humans. 

This text synthesizes the available literature on the effects of great ape air sacs and highlights shortcomings of previous approaches in understanding the roles of these intriguing organs in vocal communication and evolution. Several points of yet unattended nuance are highlighted. The interpretation that A. afarensis possessed air sacs is based on a single hyoid likely belonging to a juvenile female, and australopiths likely exhibited a combination of sexually dimorphic traits unique among primates, hindering inferences at the level of species or genera. While much of the modern literature on air sacs asserts a meaningful relationship between their presence and evolving speech production capacities, no empirical works analyze non-human great ape vocalizations from a viewpoint of evaluating air sac function hypotheses. We conclude that there is little support for the hypothesis that air sacs seriously impede potential vowel production by extant great apes. 

Nonhuman great apes have been claimed to be unable to learn human words due to a lack of the necessary neural circuitry. We recovered original footage of two enculturated chimpanzees uttering the word “mama” and subjected recordings to phonetic analysis. Our analyses demonstrate that chimpanzees are capable of syllabic production, achieving consonant-to-vowel phonetic contrasts via the simultaneous recruitment and coupling of voice, jaw and lips. In an online experiment, human listeners naive to the recordings’ origins reliably perceived chimpanzee utterances as syllabic utterances, primarily as “ma-ma”, among foil syllables. Our findings demonstrate that in the absence of direct data-driven examination, great ape vocal production capacities have been underestimated. Chimpanzees possess the neural building blocks necessary for speech.

Despite decades of research, the field of language evolution lacks a cohesive integrative account, capable of explicating possible linguistic evolution throughout the development of modern humans. We review archaeological findings in search of a timeline during which features of the modern human articulatory morphology emerged. Rudimentary systems of speech may have driven selection for a vocal tract “optimal” for speech in early humans. However, a range of other factors have also enacted substantial morphological changes to the would-be speech articulators. The incorporation of processed and (ultimately) cooked food in the Homo lineage likely facilitated significant reduction of mandible and masticatory muscles, decreased the time spent masticating, and may have been maintainable in the lineage because food processing had already been outsourced to the hands and rudimentary stone tools (reducing selection pressure for robust jaws). The articulatory anatomy of early human ancestors is limited with regard to human speech sounds, but theoretically allows for a greater range of sounds than are observed in nature. We suggest that with decreased pressure to maintain anatomical elements required for mastication of foods that are mechanically challenging to eat, the would-be articulatory complex of human ancestors may have become pre-adapted for the development toward fully modern human speech.