Evolution of the human tongue and emergence of speech biomechanics

Correcting the record: Phonetic potential of primate vocal tracts and the legacy of Philip Lieberman (1934-2022)

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.

What's special about human speech? A student exercise for comparing speech production between humans and chimpanzees

Modern humans and chimpanzees share a common ancestor on the phylogenetic tree, yet chimpanzees do not spontaneously produce speech or speech sounds. The lab exercise presented in this paper was developed for undergraduate students in a course entitled "What's Special About Human Speech?" The exercise is based on acoustic analyses of the words "cup" and "papa" as spoken by Viki, a home-raised, speech-trained chimpanzee, as well as the words spoken by a human. The analyses allow students to relate differences in articulation and vocal abilities between Viki and humans to the known anatomical differences in their vocal systems. Anatomical and articulation differences between humans and Viki include (1) potential tongue movements, (2) presence or absence of laryngeal air sacs, (3) presence or absence of vocal membranes, and (4) exhalation vs inhalation during production.

Sketches of chimpanzee (Pan troglodytes) hoo's: vowels by any other name?

In human speech, the close back rounded vowel /u/ (the vowel in "boot") is articulated with the tongue arched toward the dorsal boundary of the hard palate, with the pharyngeal cavity open. Acoustic and perceptual properties of chimpanzee (Pan troglodytes) hoo's are similar to those of the human vowel /u/. However, the vocal tract morphology of chimpanzees likely limits their phonetic capabilities, so that it is unlikely, or even impossible, that their articulation is comparable to that of a human. To determine how qualities of the vowel /u/ may be achieved given the chimpanzee vocal tract, we calculated transfer functions of the vocal tract area for tube models of vocal tract configurations in which vocal tract length, length and area of a laryngeal air sac simulacrum, length of lip protrusion, and area of lip opening were systematically varied. The method described is principally acoustic; we make no claim as to the actual shape of the chimpanzee vocal tract during call production. Nonetheless, we demonstrate that it may be possible to achieve the acoustic and perceptual qualities of back vowels without a reconfigured human vocal tract. The results, while tentative, suggest that the production of hoo's by chimpanzees, while achieving comparable vowel-like qualities to the human /u/, may involve articulatory gestures that are beyond the range of the human articulators. The purpose of this study was to (1) stimulate further simulation research on great ape articulation, and (2) show that apparently vowel-like phenomena in nature are not necessarily indicative of evolutionary continuity per se.