Reconstructed fossil vocal tracts and the production of speech. Phylogenetic and ontogenetic considerations
Résumé
The end of the twentieth century and the beginning of this one saw a reorganization of research in the field of speech and language emergence (SLE). Naturalism is the core of this new approach. It consists in describing the relations between biological aspects (in every sense of the word) on the one hand and speech and language, on the other hand, by an accumulation of hypotheses and evidence derived from a wide range of data collected by means of interdisciplinary collaborations. As with studies on the origin of man, a profusion of hypotheses has arisen, which sometimes lead to very dubious developments, based on flimsy results and on too little data, and connected to related but not fully mastered or overly simplified disciplines. This is why regular critical overviews do not seem superfluous. First, we propose a classification (push and pull theory) that provides a new reading of the various theories which have been proposed for half a century. In the present state of knowledge, it is not possible to infer when our ancestors acquired the faculty of speech and language: control of speech articulators, coordination between larynx and vocal tract, phonology, syntax, semantics, and recursion. Some longstanding questions remain unanswered: Why is our species alone in having speech and language? Many other questions are (for now) badly formulated problems: we do not have sufficient data to answer them. Perhaps these questions will remain unanswered. But we think that the following question can be answered: If we suppose that our ancestors (and distant cousins) controlled their larynx and vocal tract in the same way as present-day humans, did the geometry of their vocal tract allow them to produce the universal sound structures of the languages spoken today? We analyzed 32 skulls from the present to 1.6 Ma (million years) BP (before present) of fossil hominids available at the Musée de l'Homme in Paris or in the literature: (1) 10-30 ka (thousand years) BP: modern humans: Paleolithic; (2) 90-200 ka BP: anatomically modern humans; (3) 45-90 ka BP: Neanderthals; (4) 1.6 Ma BP: Homo ergaster. These skulls are all well preserved and most possess a mandible but the vertebral column has been reconstructed from some fossil vertebrae. We attempt to: (1) identify the position of the hyoid bone and glottis; (2) reconstruct a vocal tract in a plausible way using an articulatory model; (3) quantify the acoustic capabilities of this reconstructed vocal tract. For this purpose, we combine phylogeny and ontogeny. We can now state that our ancestors and distant cousins were equipped with a vocal tract that could produce the same variety of vowel sounds as we can today: mainly the vowels /i a u/. Vocal tract morphology has allowed for the emergence and production of speech for several hundred thousand years. But how can we know to what extent earlier hominids mastered the control skills needed to produce speech? New lines of research are proposed in which orofacial abilities necessary to the emergence of speech are linked to a precursor mechanism dedicated to feeding (masticating-swallowing gestures).