Résumé
This chapter focuses on the emergence of speech during human evolution, revisiting exaptation hypotheses (Fitch, 2010; MacNeilage, 1998) with new data from comparison with baboons. Speech necessarily evolved to be compatible with aero-digestive anatomy, reusing its functions of suction, chewing and swallowing. The tongue is involved with every feeding gesture, and also has a central position for speech. We analyze the evolution of the tongue position taking into account the distinction between the morphogenetic fields of HOX and non-HOX genes involved in the development of the pharyngeal arches and the cephalic structures, anatomical and neurological components, and functional support for breathing and swallowing. The hyoid bone is the locus of insertion of the tongue muscles as well as a precise marker of the glottis position. It is not fixed because it partly depends on the development of the facial area controlled by non-HOX genes. In contrast, the vertebral column has stable dimensions because it is controlled by HOX genes. After a detailed presentation of a baboon head dissection, we present a new method for mapping hyoid bone position relative to the vertebral axis, applied to MRI images. This is compared to a set of radiographs of 3-7.5 year human children. We observe that the hyoid bone is 1 vertebra lower in human infants than in adult baboons. The normalized oral cavity length is shorter, in agreement with prognathism reduction as controlled by non-HOX genes. Using the cervical vertebrae and their axis as a reference allows the conclusion that there is indeed laryngeal descent from baboons to humans and that it is accompanied by compensatory facial shortening. This preserves the vocal tract length as well as the relationship between the tongue and the oropharyngeal cavity, which is important for swallowing and other feeding gestures.