The human larynx descends during infancy and the early juvenile periods, and this greatly contributes to the morphological foundations of speech development. This developmental phenomenon is believed to be unique to humans. This concept has formed a basis for paleoanthropological studies on the origin and evolution of human speech. We used magnetic resonance imaging to study the development of three living chimpanzees and found that their larynges also descend during infancy, as in human infants. This descent was completed primarily through the rapid descent of the laryngeal skeleton relative to the hyoid, but it was not accompanied by the descent of the hyoid itself. The descent is possibly associated with developmental changes of the swallowing mechanism. Moreover, it contributes physically to an increased independence between the processes of phonation and articulation for vocalization. Thus, the descent of the larynx and the morphological foundations for speech production must have evolved in part during hominoid evolution, and not in a single shift during hominid evolution. In the human neonate, the hyoid bone and larynx are positioned as high as in other mammals (1-3). However, they descend gradually during postnatal life (1-6). This descent is completed through the descent of the laryngeal skeleton relative to the hyoid and the descent of the hyoid relative to the mandible and cranial base (4-6). Thus, the human supralaryngeal vocal tract (SVT) develops to form a double resonator system with equally long horizontal [SVT H , from the posterior oropharyngeal wall (POW) to the lips] and vertical (SVT V , from the vocal folds to the velum) components (2, 6). Acoustically, such a configuration, in combination with the tongue's mobility, enables humans to produce complex speech sounds (7,8).It is commonly assumed that this developmental descent evolved as an adaptation for speech in a single shift in the human lineage, in combination with decreased prognathism and flexure of the cranial base (2, 9, 10). This concept has formed a basis for paleoanthropological studies on the origin and evolution of human speech, in which the ''unique'' morphological features related to speech have been examined through comparisons with extant primates (2,(11)(12)(13)(14)(15)(16). Thus, the evolution of the morphological basis for human speech has been regarded as synonymous with the evolution of the developmental descent of the larynx. Nonetheless, there are few comparative studies on the developmental changes of humans and non-human mammals (17-19), and it is unclear how and when the unique features of the speech apparatus of adult humans appear and develop during growth. Subjects and MethodsMagnetic Resonance Imaging (MRI) Procedures. We used MRI technology to examine the developmental changes of the SVT shape in three living chimpanzee infants, named Ayumu (male), Cleo (female), and Pal (female). They were born in 2000 and were reared by their biological mothers in the Kyoto University Primate Research Institute (KUPRI) (20,2...

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We are flat-faced hominins with an external nose that protrudes from the face. This feature was derived in the genus Homo, along with facial flattening and reorientation to form a high nasal cavity. The nasal passage conditions the inhaled air in terms of temperature and humidity to match the conditions required in the lung, and its anatomical variation is believed to be evolutionarily sensitive to the ambient atmospheric conditions of a given habitat. In this study, we used computational fluid dynamics (CFD) with three-dimensional topology models of the nasal passage under the same simulation conditions, to investigate air-conditioning performance in humans, chimpanzees, and macaques. The CFD simulation showed a horizontal straight flow of inhaled air in chimpanzees and macaques, contrasting with the upward and curved flow in humans. The inhaled air is conditioned poorly in humans compared with nonhuman primates. Virtual modifications to the human external nose topology, in which the nasal vestibule and valve are modified to resemble those of chimpanzees, change the airflow to be horizontal, but have little influence on the air-conditioning performance in humans. These findings suggest that morphological variation of the nasal passage topology was only weakly sensitive to the ambient atmosphere conditions; rather, the high nasal cavity in humans was formed simply by evolutionary facial reorganization in the divergence of Homo from the other hominin lineages, impairing the air-conditioning performance. Even though the inhaled air is not adjusted well within the nasal cavity in humans, it can be fully conditioned subsequently in the pharyngeal cavity, which is lengthened in the flat-faced Homo. Thus, the air-conditioning faculty in the nasal passages was probably impaired in early Homo members, although they have survived successfully under the fluctuating climate of the Plio-Pleistocene, and then they moved “Out of Africa” to explore the more severe climates of Eurasia.

Diversifications in primate vocalization, including human speech, are believed to reflect evolutionary modifications in vocal anatomy and physiology. Gibbon song is acoustically unique, comprising loud, melodious, penetrating pure tone-like calls. In a white-handed gibbon, Hylobates lar, the fundamental frequency (f(0) ) of song sounds is amplified distinctively from the higher harmonics in normal air. In a helium-enriched atmosphere, f(0) does not shift, but it is significantly suppressed and 2f(0) is emphasized. This implies that the source is independent of the resonance filter of the supralaryngeal vocal tract (SVT) in gibbons, in contrast to musical wind instruments, in which the filter primarily determines f(0) . Acoustic simulation further supported that gibbons' singing is produced analogously to professional human soprano singing, in which a precise tuning of the first formant (F(1) ) of the SVT to f(0) amplifies exclusively the f(0) component of the source. Thus, in gibbons, as in humans, dynamic control over the vocal tract configuration, rather than anatomical modifications, has been a dominant factor in determining call structure. The varied dynamic movements were adopted in response to unique social and ecological pressures in gibbons, allowing monogamous gibbons to produce pure-tonal melodious songs in the dense tropical forests with poor visibility.

The configuration of the supralaryngeal vocal tract depends on the nonuniform growth of the oral and pharyngeal portion. The human pharynx develops to form a unique configuration, with the epiglottis losing contact with the velum. This configuration develops from the great descent of the larynx relative to the palate, which is accomplished through both the descent of the laryngeal skeleton relative to the hyoid and the descent of the hyoid relative to the palate. Chimpanzees show both processes of laryngeal descent, as in humans, but the evolutionary path before the divergence of the human and chimpanzee lineages is unclear. The development of laryngeal descent in six living Japanese macaque monkeys, Macaca fuscata, was examined monthly during the first three years of life using magnetic resonance imaging, to delineate the present or absence of these two processes and their contributions to the development of the pharyngeal topology. The macaque shows descent of the hyoid relative to the palate, but lacks the descent of the laryngeal skeleton relative to the hyoid and that of the EG from the VL. We argue that the former descent is simply a morphological consequence of mandibular growth and that the latter pair of descents arose in a common ancestor of extant hominoids. Thus, the evolutionary path of the great descent of the larynx is likely to be explained by a model comprising multiple and mosaic evolutionary pathways, wherein these developmental phenomena may have contributed secondarily to the faculty of speech in the human lineage.