Development of the Supralaryngeal Vocal Tract in Japanese Macaques: Implications for the Evolution of the Descent of the Larynx Takeshi Nishimura, 1 * Takao Oishi, 2 Juri Suzuki, 3 Keiji Matsuda, 4 and Toshimitsu Takahashi 5 1 Laboratory of Physical Anthropology, Department of Zoology, Graduate School of Science, Kyoto University, Sakyo, Kyoto 606-8502, Japan 2 Department of Cellular and Molecular Biology, Primate Research Institute, Kyoto University, Inuyama, Aichi 484-8506, Japan 3 Center for Human Evolution Modeling Research, Primate Research Institute, Kyoto University, Inuyama, Aichi 484-8506, Japan 4 Systems Neuroscience Group, Neuroscience Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaragi 305-8568, Japan 5 Department of Neurophysiology, Juntendo University, Graduate School of Medicine, Bunkyo, Tokyo 113-8421, Japan KEY WORDS descent of the hyoid; hyo-laryngeal complex; epiglottis; Macaca fuscata; MRI ABSTRACT 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 la- ryngeal 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 evo- lutionary path before the divergence of the human and chimpanzee lineages is unclear. The development of la- ryngeal 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 pharyn- geal topology. The macaque shows descent of the hyoid relative to the palate, but lacks the descent of the laryn- geal 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 ances- tor 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. Am J Phys Anthropol 135:182–194, 2008. V V C 2007 Wiley-Liss, Inc. Adult humans have a unique anatomy of the supralar- yngeal vocal tract (SVT), with equally long horizontal oral and vertical pharyngeal cavities, in contrast to all nonhuman primates having a pharynx much shorter than the oral cavity (Negus, 1949; Lieberman, 1984). Such a configuration of human SVT depends on the nonuniform growth of the oral and pharyngeal portions with a faster growth rate in the latter than the former (Negus, 1949; Lieberman, 1984; Crelin, 1987). Adult humans have a unique anatomy of the pharynx, with the epiglottis losing contact with the velum during development. This configu- ration secures a long oropharyngeal region facing the dor- sal surface of the mobile tongue, rostral to the laryngo- pharyngeal region that faces the epiglottis (Lieberman, 1984; Crelin, 1987; Zemlin, 1988). This anatomy of the human pharynx is formed ontogenetically with the great descent of the larynx relative to the palate (Negus, 1949; Roche and Barkla, 1965; Lieberman, 1984; Crelin, 1987; Westhorpe, 1987; Zemlin, 1988). The laryngeal skeleton is suspended from the hyoid apparatus and the hyoid is in turn suspended from the mandible and cranial base by muscles and ligaments (Zemlin, 1988; Williams, 1995). Anatomically, two processes accomplish the great descent of the larynx in humans: the descent of the laryngeal skeleton relative to the hyoid, and the descent of the hyoid relative to the palate. One or both of them cause the epiglottis—attached to the thyroid cartilage of the la- ryngeal skeleton—to descend along the neck and to lose contact with the velum. All nonhuman primates have a pharynx much shorter than the oral cavity (Negus, 1949; Laitman and Reiden- berg, 1993), although they show the descent of the lar- ynx relative to the palate that lengthens the pharyngeal cavity during growth (Flu ¨ gel and Rohen, 1991). Such a SVT configuration makes the great descent of the larynx Grant sponsor: Japan Society for the Promotion of Science; Grant number: 16000326; Grant sponsor: Ministry of Education, Culture, Sports, Science and Technology of Japan; Grant number: A14; Grant sponsor: Primate Research Institute, Kyoto University; Grant num- bers: 4-1 (2004), 3-1 (2005), and 1-6 (2006). *Correspondence to: Takeshi Nishimura, D.Sc., Primate Research Institute, Kyoto University, Kanrin, Inuyama, Aichi 484-8506, Japan. E-mail: nishimur@pri.kyoto-u.ac.jp Received 24 July 2006; accepted 17 August 2007 DOI 10.1002/ajpa.20719 Published online 25 October 2007 in Wiley InterScience (www.interscience.wiley.com). V V C 2007 WILEY-LISS, INC. AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 135:182–194 (2008)