Evolution of Epiglottis and Preepiglottic Space of Primate Larynx as the Vocal Tract Is Acquired.

Objectives: Comparative histoanatomy regarding the evolution of epiglottis and preepiglottic space (PES) of primate larynx as the supralaryngeal vocal tract (SVT) is acquired was investigated.

Methods: Two lemur, one galago, two macaque, two gorilla, and two chimpanzee normal larynges were investigated and compared with human larynges using the whole-organ serial section technique.

Results: Nonhuman primates with the limited pharyngeal space of SVT (lemurs, galago, and macaques, phyletic distant relative of humans) did not have PES. Their epiglottis was composed of cartilage that is not very flexible. Nonhuman primates with the lengthened pharyngeal space of SVT (gorillas and chimpanzees, hominids, phyletic close relative of humans) had PES. Furthermore, the PES of chimpanzees (phyletic closest relative of humans) existed astride the epiglottis similar to the human PES. The hominid epiglottises were composed of elastic cartilage, which is flexible, and play the role of retroflection. These histoanatomical structures of the epiglottis and PES reflect the fact that those features evolved as the pharyngeal space of SVT was lengthened.

Conclusions: The results of this study are consistent with the hypothesis that, in the process of evolution, the histoanatomical structures of epiglottis and PES changed and allowed the larynx to descend and the pharyngeal space of the SVT to be lengthened. This may facilitate speech production in humans. Moreover, the distribution of the human PES allows the epiglottis to more effectively play the role of retroflection during swallowing in order to prevent aspiration, which may have been derived from primate swallowing physiology.