The closure of the airway by the larynx has three functional ends : firstly the prevention of error in deglutition; secondly, trapping of the exhaled air flow inside the thoracic cavity (air trapping); and thirdly, phonation. These three functions were all present in the tertiary period, ca. 65 million years ago. The second function, air trapping, enabled the fixation of the thoracic cage, which is essential for obtaining the supporting points for the shoulder musculature during movements of the upper extremities. This morphological innovation originated in the three-dimesional, discontinuous arboreal habitat of the early mammals, and further evoked in tree-dwelling primates as a safety device to prevent falling from trees. The specificity of the primate larynx is the completely closure of the vocal cords during the air trapping. Although the complete closure of the vocal cords is expressed in humans as adaptation for the production of primary tones, its fundamental origin goes back to the three-dimensional arboreal locomotor adaptation in the Tertiary arboreal primates.
