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Rethinking the Evolution of Speech

May 29 2003

New findings in chimpanzees challenge current assumptions regarding the evolution of human speech, report the authors of “Descent of the larynx in chimpanzee infants.” For decades anthropologists have assumed that repositioning of the larynx, or voicebox, during infancy was a uniquely human trait that enabled humans to produce complex speech sounds. Now, Takeshi Nishimura and colleagues demonstrate that the chimpanzee larynx is similarly repositioned. The researchers used magnetic resonance imaging to study three infant chimpanzees during the first 2 years of life. In each chimpanzee, the larynx descended to a point between the pharynx and lungs, similar to humans. These results imply that the evolution of the human vocal system may have occurred in two steps. The first step—the descent of the larynx relative to the hyoid bone, a u-shaped bone in the upper neck—likely occurred before the human and chimpanzee lineages split. The second step—the descent of the hyoid bone relative to the skull--appears to have occurred only in humans, and further enabled complex vocalizations. Although the first step is a prerequisite for speech production, the authors speculate that it may have resulted from changes in the swallowing mechanism.

“Descent of the larynx in chimpanzee infants” by Takeshi Nishimura, Akichika Mikami, Juri Suzuki, and Tetsuro Matsuzawa, Published online 05 29 03, Proc. Natl. Acad. Sci. USA , 10.1073/pnas.1231107100

Abstract © 2003 PNAS