Conventional wisdom holds that the repositioning of the human larynx that occurs during infancy--a key morphological prerequisite to speech--is particular to our kind. But Takeshi Nishimura of Kyoto University in Japan and colleagues have discovered that this southward migration of the larynx to a spot between the pharynx and the lungs occurs in our speechless relative the chimpanzee, too. The team employed magnetic resonance imaging (see image) to track development in three chimps during the first two years of life.
The findings indicate that rather than evolving in a single shift in the human lineage, the anatomical foundations for speech production were laid down piecemeal, starting earlier in our evolutionary history than previously thought. The descent of the larynx, for one, appears to have occurred prior to the point at which humans and chimps diverged from their common ancestor. A second critical development--the descent of the hyoid bone--occurred only in the human line.
The authors note that although these two changes to the vocal tract have the combined effect of facilitating speech, they may have evolved under selection pressures unrelated to talking. The repositioning of the larynx, they propose, could have been an adaptation to changes in the ape swallowing mechanism; the descent of the hyoid, for its part, may have accompanied changes in the architecture of the hominid skull.