Tokyo: An unexpected discovery has overturned a long-held assumption that the human capacity for speech evolved as a result of a unique positioning of the larynx, or voice box.

A team of Japanese researchers has revealed for the first time that in chimpanzee infants the larynx also descends closer to the lungs after birth, according to a report in ‘News in Science’.

The research team, lead by Dr Takeshi Nishimura of the Primate Research Institute at Kyoto University, monitored in detail how the vocal tracts of three infant chimps - born in 2000 and reared by their biological mothers at the institute - developed over the first two years of their lives.

Using magnetic resonance imaging technology, the team found that all the chimps' larynxes rapidly descended over that period from their original birth positions to be repositioned much lower in the neck - at a point between the pharynx and lungs, the report said.

Until now it was thought that this happened only in humans. This repositioning was considered the anatomical basis for the generation and articulation of the complex sounds that comprise speech in humans. The finding suggests that the evolution of the human vocal system may have occurred in two steps, not one as originally thought.

The first step - the descent of the larynx relative to the hyoid bone, a U-shaped bone in the upper neck - is likely to have occurred before the human and chimpanzee lineages split about six million years ago. 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 pre-requisite for speech production, the researchers speculate that it may have resulted from changes in the swallowing mechanism, the report noted. In newborn humans, the higher initial positioning of the larynx enables them suckle and breathe simultaneously. The subsequent anatomical changes increase the risk of choking, because air and food must then travel a common pathway behind the tongue - suggesting that the acquisition of the power of speech came at a safety cost to humans, it added.

ANI



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