The researchers found that the oral-facial component of human speech mirrors the rhythm, development and internal dynamics of lip smacking, a friendly back-and-forth gesture performed by primates such as chimpanzees, baboons and macaques. The studies also show that the mechanics of primate lip smacking are distinct from those of chewing, similar to the separate mechanics of human speech and chewing.

These parallels suggest that in primates chewing and lip smacking — as with chewing and speech-related facial movement in humans — have separate neural controls, explained Asif Ghazanfar, an associate professor of psychology and the Princeton Neuroscience Institute, and a lead researcher for both studies. With further study, the neural pathway in primates from the brain to facial mechanics could help illuminate the neurological basis of speech disorders in humans, he said.

Ghazanfar and his colleagues first reported in the journal Developmental Science that lip smacking undergoes the same developmental trajectory from infancy to adulthood in rhesus macaques that speech-related mouth movement does in humans. Infant macaques smacked their lips slowly and with an inconsistent rhythm, similar to the documented pace of babbling in human infants. By adulthood, however, lip smacking has a distinct rhythm and a faster pace averaging 5 hertz, or cycles per second — the same as adult humans producing speech.