Scientists Say Monkey Lip Smacks Provide Insight Into Evolution of Human Speech
Posted on June 2, 2012
Scientists say monkey lip smacks could provide valuable insight into the evolution of human speech. Scientists have traditionally sought the evolutionary origins of human speech in primate vocalizations, such as monkey coos or chimpanzee hoots. But unlike these primate calls, human speech is produced using rapid, controlled movements of the tongue, lips and jaw.
Researchers at Princeton and the University of Vienna used x-ray movies to investigate lip-smacking gestures in macaque monkeys. Tecumseh Fitch, head of the Department of Cognitive Biology at the University of Vienna is pictured above with Saimiri.
Lip smacks are made by many monkey species in friendly, face-to-face situations, such as between mothers and their infants. Although lip-smacking makes a quiet sound (similar to "p p p p"), it is not accompanied by phonation, which is produced by vocal cord vibration in the "voice box" or larynx.
To the human eye, lip-smacking appears to involve simply the rapid opening and closing of the lips, but the x-ray movies show that lip-smacking is actually a complex behavior, requiring rapid, coordinated movements of the lips, jaw, tongue and hyoid bone. Lip-smacking movements occur at a rate of about 5 cycles per second, the same as speech, and are much faster than chewing movements (2.5 cycles per second). Therefore, the researchers say that although monkey lip smacking resembles "fake chewing," it is actually much more like speech.
Peter MacNeilage and his colleagues have a hypothesis that the roots of human speech do not lie in primate vocalizations, but are closer to these facial signals and lip smacks used for communication by monkeys. The scientists say the alternation between vowel and consonant that generates speech syllables is very similar to the movements underlying lip-smacking.
The research was published here in Current Biology.