p274
Precursor elements to speak (cf footnote 90) another example
Asif A. Ghazanfar et al., Cineradiography of Monkey Lip-Smacking Reveals Putative Precursors of Speech Dynamics
CB 22 1176 (2012)
A key feature of speech is its stereotypical 5 Hz rhythm. One theory posits that this rhythm evolved through the modification of rhythmic facial movements in ancestral primates. If the hypothesis has any validity, then a comparative approach may shed some light.
Using cineradiography (X-ray movies) to characterize and quantify the internal dynamics of the macaque monkey vocal tract during lip-smacking (a rhythmic facial expression) versus chewing. the lips, tongue, and hyoid move with a speech-like 5 Hz rhythm during lip-smacking, but not during chewing. Most importantly, the functional coordination between these structures was distinct for each behavior.