Evolution of the speech-ready brain: The voice/jaw connection in the human motor cortex

A prominent model of the origins of speech, known as the frame/content theory, posits that oscillatory lowering and raising of the jaw provided an evolutionary scaffold for the development of syllable structure in speech. Because such oscillations are nonvocal in most nonhuman primates, the evolution of speech required the addition of vocalization onto this scaffold in order to turn such jaw oscillations into vocalized syllables. In the present functional MRI study, we demonstrate overlapping somatotopic representations between the larynx and the jaw muscles in the human primary motor cortex. This proximity between the larynx and jaw in the brain might support the coupling between vocalization and jaw oscillations to generate syllable structure. This model suggests that humans inherited voluntary control of jaw oscillations from ancestral species, but added voluntary control of vocalization onto this via the evolution of a new brain area that came to be situated near the jaw region in the human motor cortex.

Automated summary

The frame/content theory of the origins of speech proposes that jaw oscillations provided an evolutionary scaffold for syllable structure, with the human primary motor cortex showing overlapping representations of the larynx and jaw muscles, supporting the coupling between vocalization and jaw oscillations to generate syllables. This suggests that humans inherited voluntary control of jaw oscillations from ancestral species and added voluntary control of vocalization via the evolution of a new brain area near the jaw region in the human motor cortex.