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 non‐vocal in most non‐human
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.