The species-specific organizational property of speech is a
continual mouth open-close alternation, the two phases of which are
subject to continual articulatory modulation. The cycle constitutes
the syllable, and the open and closed phases are segments –
vowels and consonants, respectively. The fact that segmental serial
ordering errors in normal adults obey syllable structure constraints
suggests that syllabic “frames” and segmental
“content” elements are separately controlled in the
speech production process. The frames may derive from cycles of
mandibular oscillation present in humans from babbling onset, which
are responsible for the open-close alternation. These communication-
related frames perhaps first evolved when the ingestion-related
cyclicities of mandibular oscillation (associated with mastication
[chewing] sucking and licking) took on communicative significance
as lipsmacks, tonguesmacks, and teeth chatters – displays that
are prominent in many nonhuman primates. The new role of Broca's
area and its surround in human vocal communication may have derived
from its evolutionary history as the main cortical center for the
control of ingestive processes. The frame and content components
of speech may have subsequently evolved separate realizations
within two general purpose primate motor control systems: (1) a
motivation-related medial “intrinsic” system, including
anterior cingulate cortex and the supplementary motor area, for
self-generated behavior, formerly responsible for ancestral
vocalization control and now also responsible for frames, and
(2) a lateral “extrinsic” system, including Broca's
area and surround, and Wernicke's area, specialized for response
to external input (and therefore the emergent vocal learning capacity)
and more responsible for content.