Evolutionary Background of Precise Vocal-Respiratory Temporal Coordination in Human Infants.

Human infants acquire speech through prolonged postnatal development. Sequential vocalizations, including speech, can be viewed as motor patterns consisting of alternating sound and silence. This temporal structure depends on the coordination of respiratory airflow and laryngeal activity, yet relatively little attention has been paid to how such timing control develops in infancy. In this review, we examine the development of vocal timing across two contexts: spontaneous vocalizations produced in nonsocial settings and vocal exchanges during face-to-face interactions. We highlight a developmental transition around the second half of the first year, in which vocal timing first becomes relatively stabilized in relation to respiratory maturation and is then increasingly used flexibly in social interaction. To place this transition in an evolutionary context, we review comparative findings from songbirds and marmosets and consider swallowing as another vital behavior requiring tight laryngeal-respiratory coordination. In humans, rapid postnatal brain growth may promote early complementary feeding, increasing the need for flexible regulation of swallowing timing, while laryngeal descent may heighten the need for airway protection. These pressures may have provided a physiological background for the later elaboration of flexible vocal timing control.