Some very practical questions about how children learn the first language compel us to study brain functions and how these functions evolve. They also bring the studies of linguistics and neurology together. The purpose of this paper is to relate some of the research that describes language acquisition with the research about the early development of the human brain, particularly the sensory systems that are involved in speech. Three interrelated propositions that are considered are: (1) that structures for speech are genetically transmitted, including distinct cognitive and affective mechanisms in the brain; (2) that critical periods in development render the child susceptible to particular environmental stimulation according to a predictable sequence which peaks and declines; and (3) that language itself supports and facilitates thought including special learning, and action on objects. These propositions are considered within the chronology of language development, beginning with neonatal response to sound, through sensorimotor integration in infancy, and into the rapid acquisition of propositional language in early childhood. (MKM)