[excerpt from Philip Gerard's "The Speaking Creature", Ecotone: reimagining place: The Evolution Issue, University of North Carolina at Wilmington, 2008]
Like all other mammals, human babies are born with the larynx high up inside the neck. An anatomically good thing, too: Otherwise a newborn would not be able to nurse and breathe at the same time. By about the time an infant is weaned, eighteen months or so, the larynx begins its descent. The baby loses its ability to eat or drink and breathe at the same time -- breathing pauses automatically during swallowing. The voice changes. The gradual descent of the larynx opens up more pharyngeal space above the vocal cords (the so-called "voice box"), so the infant can make a wider variety of noises, much more distinctly human, all of them . . . much louder. By late puberty, age fourteen or so, the larynx reaches its final position, very low down in the neck column. A boy's voice finally stops cracking and settles into a lower register.