The Lowered Larynx
In human babies, as in all other primates and non-marine mammals, the windpipe is quite separate from the oesophagus, and they can therefore breathe and swallow at the same time. As the child develops, this arrangement changes. The larynx descends in the throat until the windpipe and foodpipe are lying side by side. In the past, anthropologists have linked this to the evolution of speech but now most of them agree that this explanation is unlikely. In fact, a similar arrangement can be found in sealions and walruses, and its purpose from an evolutionary point of view is that it allows them - and us - to inhale great lungfuls of air through the mouth. Apes, which breathe almost entirely through their nose, cannot do this. The advantage of the lowered larynx for a creature which was evolving to cope with a semi-aquatic lifestyle is that it would have been able to take deeper breaths and therefore stay under water for long periods. This theory is far more likely than the largely discredited idea that speech itself somehow caused a dramatic rearrangement of the larynx.