Humans are able to produce a wide variety of sounds in part because they have descended voice boxes that makes room for longer vocal tracts.
Evolutionarily, this appears to have occurred before, and thus may have facilitated the evolution of language.
But why would our larynxes descend in the first place?
One theory is that longer vocal tracts enabled us to make lower noises in order to sound larger and more threatening. Red Deer, for example, use a descended vocal tract for this purpose. The theory is supported by the fact that in males the voice box descends further during puberty (when boys males start fighting for access to girls).
One anatomical problem that arose as a result of lowered voice boxes is that the passageways for food and air became connected. This connection gave rise to an epiglottis which either covers the trachea or the esophagus.
As a result, we are the only primates who cannot eat and breathe at the same time.
No doubt you are now wondering how come babies don't either starve or suffocate when nursing.
The answer there is that the voice box doesn't descend until about nine months of age, thereby allowing hungry babies to suck and breathe simultaneously.
All of which explains why babies have such high squeaky voices.
- "The Evolution of Speech: a Comparative Review", by W. Tecumseh Fitch, in Trends In Cognitive Science, Vol 4, No 7, July 2000
- "Postnatal descent of the epiglottis in man", by Sasaki, C.T. et al. (1977) Arch. Otolaryngol. 103, 169–171