The particular shape of the vocal tract, that is, its fitness for articulating language
sounds, is probably due to what in evolutionary biology is called exaptation: a large
change in function is accomplished with little change in structure, or, expressed in a
different way, old tools are used for new purposes.
It has not been possible to find anatomical changes in the vocal tract that can
only be understood as evolutionary adaptions to language. The vocal tract most
probably acquired its present shape for reasons that are irrelevant for language, like
adaption to a different diet and walking upright, but this new shape turned out to be an
almost perfect tool for talking.