Adam Benton recently posted "When did Language evolve" on his blog Evoanth which looks at the latest information about the evolution of the hyoid bone, which is linked to the tongue and other muscles which are involved in speech.
Clearly there is a relationship between our ability to vocalise and our use of language - but a key question is what came first. Did early humans use imitation animal calls to help in hunting and then use a feature that already existed to communicate - or did our vocal tract evolve because we were already using a simple language and a clearer voice make things better. If we think of it in terms of information flow one thing seems obvious - our ability to makes a wide range of sounds, with clicks, whistles and a wide frequency range is far more than is needed to support our natural language. In the animal kingdom a number of birds, such as the Myna bird,are very good at imitating the sounds it hears - and also has the Fox2 gene supposed to be responsible.
However my own feeling is that natural language is almost entirely due to cultural evolution and I posted the following comment to Adam's post.