This fascinating article explains that scientists have discovered similarities between the way that a human voice operates and the way that a jet engine operates. This has to do with vortices (areas of rotational motion) and airflows and the structure above a person’s vocal cords. Apparently, all of this explains why (or should I say, how) it is that every person’s voice is different.
Of course, every little boy (and every former little boy) who has ever pretended to be a jet plane, will say that he’s suspected this connection all along. Perhaps, anyway, it explains the affinity between the two? Another fascinating fact is that the animal whose vocal structure is most similar to ours is the dog. The scientists are hoping to study more about how a dog’s "voice" works in order to discover ways to help people with vocal problems.
This is all very interesting and compelling. But, of course, understanding why (or, again, how) it is that we have voice will not answer the question of why we have speech. I will venture a guess that it will be a long time before a study can answer that one.