While the air sac makes possible booming vocalizations, it may have prevented the production of the rich variety of distinctive sounds that is exploited in spoken language. A paper published last month in the Journal of Human Evolution, "Loss of air sacs improved hominin speech abilities," explores this relationship. From the abstract:
Air sacs are a feature of the vocal tract of all great apes, except humans. Because the presence or absence of air sacs is correlated with the anatomy of the hyoid bone, a probable minimum and maximum date of the loss of air sacs can be estimated from fossil hyoid bones. Australopithecus afarensis still had air sacs about 3.3 Ma, while Homo heidelbergensis, some 600 000 years ago and Homo neandethalensis some 60 000 years ago, did no longer. The reduced distinctiveness of articulations produced with an air sac is in line with the hypothesis that air sacs were selected against because of the evolution of complex vocal communication. This relation between complex vocal communication and fossil evidence may help to get a firmer estimate of when speech first evolved.
The researcher, Bart de Boer of the University of Amsterdam, attempted to recreate the sounds of our inarticulate ancestors by forcing air through artificial vocal tracts of plastic tubing, some of which contained an extra chamber to model the air sac. The results can be heard here courtesy of New Scientist.