The main differences between the erectus vocal apparatus and the sapiens apparatus were in the hyoid bone and pre-Homo vestiges, such as air sacs in the centre of the larynx. Tecumseh Fitch was one of the first biologists to point out the relevance of air sacs to human vocalisation. Their effect would have been to render many sounds emitted less clear than they are in sapiens. The evidence that they had air sacs is based on luck in finding fossils of erectus hyoid bones.
