 Negus, however, pointed out that the descent of the larynx reflected the reshaping and descent of the human tongue into the pharynx.This process is not complete until age six to eight years.Some researchers, such as Philip Lieberman, Dennis Klatt, Bart de Boer and Kenneth Stevens using computer-modeling techniques have suggested that the species-specific human tongue allows the vocal tract (the airway above the larynx) to assume the shapes necessary to produce speech sounds that enhance the robustness of human speech.