Comparison of orangutan, chimpanzee and human vocal anatomy (a–c, respectively). Red indicates the tongue body,yellow the larynx and blue the air sacs (apes only). Note the longer oral cavity and much lower larynx in the humans (c), with concomitant distortion of tongue shape compared with orangutans (a) and chimpanzees (b). These differences allow a much greater range of sounds to be produced by humans, which would have been significant in the evolution of speech.
Excerpted from: Origin of language