A new paper in Science Advances (Fitch, de Boer, Mathur & Ghazanfar, 2016) uses models informed by x-rays of macaque vocal tracts to claim that monkeys have the tools neccessary to make enough speech sounds to create a productive spoken language. This means that the evolution of speech is likely to be primarily dependent on cognitive adaptation, rather than adaptation of the vocal tract.
For four decades, the inability of nonhuman primates to produce human speech sounds has been claimed to stem from limitations in their vocal tract anatomy, a conclusion based on plaster casts made from the vocal tract of a monkey cadaver. We used x-ray videos to quantify vocal tract dynamics in living macaques during vocalization, facial displays, and feeding. We demonstrate that the macaque vocal tract could easily produce an adequate range of speech sounds to support spoken language, showing that previous techniques based on postmortem samples drastically underestimated primate vocal capabilities. Our findings imply that the evolution of human speech capabilities required neural changes rather than modifications of vocal anatomy. Macaques have a speech-ready vocal tract but lack a speech-ready brain to control it.