An international team of scientists, led by Tecumseh Fitch at the University of Vienna and Asif Ghazanfar at Princeton University, has now looked inside monkeys' vocal tracts with x-rays, and found them to be much more flexible than thought before. The study indicates that the limitations that keep nonhuman primates from speaking are in their brains, rather than their vocal anatomy. The scientists used x-ray video to see within the mouth and throat of macaque monkeys induced to vocalize, eat food, or make facial expressions.
They then used these x-rays to build a computer model of a monkey vocal tract, allowing them to answer the question "what would monkey speech sound like, if a human brain were in control?" This showed that monkeys could easily produce many different sounds, enough to produce thousands of distinct words.
This implies that a basic form of spoken language could have evolved at any time in human evolution, without requiring any changes in vocal anatomy.
Publication in "Science Advances": W. T. Fitch, B. de Boer, N. Mathur, A. A. Ghazanfar, Monkey vocal tracts are speechready. Sci. Adv. 2, e1600723 (2016). DOI: 10.1126/sciadv.1600723