Bart de Boer of the VUB Artificial Intelligence Laboratory in Belgium then turned the information into a computer model that could predict and simulate a macaque's vocal range based on the physical attributes. Human speech stems from a source sound produced by the larynx that is changed by the positions of the vocal anatomy such as the lips and tongue. They found that a macaque could produce comprehensible vowel sounds -- and even full sentences -- with its vocal tract if it had the neural ability to speak. "This new result tells us that there's still a big mystery concerning where human speech came from," said Laurie Santos, a psychology professor at Yale University. "The paper opens whole new doors for finding the key to the uniqueness of humans' unparalleled language ability. "If a species as old as a macaque has a vocal tract capable of speech, then we really need to find the reason that this didn't translate for later primates into the kind of speech sounds that humans produce.