Only humans speak. No other animal has anything approaching the complexity of language. The natural communication systems of other primates (monkeys and apes) are call systems. These vocal systems consist of a limited number of sounds—calls— that are produced only when particular environmental stimuli are encountered. Such calls may be varied in intensity and duration, but they are much less flexible than language because they are automatic and can’t be combined. When primates encounter food and danger simultaneously, they can make only one call. They can’t combine the calls for food and danger into a single utterance, indicating that both are present.
At some point in human evolution, however, our ancestors began to combine calls and to understand the combinations. The number of calls also expanded, eventually becoming too great to be transmitted even partly through the genes. Communication came to rely almost totally on learning.
Although wild primates use call systems, the vocal tract of apes is not suitable for speech. Until the 1960s, attempts to teach spoken language to apes suggested that they lack linguistic abilities. In the 1950s, a couple raised a chimpanzee, Viki, as a member of their family and systematically tried to teach her to speak. However, Viki learned only four words (“mama,” “papa,” “up,” and “cup”).