Tia, an Australian cockatoo, sounds almost human when she says "hallo," "bye bye," "I love you," and dozens of other words and phrases. She even utters them in the correct context. So why doesn't this count as language? The only meaningful distinction between human speech and nonhuman animal vocalizations, says biologist Marc Hauser of Harvard University, is the power of recursion: Humans alone can take discrete elements such as words or numbers and combine them to create an infinite variety of expressions.
Hauser, working with linguist Noam Chomsky of MIT and psychologist W. Tecumseh Fitch of the University of St. Andrews in Scotland, has observed that nearly all of the ways humans produce and perceive sounds have close parallels within the animal kingdom. Birds learn their songs by listening to others of their species. Rats and pigeons can acquire the concept of numbers, although they cannot count very high. Many primates can distinguish between two different human languages on the basis of rhythm. Even the descended larynx deemed necessary for human speech is found in other mammals—in red and fallow deer, for example. Only the skill of recursion sets us apart, Hauser claims.
Hauser speculates that humans may have developed the power of recursion because of a growing need to engage in transactions involving complex numbers. "One pressure for such a system might be when we evolved the capacity for complicated social exchange. Suppose I give you 24 mongongo nuts and later you give back 22. If I can quantify that, I'm going to say, no, that's not fair, you owe me two. That recursive element seems to have created a revolution in terms of what we can communicate about," he says.