"The sounds uttered by birds offer in several respects the nearest analogy（类似） to language," Charles Darwin wrote in "The Descent of Man" (1871), while contemplating how humans learned to speak. Language, he speculated, might have had its origins in singing, which "might have given rise to words expressive of various complex emotions." Now researchers from MIT, along with a scholar from the University of Tokyo, say that Darwin was on the right path. The balance of evidence, they believe, suggests that human language is a grafting of two communication forms found elsewhere in the animal kingdom: first, the elaborate（详尽的） songs of birds, and second, the more utilitarian（功利的）, information-bearing types of expression seen in a diversity of other animals.

"It's this adventitious combination that triggered human language," says Shigeru Miyagawa, a professor of linguistics in MIT's Department of Linguistics and Philosophy, and co-author of a new paper published in the journal Frontiers in Psychology.

The idea builds upon Miyagawa's conclusion, detailed in his previous work, that there are two "layers" in all human languages: an "expression" layer, which involves the changeable organization of sentences, and a "lexical（词汇的）" layer, which relates to the core content of a sentence. His conclusion is based on earlier work by linguists including Noam Chomsky, Kenneth Hale and Samuel Jay Keyser.

Based on an analysis of animal communication, and using Miyagawa's framework, the authors say that birdsong closely resembles the expression layer of human sentences -- whereas the communicative waggles of bees, or the short, audible messages of primates, are more like the lexical layer. At some point, between 50,000 and 80,000 years ago, humans may have merged these two types of expression into a uniquely sophisticated form of language.

"There were these two pre-existing systems," Miyagawa says, "like apples and oranges that just happened to be put together."

These kinds of adaptations of existing structures are common in natural history, notes Robert Berwick, a co-author of the paper, who is a professor of computational linguistics in MIT's Laboratory for Information and Decision Systems, in the Department of Electrical Engineering and Computer Science.

"When something new evolves, it is often built out of old parts," Berwick says. "We see this over and over again in evolution. Old structures can change just a little bit, and acquire radically（根本上） new functions."