“The sounds uttered by birds offer in several respects the nearest analogy to language,” Charles Darwin wrote in The Descent of Man (1871). Now MIT researchers say Darwin was on the right path: the balance of evidence suggests that human language draws on both the elaborate songs of birds and the more utilitarian expression seen in other animals. “It’s this adventitious combination that triggered human language,” says linguistics professor Shigeru Miyagawa, coauthor of the team’s paper in Frontiers in Psychology.
The idea builds upon Miyagawa’s view (based on earlier work by linguists including Noam Chomsky, Kenneth Hale, and Samuel Jay Keyser) that all human languages have an “expression” layer, which involves the changeable organization of sentences, and a “lexical” layer, which relates to the core content of a sentence.
Take a sentence like “Todd saw a condor.” The expression layer is where elements can be rearranged to add complexity or ask questions: “When did Todd see a condor?” But the lexical layer retains the same core elements: the subject, “Todd,” the verb, “to see,” and the object, “condor.”
An analysis of animal communication, the authors say, suggests that birdsong resembles the expression layer; “holistic” melodies contain just one meaning, but some birds loop back to parts of previous melodies, allowing for greater variation. The communication systems of other animals—the simple sounds of nonhuman primates, the way bees waggle to tell each other about sources of food—are more like the lexical layer. At some point, between 50,000 and 80,000 years ago, humans may have merged these forms into a sophisticated type of language that lets us generate an infinite number of sentences.
Such adaptations are common in natural history, says Robert Berwick, a professor of computational linguistics and a coauthor of the paper.
“When something new evolves, it is often built out of old parts,” Berwick says. “We see this over and over again in evolution.” The researchers point out that birds and humans learn languages at the same phase of life, using the same part of the brain, and employ “a finite number of stress patterns” in speech.
“It’s just a hypothesis,” Berwick says, noting that the researchers would like to see further studies of bees, birds, and primates. “But it’s a way to make explicit what Darwin was talking about very vaguely, because we know more about language now.”