How human language could have evolved from birdsong

''The sounds uttered by birds offer in several respects the nearest analogy 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 songs of birds, and second, the more utilitarian, 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, The emergence of hierarchical structure in human language, published in the journal Frontiers in Psychology.

The new paper, was co-written by Miyagawa, Berwick and Kazuo Okanoya, a biopsychologist at the University of Tokyo who is an expert on animal communication.

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 organisation of sentences, and a ''lexical'' layer, which relates to the core content of a sentence.