Researchers from Massachusetts Institute of Technology, along with a scholar from the University of Tokyo, say that Darwin was right when he wrote in "The Descent of Man" (1871) contemplating how humans learnt to speak language, could have had its origins in singing.
The balance of evidence, researchers 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," co-author and linguistics professor in MIT's Department of Linguistics and Philosophy, Shigeru Miyagawa said.
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.
The study 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.
"When something new evolves, it is often built out of old parts," co-author of the paper Robert Berwick said. "We see this over and over again in evolution. Old structures can change just a little bit, and acquire radically new functions."
Birdsong lacks a lexical structure. Instead, birds sing learnt melodies with what Berwick calls a "holistic" structure; the entire song has one meaning, whether about mating, territory or other things.
While the Bengalese finch, can loop back to parts of previous melodies, bees communicate visually, using precise waggles to indicate sources of foods to their peers; other primates can make a range of sounds, comprising warnings about predators and other messages.
Humans can communicate essential information, like bees or primates ¿ but like birds, we also have a melodic capacity and an ability to recombine parts of our uttered language.
"It's not a very long step to say that what got joined together was the ability to construct these complex patterns, like a song, but with words," Berwick says.
The study notes some of the "striking parallels" between language acquisition in birds and humans include the phase of life when each is best at picking up languages, and the part of the brain used for language.
"Human language is not just freeform, but it is rule-based," Miyagawa says. "If we are right, human language has a very heavy constraint on what it can and cannot do, based on its antecedents in nature."
The paper "The Emergence of Hierarchical Structure in Human Language," was co-written by Miyagawa, Berwick and Kazuo Okanoya was published in the journal Frontiers in Psychology.