The silvery gibbon - an endangered primate found on an Indonesian island - sings long and complicated songs to invaders, mates, and family. Researchers now suggest that these apes may hold  clues to how language developed in humans.

"How did human language arise? It's far enough in the past that we can't just go back and figure it out directly," linguist Shigeru Miyagawa from the Massachusetts Institute of Technology (MIT) said in a statement. "The best we can do is come up with a theory that is broadly compatible with what we know about human language and other similar systems in nature."

According to Miyagawa and his colleagues, humans likely picked up a sense of melody from listening to bird songs. Content-carrying parts of language however, with complex meanings, was far more likely derived from other primates. Somehow, over the last 100,000 years, those learned qualities were combined and adapted by a young humanity.

In a paper co-authored by Miyagawa, MIT researchers analyzed functions in bird and gibbon song, comparing them to human speech patterns. They conclude that the human language is based around two layers - the "expressive layer" and the "lexical layer," that when combined, do away with the limitations that birds and gibbons face when trying to convey messages.

According to the study, the expressive layer consists of "the mutable structure of sentences," while the lexical layer is where the "core content" of a message resides. However, both are necessary to convey complex meaning, according to a MIT release.

The researchers also found that combined, these factors allow sentences to build in meaning rather than simply convey more meanings. In an over-simplified explanation: this allows for a message to become more detailed and complex about one thing, rather than just talking about several simple things.

"As we find more evidence that other animals don't seem to posses this kind of system, it bolsters our case for saying these two elements were brought together in humans," co-author Robert Berwick concludes.

The study was published in Frontiers in Psychology on June 9