Evening grosbeak: Its population has seen the second biggest drop among American birds.
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,Shigeru Miyagawa, a professor of linguistics in MITs Department of Linguistics and Philosophy, said.
The idea builds upon Miyagawas 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 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 Miyagawas framework, the authors said 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, like apples and oranges that just happened to be put together, Miyagawa said.
Birdsong lacks a lexical structure. Instead, birds sing learned melodies with what co author Robert Berwick calls a holistic structure; the entire song has one meaning, whether about mating, territory or other things.
By contrast, other types of animals have bare-bones modes of expression without the same melodic capacity. 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, according to Miyagawa, Berwick and another co-author Kazuo Okanoya, fruitfully combined these systems.
We 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.
For this reason, our finite vocabularies can generate a seemingly infinite string of words. Indeed, the researchers suggest that humans first had the ability to sing, as Darwin conjectured, and then managed to integrate specific lexical elements into those songs.
Its 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.
As they note in the paper, 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.
Another similarity, Berwick notes, relates to an insight of celebrated MIT professor emeritus of linguistics Morris Halle, who, as Berwick puts it, observed that ?all human languages have a finite number of stress patterns, a certain number of beat patterns. Well, in birdsong, there is also this limited number of beat patterns.
The findings are published in the journal Frontiers in Psychology.