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With its complex interweaving of symbols, structure, and meaning, human language stands apart from other forms of animal communication. But where did it come from? A new paper suggests that researchers look to bird songs and monkey calls to understand how human language might have evolved from simpler, preexisting abilities.

One reason that human language is so unique is that it has two layers, says Shigeru Miyagawa, a linguist at the Massachusetts Institute of Technology (MIT) in Cambridge. First, there are the words we use, which Miyagawa calls the lexical structure. "Mango," "Amanda," and "eat" are all components of the lexical structure. The rules governing how we put those words together make up the second layer, which Miyagawa calls the expression structure. Take these three sentences: "Amanda eats the mango," "Eat the mango, Amanda," and "Did Amanda eat the mango?" Their lexical structure—the words they use—is essentially identical. What gives the sentences different meanings is the variation in their expression structure, or the different ways those words fit together.

The more Miyagawa studied the distinction between lexical structure and expression structure, "the more I started to think, 'Gee, these two systems are really fundamentally different,' " he says. "They almost seem like two different systems that just happen to be put together," perhaps through evolution.

One preliminary test of his hypothesis, Miyagawa knew, would be to show that the two systems exist separately in nature. So he started studying the many ways that animals communicate, looking for examples of lexical or expressive structures.

Vervet monkeys (Chlorocebus pygerythrus), for example, use different alarm calls to refer to different types of predators, such as snakes and leopards. "That's already headed in the direction of the way in which we use different words," says co-author Robert Berwick, a computational linguist at MIT. Still, the monkeys always use the calls in the same context—to warn others about predators that currently pose a threat. They can't arrange the calls in new patterns to talk about predators they saw yesterday, predators they expect to see tomorrow, or the abstract idea of "predator." Vervet monkeys have a lexical structure but no expressive structure.

Songbirds, on the other hand, appear to communicate using only an expressive structure. Nightingales (Luscinia megarhynchos), for example, can manipulate the patterns of their songs to form up to 200 different melodies. But no individual note has any particular meaning, the way the words in a sentence do. What's more, an entire song always communicates the same message—the bird's identity, location, and sexual availability. Birdsong is what Berwick calls a "holistic signal." A song's structure may vary, but it "means the same thing every time," he says.

Because we see independent lexical and expressive structures in animals, it's possible that human language could have evolved through the combination of similar preexisting systems, Miyagawa, Berwick, and University of Tokyo biopsychologist and co-author Kazuo Okanoya report online this month in Frontiers in Psychology.

"Evolution can work in different ways, and one way would be that existing mechanisms are combined into something new," agrees Johan Bolhuis, a biologist at Utrecht University in the Netherlands who was not involved with the study. "That's something we see a lot in evolution, and it might have happened [with language] too." Still, he cautions, "there are lots of aspects [of the research] that are very speculative."

Tecumseh Fitch, an evolutionary biologist at the University of Vienna who was not involved in the research, particularly warns against concluding that birdsong is an "antecedent" of human language, because we know the two communication systems did not evolve from a common ancestor. The paper's basic idea, however, is "an interesting and plausible speculation. And now our job, as with all hypotheses, is to [find] the testable predications and go about testing them."

Posted in Brain & Behavior, Plants & Animals