Homo Grammaticus

Mathematics has a say about how human language evolved.

Whenever I tell my four-year-old a dream, he also tells me a dream. His is often similar to mine. He doesn't distinguish between a story and a dream. Both my four- and my six-year-old have their own fantasy realms. Sometimes, when a fact proves contrary to their expectation, they hold comfortably to their version of reality in a different world. Their language is limited neither to actual experience nor to the context of this world. We can talk about everything.

Producing the sounds we use in an ordinary conversation is an anatomical feat. The motions of various parts of our vocal tract are coordinated within millimeters and timed within hundredths of a second. On the receptive end, a listener must process a stream of sounds instantaneously. When it comes to words, a six-year-old has a lexicon, or word store, of about 13,000. The rate of word learning in humans comes to about one word every ninety waking minutes from age one to age seventeen. This leaves a seventeen-year-old with about 50,000 words stored in her mental lexicon. When it comes to grammar, a four-year-old knows how to avoid 95 percent of the mistakes he could make. Children acquire the grammatical rules of their native language spontaneously and without formal education. All they need is the opportunity to talk to someone and to hear examples of sentences.

I could prove to you mathematically that what children do in acquiring language is not possible unless we add a further assumption: children must have a built-in sense of what grammar is. The linguist Noam Chomsky called this innate mechanism universal grammar. It is written in our genes and generated by neuronal circuitry in our brain.

Grammar is the computational system of human language. As used by linguists, the term "grammar" encompasses the patterns inherent in speech sounds, in word forms, and in sentence structures (syntax). All human languages use complex grammar. Grammar is what enables us to produce an infinite number of meaningful sentences, and it is what allows a child to speak sentences he has never heard before. The computations that are necessary for formulating or interpreting sentences cannot, so far, be performed by any computer, but they flow through our brain's language processor without conscious effort on our part. We can talk and listen without thinking about it.

The aim of my own work on language is to outline the fundamental principles that determine how natural selection shaped animal communication and led from there to human language. The main forces of evolution--mutation and natural selection--can be described by precise mathematical equations. As early as 1906, Oxford zoologist Walter Weldon remarked that "Darwinian evolution is intrinsically a mathematical theory and can only be tested by mathematical and statistical techniques." Hence, I and my colleagues at the Institute for Advanced Studies in Princeton are using mathematics to find out how language evolved.

Language was the most important evolutionary event in the history of the human species. Indeed, grammatical language defines humanity. The complex vocalizations of mammals such as dolphins and primates have been the subject of many studies, but so far, no natural animal communication appears to have a power of expression that is in any way close to human language. Animal communication can be based on a limited repertoire of calls (for example, warning or territorial calls) or consist of variations on a theme (such as bird-songs) or be a continuous, analog signal (the honeybee's dance, which transmits information on food sources). But the grammar inherent in human language enables us, in the words of Wilhelm von Humboldt, to "make infinite use of finite means." Language has changed us and the appearance of the planet and is responsible for the acceleration of cultural evolution during the last few millennia.

Human language originated after our human ancestors diverged from our closest relatives, the chimpanzees, about 5 to 7 million years ago. Since all currently living Homo sapiens have the same language ability, the most recent date for the origin of language would be the time of our last common ancestors, who lived in Africa perhaps 150,000 to 200,000 years ago. Evolution would not have had enough time to build our language ability from scratch but must instead have used existing structures that may originally have been employed for other purposes. Neuroanatomists describe certain areas in the brains of monkeys, for instance, that correspond to the human language areas but that are apparently not involved in producing calls or gestures. Monkeys use these brain regions to interpret sounds and control facial muscles. Evolution may have had an easy game here in adopting these structures to generate the neuronal circuits that control speech production and speech interpretation in humans.

Language evolved as a means of communicating information between individuals. In order to communicate on a basic level, a population of individual animals or hominids must discover that specific signals can be associated with specific referents--things being referred to--such as people, objects, actions, places, times, and events. A wolf, for example, may whine, growl, or howl, and this sound (along with extensive body language) can convey certain information to the members of its pack. We can imagine early hominids--perhaps Lucy and her fellow Australopithecus afarensis, who lived 4 million years ago--being capable of making a variety of sounds and transferring information about their world. If a wolf cub fails to learn the sounds and signals of its society, its life may be short. Similarly, hominids that were best able to transmit--and to hear and interpret--specific signals presumably benefited from this trait. They were fitter in the evolutionary sense, surviving longer and having offspring that knew how to communicate.