THE BIOLOGY OF LANGUAGE ACQUISITION

Human language is made possible by special adaptations of the human mind and body that occurred in the course of human evolution, and which are put to use by children in acquiring their mother tongue.   

• Evolution of Language 

 Most obviously, the shape of the human vocal tract seems to have been modified in evolution for the demands of speech. Our larynxes are low in our throats, and our vocal tracts have a sharp right angle bend that creates two independently-modifiable resonant cavities (the mouth and the pharynx or throat) that defines a large two-dimensional range of vowel sounds (see the chapter by Liberman).

It is tempting to think that if language evolved by gradual Darwinian natural selection, we must be able to find some precursor of it in our closest relatives, the chimpanzees. In several famous and controversial demonstrations, chimpanzees have been taught some hand-signs based on American Sign Language, to manipulate colored switches or tokens, and to understand some spoken commands (Gardner & Gardner, 1969; Premack & Premack, 1983; Savage-Rumbaugh, 1991). Whether one wants to call their abilities "language" is not really a scientific question, but a matter of definition: how far we are willing to stretch the meaning of the word "language". 

The scientific question is whether the chimps' abilities are homologous to human language that is, whether the two systems show the same basic organization owing to descent from a single system in their common ancestor. For example, biologists don't debate whether the wing-like structures of gliding rodents may be called "genuine wings" or something else (a boring question of definitions). It's clear that these structures are not homologous to the wings of bats, because they have a fundamentally different anatomical plan, reflecting a different evolutionary history. Bats' wings are modifications of the hands of the common mammalian ancestor; flying squirrels' wings are modifications of its rib cage. The two structures are merely analogous: similar in function.

This lack of homology does not, by the way, cast doubt on a gradualistic Darwinian account of language evolution. Humans did not evolve directly from chimpanzees. Both derived from common ancestor, probably around 6-7 million years ago. This leaves about 300,000 generations in which language could have evolved gradually in the lineage leading to humans, after it split off from the lineage leading to chimpanzees. Presumably language evolved in the human lineage for two reasons: our ancestors developed technology and knowledge of the local environment in their lifetimes, and were involved in extensive reciprocal cooperation. This allowed them to benefit by sharing hard-won knowledge with their kin and exchanging it with their neighbors (Pinker & Bloom, 1990).

•  Dissociations between Language and General Intelligence

 Humans evolved brain circuitry, mostly in the left hemisphere surrounding the sylvian fissure, that appears to be designed for language, though how exactly their internal wiring gives rise to rules of language is unknown (see the Chapter by Zurif). The brain mechanisms underlying language are not just those allowing us to be smart in general. Strokes often leave adults with catastrophic losses in language (see the Chapter by Zurif, and Pinker, 1994a). By definition, Specifically Language Impaired people show such deficits despite the absence of cognitive problems like retardation, sensory problems like hearing loss, or social problems like autism. 

More interestingly, there are syndromes showing the opposite dissociation, where intact language coexists with severe retardation. These cases show that language development does not depend on fully functioning general intelligence. One example comes from children with Spina Bifida, a malformation of the vertebrae that leaves the spinal cord unprotected, often resulting in hydrocephalus, an increase in pressure in the cerebrospinal fluid filling the ventricles (large cavities) of the brain, distending the brain from within. Hydrocephalic children occasionally end up significantly retarded but can carry on long, articulate, and fully grammatical conversations, in which they earnestly recount vivid events that are, in fact, products of their imaginations (Cromer, 1992; Curtiss, 1989; Pinker, 1994a). Another example is Williams Syndrome, an inherited condition involving physical abnormalities, significant retardation (the average IQ is about 50), incompetence at simple everyday tasks (tying shoelaces, finding one's way, adding two numbers, and retrieving items from a cupboard), social warmth and gregariousness, and fluent, articulate language abilities (Bellugi, et al., 1990).

• Maturation of the Language System
  As the chapter by Newport and Gleitman suggests, the maturation of language circuits during a child's early years may be a driving force underlying the course of language acquisition (Pinker, 1994, Chapter 9; Bates, Thal, & Janowsky, 1992; Locke, 1992; Huttenlocher, 1990). Before birth, virtually all the neurons (nerve cells) are formed, and they migrate into their proper locations in the brain. But head size, brain weight, and thickness of the cerebral cortex (gray matter), where the synapses (junctions) subserving mental computation take place, continue to increase rapidly in the year after birth. Long-distance connections (white matter) are not complete until nine months, and they continue to grow their speed-inducing myelin insulation throughout childhood. Synapses continue to develop, peaking in number between nine months and two years (depending on the brain region), at which point the child has 50% more synapses than the adult. Many explanations have been advanced for children's superiority: they can exploit the special ways that their mothers talk them, they make errors unself-consciously, they are more motivated to communicate, they like to conform, they are not xenophobic or set in their ways, and they have no first language to interfere.

joesat ^_^ 

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