Box 4-1: Through a Glass, Darwinian
At birth the brain has approximately one hundred billion neurons with over 50 trillion synaptic connections. Approximately one half of the 80,000 different genes in the human gene known are involved in forming and maintaining the central nervous system. In the first few months of life the number of synaptic connections in the human brain increases to more than one thousand trillion. The brains capacity to make new synaptic connections steadily decreases overtime. Although some capacity to make new connections exists throughout life (otherwise we would not be capable of any new learning) many neural systems, especially language acquisition systems have lost most of their plasticity by the end of certain sensitive periods in childhood. For example, an infant’s auditory map for phoneme recognition appears to be almost fully completed by 12 months of age.
Human infants are capable of making a large number of categorical distinctions between phonemes during the first few months of life, in some cases from the age of one month, including distinctions not made in their native language (Jusczyc, 1997). From as early as the first month, neurons in the auditory cortex are generally sensitive to auditory stimulation and process this information according to innately determined schematic patterns. At four and one-half years of age infants have a detailed representation of the sound patterns of their own names and will consistently show a preference for their names over control words.
At six months of age, babies can respond to every phoneme uttered in languages as diverse as Hindi and Nthlakampx, a native American language with certain consonant combinations that are impossible to distinguish for nonnative speakers (Werker & Desjardins, 1995). Six moth old infants are starting to show a preference for words that have a prosodic organization typical of words in their native language (Jusczyc, 1997). At seven and one-half months of age, children have the capacity to detect the sound patterns of certain words in fluent speech contexts. By the time children are nine months old they show a preference for listening to their native language. By the time children are ten months old they are adept at making discriminations between phonemes in their native language and are starting to be insensitive the differences between the phonemes of foreign languages. In fact, at ten months of age, they have lost nearly two thirds of the capacity they possessed at six months (Werker & Desjardins, 1995).
As the child hears patterns of word sounds in his or her native language, certain clusters of neurons in the auditory cortex are recruited to respond to each phoneme(Jusczyc, 1997). Certain clusters only fire in response to a particular sound such as “ma”. If one sound is clearly distinct from another as “row” and “low” are in English then the neural clusters that identify one sound will lie far from those that identify the other sound. If the sounds are deemed by a particular language to be virtually identical as “row” and “low” are in Japanese then the two sets of neurons are so physically close that the infant will have trouble distinguishing the two phonemes. After a child’s auditory map is formed at 12 months he or she will be unable to pick up phonemes he has not heard thousands of times because no clusters of neurons have been assigned the task of responding to those particular sounds. In other words, they are functionally deaf to sounds not present in their native language. With increasing age, there are fewer and fewer uncommitted neurons available for responding to new phonemes. Consequently, with each passing year, learning a new language becomes more and more a daunting task.
The fact that the early neurological development of our auditory cortex selectively discards innate wide ranging sensitivity in favor of a much narrower spectrum of phonemic sensitivity may shed some light on language evolution. Why did we develop this added twist in neurological development when it is clear that we already had the genetic programs to create an auditory system capable of parsing speech sounds into useful perceptual categories? Why is valuable developmental energy expended to achieve the narrow, specialized capabilities of being attuned to one specific language or dialect? Why is there so much inherent plasticity in this type of neural organization?
The open-ended functioning of this system suggests that it has evolved to meet the exigencies of a rapidly changing auditory communication system. If auditory communication (proto-language/language) evolved simply to facilitate information exchange between group members the system would work more efficiently if phonemic/semantic variation were kept to a minimum. The pattern of neurological development that produces our language abilities strongly indicates that it is a system designed (evolved) to cope with continuously changing linguistic environments. A communication system where the symbols and their referents are continuously changing makes no sense in the context of normal environmental selective pressures. This pattern makes sense only if we evoke the other form of selective pressure that Darwin noted i.e., sexual selection. Specifically selection driven by female choice.
What sort of selection criteria might ancestral females have been using that specifically influenced language evolution? Based upon what is known about women’s current mate selection criteria, they were probably verbal ‘displays” that evidenced intelligence, creativity, kindness, devotion and commitment. A process of runaway selection for ever more creative displays may have kicked in at some point. Novel pronunciations for existing words would have been continuously introduced (look at how modern poets and singers take literary license change the pronunciation of words) as well as completely new words to label new concepts. Some theorists have suggested that these primordial courtship discourses were sung instead of recited. Thus explaining the basis of another mysterious human capacity—musical ability (see “Through a Glass, Darwinian” in Chapters 5 & 6).