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).