As neurologist Oliver Sacks described him in Seeing Voices: Joseph saw, distinguished, categorized, used; he had no problems with perceptual categorization or generalization, but he could not, it seemed, go much beyond this, hold abstract ideas in mind, reect, play, plan. He seemed completely literal--unable to juggle images or hypotheses or possibilities, unable to enter an imaginative or figurative realm.... He seemed, like an animal, or an infant, to be stuck in the present, to be confined to literal and immediate perception, though made aware of this by a consciousness that no infant could have.
To understand why humans are so intelligent, we need to understand how our ancestors remodeled the apes symbolic repertoire and enhanced it by inventing syntax. Wild chimpanzees use about three dozen different vocalizations to convey about three dozen different meanings. They may repeat a sound to intensify its meaning, but they do not string together three sounds to add a new word to their vocabulary.
Speakers of English also use about three dozen vocalizations, called phonemes. Yet only their combinations have content: we string together meaningless sounds to make meaningful words. No one has yet explained how our ancestors got over the hump of replacing one sound/one meaning with a sequential combinatorial system of meaningless phonemes, but it is probably one of the most important advances that took place during ape-to-human evolution.
Furthermore, human language uses strings of strings, such as the word phrases that make up this sentence. The simplest ways of generating short sentences, as in pidgins and the utterances of a two-year-old, are known as protolanguage. In a protolanguage, the association of the words carries the message. Syntax is not needed if the sentences are short.
Our closest animal cousins, the common chimpanzee and the bonobo (pygmy chimpanzee), can achieve surprising levels of language comprehension when motivated by skilled teachers. Kanzi, the most accomplished bonobo, can interpret sentences he has never heard before, such as Go to the office and bring back the red ball, about as well as a 2.5-year-old child. Neither Kanzi nor the child constructs such sentences independently, but each can demonstrate understanding.
With a years experience in comprehension, the child starts constructing fancier sentences. The rhyme about the house that Jack built (This is the farmer sowing the corn /That kept the cock that crowed in the morn /... That lay in the house that Jack built) is an extreme case of nesting word phrases inside one another, yet even preschoolers understand how that changes its meaning.
Syntax has treelike rules of reference that enable us to communicate quickly--sometimes with fewer than 100 sounds strung together--who did what to whom, where, when, why and how. Generating and speaking a long, unique sentence demonstrate whether you know the rules of syntax well enough to avoid ambiguities. Even children of low intelligence acquire syntax effortlessly by listening, although intelligent deaf children like Joseph may miss out.
Something close to verbal syntax also seems to contribute to another outstanding feature of human intelligence, the ability to plan. Aside from hormonally triggered preparations for winter and mating, animals exhibit surprisingly little evidence of planning more than a few minutes ahead. Some chimpanzees use long twigs to pull termites from their nests, yet as author Jacob Bronowski observed, none of the termite-fishing chimps spends the evening going round and tearing off a nice tidy supply of a dozen probes for tomorrow.
SHORT-TERM PLANNING does occur to an extent, and it seems to allow a crucial increment in social intelligence. Deception is seen in apes but seldom in monkeys. A chimp may give a call signaling that she has found food at one location, then quietly circle back through the dense forest to where she actually found the food. While the other chimps beat the bushes at the site of the food cry, she eats without sharing.