ABSTRACT
Apes trained in sign language, or using symbolic keyboards, often seem to demonstrate representational capacity, vocabulary, and elements of grammar. These are useful categories in initial considerations of language evolution.
Secondary representation is the ability to separate a primary representation (e.g., the perception of a banana) from its referent (an actual banana) for hypothetical purposes (using a banana as a symbolic telephone). Chimpanzees use secondary representations when visualizing a selected leafy twig as a stripped termite probe, or when cradling a log like a "baby". Monkeys lack this ability.
Ape language studies provide both a symbolic means of reference (signs or keyboard symbols), and instruction in comprehension. They have demonstrated acquired vocabularies of hundreds of signs or keyboard symbols. Combinations are often meaningless or repetitious, and acquisition is slow, but in many cases meaningful intent is clear. To many cognitive psychologists, these communications of meaning denote language.
To a linguist, however, language's hallmark is grammar (syntax). Here ape abilities fall short. Long combinations produce little evidence of consistent ordering. Two-sign combinations, however, do show some consistency. That suggests that natural ape gestures, often communicating an action and implying an actor, may have originated grammar.
Anatomical comparisons provide further insight. Our Broca's area was preceded by primate area F5, which is involved in sequential actions with grammar-like structure. Significantly, unlike other apes, both chimpanzee species frequently pair gestures with vocalizations, suggesting that early human language was both gestural and vocal.
Other observations indicate that language emerged from a 32-million-year background. There were then mirror cells, needed to copy gestures, and leftward asymmetry of both the temporal and inferior parietal lobes. By 17 million years ago, symbolic capacity and a simple "vocabulary" of natural communication gestures appeared. By 7.5 million years ago, gestures and vocalizations were co-produced.
Sometime after 6 million years ago, laryngeal control increased, and connections improved between Broca's area and the temporal lobe. By 2.4 million years ago well-defined gyri marked Broca's area, and inferior parietal cortex enlarged. About 1.9 million years ago the jaw began shortening, and the tongue reoriented. Crude, limited speech may have been produced and understood.
