Philip Lieberman:

"Uniquely Human" (Harvard Univ Press, 1992)

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Human language is a relatively recent evolutionary innovation that came about when speech and syntax were added to older communication systems. The function of speech and syntax is to enhance the speed of communication: speech allows humans to overcome the limitations of the mammalian auditory system and syntax allows them to overcome the limits of memory.
Two principles are recalled. Natural selection acts on individuals who each vary: species that successfully change and adapt are able to maintain a stock of varied traits coded in the genes of the individuals who make up their population. The "mosaic" principle states that parts of the body of an organism are governed by independent genes. There are no central genes who control the overall assembly of the body.
Given these principles, two phenomena can be explained: a series of small, gradual changes in structure can lead to an abrupt change in the behavior of the organism; and an abrupt change in behavior may cause an abrupt change in morphology which causes the formation of a new species (at "functional branch-points").
The structure of the brain reflects its evolutionary history. The brain consists of a set of specialized circuits with independent evolutionary histories. Unlike modular theories such as Chomsky's and Fodor's, Lieberman's "circuit model" (derived from Norman Geschwind's connectionist model) assumes that the brain bases for language are mostly language-specific and mostly located in the newest part of the brain, the neocortex. The brain consists of many specialized units that work together in different circuits (the same unit can work in many circuits). The overall circuitry reflects the evolutionary history of the brain, with units that adapted to serve a different purpose from their original one. Therefore, rapid vocal communication is responsible for the evolution of the human brain.
The theory is supported by a wealth of anthropological and neurophysiological data (particularly from Broca's and Wernicke's experiments).
Besides language, the other unique trait of the human race (and therefore of the human brain) is moral code, in particular altruism. This is also a relatively recent development, and presupposes language and cognition.

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