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The active intelligence of today's primates flowered from trends that sculpted primate brain evolution across deep time: an increase in absolute brain size but a decrease in relative brain size (RBS, the ratio of brain size to body size) in bigger-bodied compared to smaller-bodied species (reflecting developmental scaling within species), increased RBS in highly "encephalized" species, and increased complexity of brain organization in conjunction with major adaptive shifts and selection for neurological specializations. Indices that quantify encephalization are discussed, as are developmental and physiological factors that constrain brain size. Data are provided which suggest that absolute brain size and RBS increased steadily rather than erratically during the last 3 Ma of hominin evolution, and the "received wisdom" that human frontal lobes are disproportionately enlarged is questioned. Despite the enormous importance attributed to the evolution of primate brain size, the conviction remains that size alone is not enough to account for the observed diversity in primate behavior and that circuitry, neurochemistry, and subsystems (modules) were reorganized within brains to accommodate evolving behavioral repertoires (such as those entailed in language). Arguments about the relative evolutionary merits of brain size versus neurological reorganization are reviewed and, to some extent, reconciled.

DOI: 10.1007/978-3-642-39979-4_37

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