Humans and chimpanzees share a common prehominid ancestor and, together with gorillas, constitute a group of closely related primates (Falk, 1987a; Ruvolo, 1997; Deinard & Kidd, 1999). Although they have similar body sizes, humans and chimpanzees differ completely in terms of encephalization. The human brain is indeed about three times larger than the chimpanzee's. This dramatic increase in brain size started more than 2 million years ago and characterizes the human lineage. In contrast to the generally acknowledged consensus that brain size relative to body size is a better measure of increased cognitive performance than is absolute brain size, it has recently been suggested that higher cognitive capacity is more closely related to absolute brain size and that absolute brain size more closely reflects the cognitive differences between humans, great apes, and monkeys than encephalization indices (Rumbaugh et al., 1996; Gibson et al., 1998; Gibson et al., Chapter 5, this volume).
Increased brain size is nevertheless also accompanied by decreased interhemispheric transfer speed and thus decreased cognitive processing speed. In order to maintain processing power, the number of elements clustered in one hemisphere is therefore expected to increase. This principle may be at the origin of hemispheric specialization (Ringo et al., 1994). More recently, Anderson (1999) showed by analytical means that if interneural conduction time increases proportionally with interneuronal distance, spatial clustering of interneuronal connections is the only way to increase the number of synaptic events occuring in a given period of time.