Studies of encephalization often focus on taxonomic differences in the size of whole brains or of forebrain areas. This does not necessarily mean that size is the key causal variable behind differences in cognitive performance. Many other features of nervous systems, e.g., synaptic networks, neuronal density, neurotransmitter facilitation, might be equally or more important. Comparative studies on brain size have often been justified by the ease with which broad data bases could be gathered on preserved brains (Portmann, 1946, 1947a, 1947b), endocasts (Mlikovski, 1989a, 1989b, 1989c, 1990; Iwaniuk and Nelson, 2002) and fossils (Burish et al., 2004; see The Hominin Fossil Record and the Emergence of the Modern Human Central Nervous System). However, recent molecular work suggests that size may indeed be one of the crucial aspects of adaptive evolution of brains. Dorus et al. (2004) examined 214 genes in humans, macaques, mice, and rats and found that those with the highest rates of evolution in primates determine brain size. Genes not involved with the nervous system or involved in physiological rather than developmental aspects of the brain showed similar evolutionary rates in rodents and primates. This suggests that size differences may be more than easy proxies for subtler differences in anatomy and function. Disregarding for the moment the major interclass transitions in the way the nervous system is organized, comparisons within taxa suggest that differences in cognitive performance show convergent co-evolution with differences in the size of association areas and of whole brains. Within the constraints of a highly conserved neuronal and synaptic machinery, building an intelligence might be similar to moving an animal through the air. In the same manner that locusts, bats, and albatrosses have evolved convergent, independent solutions to similar problems posed by gravity and lift, honeybees, cuttlefish, crows, and chimpanzees might also have evolved convergent solutions to the common problem of flexible processing and storage of information.
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