General Considerations

This article emphasizes inferences from allometric analysis of brain/body relations and encephalization, the latter being a complex trait often attributable to convergent evolution. Although the diversity in organization of brains is at least as important, especially for understanding the phylogenetic trees, an adequate discussion of the evolution of diversified brain organization requires a more detailed review of comparative anatomy and physiology than is possible in a single article, and the conclusions, though important, are easily summarized for evolutionary neurobiology: Brain structure is appropriate to function, and specialized functions are appropriate to the environment (i.e., structure and function are adaptive). In short, the results are consistent with adaptation as a biological principle. Applied to the sizes (weight, volume, or surface area) of the subsystems in the brain, such as cortical projection areas and thalamic nuclei, this is the principle of proper mass.

Despite their simplicity, allometry and encephalization provide more unusual evolutionary insights. Allometry helps us understand the biological role of size; encephalization does the same for understanding neural information-processing capacity and its evolution. It will be enough to review the diversity of

Encyclopedia of the Human Brain Volume 2

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organization by citing a few examples, the reports of which are extremely well documented.

The issues considered in this article are also relevant for the evolution of invertebrate nervous systems. The neuron, for example, probably appeared as a specialized cell early in metazoan evolution, more than 600 million years ago (Ma), and many of its features are identical in all instances in which it functions in a synaptic nervous system. This is evidently true for small networks of neurons as well as for isolated cells. Much of what is known about neural functions as single units and in small networks was learned from giant neurons of horseshoe crabs and from networks of cells in sea slugs and roundworms. The early appearance of the adaptation is deduced from a cladistic analysis of the time of divergence of species in which it is identifiable. It is most likely that the adaptation first appeared in a pre-Cambrian metazoan species that is the ''common ancestor'' of all the living species that share the adaptation. From an evolutionist's perspective, however, very complex behavior requiring integrated neural activity and involving more extensive neural circuitry that is common to vertebrates and invertebrates is as likely to be analogous (''homoplas-tic'') rather than homologous. It may have evolved in independent evolutionary paths in the vertebrate and invertebrate groups in which it occurs.

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