The Living Brain

Most of the evolutionary evidence on living brains is from anatomical and physiological studies of brain tracts and regions compared for unique and common features across species. There is growing interest in molecular evidence (e.g., on neurotransmitters), and one can anticipate increasing emphasis on that kind of information.

Braitenberg and Schiiz have published a straightforward anatomical monograph, noteworthy for the quantitative analyses of the cerebral cortex of the mouse. Though not specifically concerned with evolutionary issues, they provided exemplars of data necessary for an evolutionary analysis. The most striking facts are on the amount of information processing machinery in the mouse, with some suggestions on the human brain. There are about 40,000,000 neurons in the 0.5-g brain of a mouse; more astonishing, there are about 80,000,000,000 synapses in its neocortex. Taking into account the packing density of neurons and synapses, they reached the conclusion that a particular volume of cortex processes the same amount of information, whether it is in a mouse or a man. This is an outstanding uniformity for evolutionary analysis since it validates the use of brain size as a "statistic" to estimate the total information processing capacity of a brain.

Uniformity is balanced by diversity. All species differ in the details of the organization of the component systems of their brains. The raccoons and their relatives (family Procyonidae) provide an outstanding example reported by W.I. Welker. The fish-handling raccoon has a much enlarged forepaw projection area in its somatosensory neocortex, with separate representation in the brain for each of the pads on the forepaw. The coati mundi, kinkajou, and most other procyonids obtain this kind of information by nosing about, exploring their environment by touching things with the sensory skin around the nostrils. Their neocortical projections from that region are comparably expanded and their forepaw projection areas are much less extensive and not as differentiated as in the raccoon. The conclusion is inescapable that reorganization of the brain, like the differentiation of the behavior that it controls, occurred as part of the speciation of procyonids as they evolved, and that raccoons branched away from the main line by their specialized adaptations in their use of forepaws. Data like these can be used for formal cladistic analyses. The mammalian phylogeny constructed from brain features is essentially the same as that based on a more complete suite of traits.

I depend on the comparative quantitative data laboriously accumulated by Stephan and his colleagues on the volumes of many brain structures in "primitive" species represented by insectivores and their relatives and in "advanced" species (primates) for many of my analyses of allometry and encephalization. Theirs are the most complete data of this sort currently available. In their sample of 76 species, there were 26 from the order Insectivora (shrews, moles, and hedgehogs), 2 Macroscelididae (elephant shrews), 3 Scan-dentia (tree shrews), and 45 primates, of which 18 are from the suborder Prosimii (lemur-like species) and 27 from the suborder Anthropoidea (simian species, including humans). The brain structures are listed in Table I. These data are especially useful because of the large number of species that are in the sample and the good sample of brain structures on which measurements were taken.

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