Brain Evolution in Vertebrates

In those groups with enlarged and elaborated brains, the more dorsal (alar plate-derived) parts of the brain tend to show more variation than the more ventral (basal plate-derived) parts. Structural elaboration is often correlated with a major exploitation of a particular sensory aspect of the world or with the gain of complex behaviors (see Butler & Hodos, 1996, and references therein). For example, mormyrid fishes utilize an expanded cerebellum and lateral line lobe, which are alar plate-derived, in their complex electrosensory communication system for individual recognition, nest building, and care of their young. Many tropical reef fish have greatly enlarged forebrains and complex territorial, courtship, and parental behaviors. Some cartilaginous fishes also have substantially enlarged fore-brains used in complex sensory processing.

Within the brain stem across amniotes (reptiles, birds, and mammals), similarities exist for many of the nuclei, but the alar plate-derived, sensory part of the trigeminal nerve is very versatile. It generally supplies touch, position sense, pain, and temperature for the face but also innervates mechanosensory and electrosensory receptors in platypuses, infrared receptors in some snakes, and magnetic-sensitive receptors in birds. The cerebellum likewise varies markedly. In primates the neocerebellar hemispheres are greatly expanded for control of limb movements as well as some aspects of sensory processing. The midbrain roof, or tectum, is also highly variable. Its rostral part, the superior colliculus, is involved in visual localization functions. Of modest size in mammals, it reaches its apogee in birds. The caudal tectum, the inferior colliculus, processes auditory stimuli and is elaborately developed in bats as part of their echolocation sonar system and in birds, such as owls, that hunt in darkness and localize their prey by sound.

Among amniotes, major differences occur in forebrain structure. In mammals, the elaborately layered neocortex (Bock & Cardew, 1999) receives sensory input relayed from dorsal thalamic nuclei, whereas in reptiles and birds, some of the telencephalic cell populations that receive thalamic input are organized as nuclei rather than in layers. Whether these nuclei are equivalent (homologous) to neocortex is an unresolved question (Karten, 1991; Northcutt & Kaas, 1995; Butler & Hodos, 1996; Puelles et al., 2000). All modern mammals are derived from an ancestral stock with somatomotor, auditory, and visual cortical regions occupying similar relative positions on the cerebral hemispheres. Within various orders of mammals, the number of cortical sensory areas has independently increased, and each area has become dedicated to the analysis of specific aspects of the sensory input (Bock & Cardew, 1999). Many primates, for example, have over 20 visual cortical areas that each analyze different aspects and combinations of the visual input. Bats have specialized auditory cortical areas for analyzing the Doppler shift in constant frequency to determine prey velocity and for analyzing frequency modulated sounds with time delay to determine range. Some mammals with prominent whiskers, such as rodents, have specialized, cylindrically shaped regions in the somatosen-sory cortex called barrels that each receive the input from a single whisker.

Neocortex in humans has few truly unique features vis a vis other primate brains. The volume of neocortex relative to the total volume of the brain is only what one would expect for a generalized primate (Passingham, 1979). Language was arguably the most important evolutionary gain for our species (Deacon, 1997), but even here, the parts of the brain used for language comprehension and motor speech have precedent areas in other primates. It is possible that small differences in the volume of cortex in a given region allow for dramatic differences in function. Current research includes new insights gleaned from comparative embryological studies, indicating that small changes in the genome and in the complex developmental program can have profound effects on the phenotype. Some of the most difficult persistent questions concern the complex relationships between cytoarchitecture and function.

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