Recent advances in the ability to investigate gene expression during brain development have revealed a number of ways in which the neocortex is patterned during development and suggested how these patterns may have been altered in the course of mammalian evolution. Accumulating evidence indicates that gradients of proteins produced from signaling centers provide a coordinate system for the position of cortical fields and incoming thala-mocortical afferents during development, and manipulation of the expression patterns of these molecules results in predictable alterations in the positions of entire cortical fields (Nakagawa et al., 1999; Bishop et al., 2000; O'Leary and Nakagawa, 2002; Fukuchi-Shimogori and Grove, 2003; Shimogori et al., 2004).
In a recent landmark study, Fukuchi-Shimogori and Grove (2001) were able to induce the development of a second, mirror-image representation of the mystacial vibrissae in mouse somatosensory cortex by generating an extra, caudally located signaling center producing the fibroblast growth factor FGF8. This result is of particular relevance for theories of brain evolution in mammals because the addition of a mirror-image cortical area, or map, adjacent to an existing sensory representation has clearly occurred repeatedly in the course of mammalian evolution as brains have become larger and more complex (Kaas, 1987,1993; Krubitzer, 1995). Thus, Fukuchi-Shimogori and Grove have been able to alter gene expression to produce a configuration of cortex that mimics a commonly observed product of brain evolution. Such studies suggest the genetic mechanisms by which relatively large-scale changes to sensory representations may have occurred.
However, my purpose is to emphasize a more gradual process of brain evolution that may often be a precursor to larger-scale changes requiring altered gene expression in the central nervous system (CNS). Here I suggest that many important evolutionary changes in brain organization occur simply by altering the development of the body. The well-documented, plastic nature of the developing nervous system could then accommodate new configurations of the sensory periphery through cascades of inductive events, largely independent of altered expression of patterning genes in the CNS. This idea is not new, or necessarily surprising. However, much attention is currently being focused on gene expression in the CNS and it seems important to consider simultaneously the sensory periphery - the interface between an animal and its environment.
My goal is briefly to summarize evidence from studies that support the instructive relationship between the sensory periphery and the CNS and to extend the discussion in some small but important ways. In particular, I will present evidence for variations in the sensory periphery that are paralleled in the brains of star-nosed moles. The mechanosensory star in this species and corresponding modular representation in cortex are the result of relatively recent selective pressure for elaboration of the somatosensory system, and the variations reported here were found in adult animals captured in the wild. These variations therefore represent the raw materials upon which natural selection is currently acting in these populations, and as such they are particularly relevant examples for the topic of brain evolution. I will also discuss evidence for another mechanism by which evolution may alter brain organization by acting in the periphery. Evidence from star-nosed moles suggests that the timing of developmental events in sensory sheets may have an important effect on the corresponding magnification of these areas in developing cortical maps. To lay the groundwork for this discussion, I first describe the whisker-barrel system in cortex of rodents and some of the variations observed in their sensory systems (see Mosaic Evolution of Brain Structure in Mammals, The Evolution of Neuron Classes in the Neocortex of Mammals, Reconstructing the Organization of Neocortex of the First Mammals and Subsequent Modifications, Encephalization: Comparative Studies of Brain Size and Structure Volume in Mammals, The Evolution of Crossed and Uncrossed Retinal Pathways in Mammals, Do All Mammals Have a Prefrontal Cortex?, The Evolution of the Basal Ganglia in Mammals and Other Vertebrates, The Evolution of the Dorsal Thalamus in Mammals).
somatosensory cortex tactile fovea
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