Comparative analyses of the spatial organization of neurons, including their configuration as minicolumns, can increase our understanding of the functionally relevant aspects of microcircuitry. The bulk of this dissertation is comprised of two histological studies of the spatial organization of neurons in humans and apes. The third component is a literature review of prefrontal cortex development and comparative studies of the region in extant hominoids. Horizontal spacing distance (HSD) and gray level ratio (GLR) were measured in layer III of four regions of cortex in all extant hominoid species : frontal pole (Brodmann area 10), and primary motor (BA 4), primary somatosensory (BA 3), and primary visual cortex (BA 17). A uniquely human pattern in BA 10 was identified, in comparison bothwith BA 10 in the apes and with the other three regions examined. To identify when in development this human specialization emerges, I then measured the spatial organization of neurons in chimpanzees and humans ranging in age from birth to eleven years, using the same methodology. My findings support the pattern of later maturation of prefrontal cortex in humans, and further suggest that prolonged prefrontal development also characterizes chimpanzees. However, as my previous work suggested that adult humans have significantly greater spacing in the frontal pole than apes, this implies that HSD continues to increase in humans after puberty, but not in chimpanzees. As discussed in the literature review in Chapter 1, the prefrontal cortex forms a large part of a neural system crucial for normal social and executive functioning in humans and other primates. The region matures later in development than more caudal areas, and some of its subdivisions exhibit more complex dendritic arborizations, which is further supported by the two histological studies in the dissertation. As a whole, this dissertation argues that human brain evolution after the split from African apes was characterized by an increase in the number and width of minicolumns and the space available for interconnectivity between neurons in the prefrontal cortex, and that specific reorganizational events in neural circuitry may have taken place as a consequence of adjusting to increases in size or as adaptive responses to specific selection pressure
