Neocortex is an important part of the brain that varies in size from a small cap on the rest of the forebrain (Kaas, 2007) to approximately 80% of the brain in humans (Azevedo et al., 2009). The varied functions of neocortex depend on the cortical areas, the so-called “organs of the brain” (Brodmann, 1909) that are specialized for processing different inputs and providing different outputs. Cortical areas can be hard to define and identify, and their exact number in any species is uncertain. However, it is clear that the number of cortical areas varies across extant taxa, from approximately 20–30 or so to perhaps more than 200 in humans (Kaas and Preuss, 2008). Because the first mammals had little neocortex and likely few cortical areas, interest in the evolution of neocortex across the great radiation of mammals has largely focused on the issue of modifying and adding cortical areas. Some of the cortical areas proposed for primates are shown in Fig. 7.1. However, areas are often composed of smaller subdivisions, the cortical columns or modules, and these subdivisions within areas modify and expand the functions of areas. Thus, an understanding of how different types of neocortex evolved depends not only on determining the numbers and types of cortical areas that exist but also on the modifications of the internal organization of areas that occur in the various lines of evolution, including modifications in columnar organization. Here we review the types of columnar subdivisions of cortical areas that have been proposed (Hendrickson, 1985; Purves et al., 1992; Mountcastle, 1997; da Costa and Martin, 2010) and then consider how and when such modules might have evolved. The phyletic distributions of the types of columns in extant mammals allow one to infer when such columns evolved (Hennig, 1966; Striedter, 2005). Primates, rodents, tree shrews, and lagomorphs are all placed within the superorder Euarchontoglires. Thus, we are especially interested in how types of columns are distributed within the primate radiation, but also whether they are present in the closest relatives of primates. Because the shapes of columns are not always columnar, they also are called modules.
One of the defining features of neocortex is that it consists of layers and various sublayers of neurons specialized for different steps in processing; neurons in radial (vertical) arrays across the layers are more densely interconnected than neurons along the layers (Casagrande and Kaas, 1994; Nieuwenhuys, 1994b; Kaas, 2010). As a result, neurons in narrow vertical arrays share many response properties, especially the location of the receptor fields of neurons on the sensory receptor surface. This arrangement has great functional importance, and it is likely responsible for the impressive flexibility and powers of neocortex. Developmentally, minicolumns reflect