Frontal Cortex Evolution in Primates

The frontal cortex can be defined as the neocortex anterior to the motor somatosensory–cortex border. This is a large region in primates, containing areas involved directly or indirectly in the control of almost every behavior. It has long been thought that the frontal cortex played an important role in primate evolution. Modern evidence supports this view. Indeed, given the diversity of functions in the region and the variety of unique behaviors exhibited by primates, it would be surprising if the frontal cortex had not evolved unique adaptations. The common ancestor of eutherian mammals probably had a small body and a small brain. Comparative work in modern mammals suggests it had a basic complement of cortical areas including primary visual, auditory, and somatosensory areas. It also probably had a primary motor area (M1). This means that we could define a frontal cortex in that mammal and that the region of the frontal cortex in modern eutherians (taken as a whole) can be thought of as being homologous. It is likely however that this broad homology obscures substantial differences in frontal cortex structure between primates and nonprimates. One piece of evidence to this effect is that the frontal cortex scales differently in primates and nonprimates. In primates, the frontal cortex hyperscales with the brain size. This can be seen in the reconstructions in Figure 1 which show the brain of a small primate, the galago, and a larger primate, the macaque. A primate with a larger brain tends to have a disproportionately large frontal cortex. In contrast, in a nonprimate order, car-nivores, the frontal cortex size does not vary systematically with brain size. This suggests that the structure and development of the frontal cortex differs substantially in the two orders. When we focus on specific cortical areas and regions, we again find that the primate frontal cortex differs in important ways from that found in other orders. A variety of evidence suggests that the two main branches of primates, strepsirrhines and anthropoids, share up to ten motor areas in the frontal cortex. Of these, only one or two have clear homologs outside primates. Most eutherians have an agranular M1, with large layer 5 pyramidal cells, somatotopy, and relatively low thresholds of stimulation. Rostral and medial to M1 is a supplementary motor area (SMA, also called M2) in primates. It is also somatotopically organized, and has slightly smaller layer 5 pyramidals than M1. …