Architectonics of the parietal and temporal association cortex in the strepsirhine primate Galago compared to the anthropoid primate Macaca.

A number of higher order association areas have been described in the parietal and temporal cortex of large-brained anthropoid primates such as Macaca. However, little is known about the evolution of these areas, and the existence of homologous areas has not yet been clearly demonstrated in other mammalian groups. We addressed this issue by comparing the myelo- and cytoarchitecture of posterior association cortex in the anthropoid Macaca to that of the small-brained, strepsirhine ("prosimian") primate Galago. Our results suggest that Galago possesses many, if not most, of the areas present in Macaca. We were able to identify regions in Galago which resemble Macaca posterior parietal area 7, superior temporal polysensory cortex (ST), inferotemporal visual cortex (IT), the temporoparietal auditory area (Tpt), and posterior parahippocampal cortex (areas TH and TF). Area 7, ST, and IT can each be subdivided further in Macaca, and for most of these subdivisions we were able to identify counterparts in Galago. However, we could not distinguish as many divisions of ST cortex in Galago as in Macaca, and it is possible that new areas arose in this region during anthropoid evolution. There also appear to be general differences in architectonic organization between these animals, with Macaca exhibiting greater development of pyramidal layer IIIc and of the internal granular layer (IV) across much of the parieto-temporal cortex. These findings suggest that many, although possibly not all, of the parietal and temporal association areas present in the modern anthropoid Macaca evolved early in primate history, prior to the divergence of the lineages leading to strepsirhines and anthropoids.