It has been suggested that pressures inherent in the primate social environment have driven the elaboration of the human and nonhuman primate neocortex. Yet, functional studies of human and nonhuman primate behavior indicate that the neural systems subserving social behavior include both critical subcortical structures and specific neocortical areas. Unfortunately, a dearth of information exists for components of this functional system, making it difficult to address the evolutionary development of these structures which are so essential to human behavior. The proposed analysis specifically attempts to address this problem by providing information about an important subcortical structure, the amygdaloid complex. The amygdaloid complex in primates plays a key role in mediating social appraisal and interaction and is highly incorporated with cortical areas involved in social information processing. As such, the aims of this research also reflect a paradigm shift in the neurosciences which have come to stress the functional links between subcortical limbic structures and neocortex, underscoring the interdependence of emotion and higher order cognition. The proposed morphometric analysis will procure unbiased volumes and neuron counts for the amygdaloid complex and five of its constituent nuclei in all six hominoid species and three Old World monkey species. Because these five amygdaloid nuclei are differentially connected to brainstem, olfactory, and neocortical areas, they are expected to show differential expansion resulting in evolutionary reorganization of the amygdaloid complex. The intellectual merit of this project lay in its intended goal to increase understanding of the evolution of the human and ape brain, specifically for regions that may have adaptive significance. While the brain is an important locus for human adaptation, scant comparative neuroanatomical information is available for our closest living relatives, the five other species of hominoids (bonobos, chimpanzees, gorillas, orangutans, and gibbons). For example, previous analyses of the amygdaloid complex have only produced volumetric measures for only four individual apes. This dearth of information limits the testability of theories of brain evolution, including those focused on the interplay between social pressures and the evolution of cognition. The proposed analysis is well suited to address these problems given that the sample, which includes thirty-five hominoid specimens, is the largest to date. Furthermore, this project will broadly impact the scientific community, providing new data, student expertise, histological series, and laboratory facilities. It will contribute to the development of new wet lab facilities in the UCSD Anthropology Department, which will allow for the production of other invaluable histological series of ape brains. Experts from UCSD and the Salk Institute will teach the student researcher multiple staining techniques to process the brains of rare primate species donated by zoos and research institutes. Results of the morphometric analyses will be available in published form to other researchers, and the series produced from the analysis will also be available for use in other anatomical analyses. Overall, the results of this research are expected to have a clear, positive impact on biological anthropology and the neurosciences, as well as the greater scientific community.
