Cerebral Cortex Advance Access published online on July 20, 2009
Cerebral Cortex, doi:10.1093/cercor/bhp138
Broca's Area Homologue in Chimpanzees (Pan troglodytes): Probabilistic Mapping, Asymmetry, and Comparison to Humans
1 Department of Anthropology, The George Washington University, Washington, DC 20052, USA, 2 Department of Psychology, Agnes Scott College, Decatur, GA 30030, USA, 3 Division of Psychobiology, Yerkes National Primate Research Center, Atlanta, GA 30322, USA, 4 Division of Biomedical Sciences and Pathobiology, Virginia-Maryland Regional College of Veterinary Medicine at Virginia Polytechnic Institute and State University, Blacksburg, VA 24036, USA, 5 Foundation for Comparative and Conservation Biology, Needmore, PA 17238, USA, 6 Department of Neuroscience, Mount Sinai School of Medicine, New York, NY 10029, USA, 7 New York Consortium in Evolutionary Primatology, New York, NY, USA
Address correspondence to Chet C. Sherwood, Department of Anthropology, The George Washington University, 2110 G Street NW, Washington, DC 20052, USA. Email: sherwood{at}gwu.edu.
Neural changes that occurred during human evolution to support language are poorly understood. As a basis of comparison to humans, we used design-based stereological methods to estimate volumes, total neuron numbers, and neuron densities in Brodmann's areas 44 and 45 in both cerebral hemispheres of 12 chimpanzees (Pan troglodytes), one of our species’ closest living relatives. We found that the degree of interindividual variation in the topographic location and quantitative cytoarchitecture of areas 44 and 45 in chimpanzees was comparable to that seen in humans from previous studies. However, in contrast to the documented asymmetries in humans, we did not find significant population-level hemispheric asymmetry for any measures of areas 44 and 45 in chimpanzees. Furthermore, there was no relationship between asymmetries of stereological data and magnetic resonance imaging–based measures of inferior frontal gyrus morphology or hand preference on 2 different behavioral tasks. These findings suggest that Broca's area in the left hemisphere expanded in relative size during human evolution, possibly as an adaptation for our species’ language abilities.
Key Words: cytoarchitecture • evolution • great ape • handedness • stereology
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