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The brains of chimpanzees show a number of similarities to those of humans, the results of two new studies suggest. Findings published in the December issue of Behavioral Neuroscience indicate that the animals have differences between the right and left sides of their brains in much the same way that humans do. In addition, it appears that the neurological basis for handedness is not unique to our species.
Hani D. Freeman of the Yerkes National Primate Research Center and her colleagues scanned the brains of 60 chimpanzees with magnetic resonance imaging (MRI) and measured two key regions of the brain's limbic system, the hippocampus and the amygdala. They found that the hippocampus, important to learning and spatial memory among other things, was asymmetrical--the right half was significantly larger than the left. What is more, the difference was larger for males than it was for females. The amygdala, on the other hand, showed no difference in size between the left and right halves of the brain. Both of these patterns also characterize human brains. "The limbic system asymmetries advance the position that asymmetries are fundamental aspects of the nervous system of all primates, and apply to more primitive systems in the brain," remarks study co-author William Hopkins of Berry College and Yerkes National Primate Research Center.
Hopkins was also a co-author of a second paper that investigated hand preference in chimps. The researchers observed 66 animals and correlated brain anatomy with three measures of handedness: reaching, feeding and fishing peanut butter out of a tube. They found that the chimps' hand preference was related to asymmetries in two brain regions associated with motor tasks, the planum temporale and the precentral gyrus. The results, the authors write, "suggest that the neurobiological basis for handedness evolved as early as five million years ago and emerged independent of systems associated with language and speech."