The human brain is a bizarre device, engendered by natural selection for one main purpose: to make decisions that enhance reproductive success. This simple point is at the heart of the complicated lessons of evolutionary biology; it enables the brain scientist to grasp a central phenomenon of human brain function: lateral cerebral specialization -- the ability of the left and right hemispheres of the brain to handle different tasks. Nowhere else in the animal kingdom is there such extensive specialization of function. Why is this? How did it come about?

Many insights to these questions come from split-brain research. It may be that the often underappreciated corpus callosum, a fiber that merely exchanges information between the two hemispheres, is the prime enabler for the human condition. Because the callosum exchanges information between the two hemispheres, it allows the brain to be more efficient. The two hemispheres can develop their own specialized functions, yet still take advantage of each other's benefits through the callosum. Mutational events, such as changing local cortical connectivity of neurons, can happen in one lateralized cortical area, with the homologous area on the other side of the brain mutation-free. As a consequence, the mutation-free side can continue to supply its cortical function to the organism's overall cognitive system. The callosum allows for a costless extension of the cortex's capacity by reducing redundancy and creating new cortical zones in which mutations build new functions.

This proposal, which comes out of work that Paul Corballis, Margaret Funnell, and I are doing at Dartmouth College, builds on a large backdrop of new discoveries in cognitive neuroscience. Over the past few years, one of the dominant facts that has emerged from split-brain research is that the left hemisphere has marked limitations in perceptual functions and that the right hemisphere has even more prominent limitations in its cognitive functions. Our model proposes that lateral specialization reflects the emergence of new skills and the retention of others. Natural selection allowed this unique state of affairs because the callosum kept these developments integrated into a constant functional system, one that was continuously improving as a decision-making device.

This new way of thinking about the role of the callosum could lead the way to understanding how new functions -- exclusively human in nature -- arise during cortical evolution. Giacomo Rizzolatti at the University of Parma has shown that neurons in a monkey's prefrontal lobe respond not only when the animal is poised to grasp a piece of food, but also when the human experimenter is about to grasp the same piece of food.

Rizzolatti and his colleagues have questioned whether such a system might be the seed for the uniquely human capacity to have a theory of mind about another human, or to put it differently, the sophisticated ability to imagine someone else's state of mind. In this context, we gain the sense that in developmental and evolutionary time a dynamic cortical system establishes the adaptations that become laterally specialized. The structure allowing for this expanded capacity within a limited cortical space is, in fact, the corpus callosum.

Michael Gazzaniga is Director of the Center for Cognitive Neuroscience at Dartmouth College.