The Master and His Emissary: The Divided Brain and the Making of the Western World

by Iain McGilchrist

You might think that as brains evolve to become larger, the interhemispheric connections would increase in tandem. But not at all: they actually decrease relative to brain size.9 The bigger the brain, the less interconnected it is. Rather than taking the opportunity to increase connectedness, evolution appears to be moving in the opposite direction. And there is a close relationship between the separation of the hemispheres on the one hand and the development of something that keeps cropping up in this unfolding story: the asymmetry of the hemispheres. Because it turns out that the greater the brain asymmetry, too, the smaller the corpus callosum, suggesting that the evolution both of brain size and of hemisphere asymmetry went hand in hand with a reduction in interhemispheric connectivity.10 And, in the ultimate case of the modern human brain, its twin hemispheres have been characterised as two autonomous systems.