Why do we speak with the left hemisphere?
Human language tends to be associated with circuitry in the left cerebral hemisphere, regardless of individual hand dominance. This may have resulted from the coevolution of language and dexterous manipulation, specifically the use of the dominant hand to direct forces and point to objects in the environment. Asymmetric manipulation of physical objects reflects a fundamental asymmetry of perceptual-motor brain circuitry, which in turn results from the nature of the mechanical interaction between the organism and its environment. The natural selection of more effective manipulative ability, especially in the form of tool use, strengthened the fundamental organismal asymmetry, leading to distinct handedness and hemispheric dominance for manipulation and language in humans. The related subject of ocular dominance is also discussed.