Two Hemispheres Serve Two Evolutionary Attentional Needs

The divided brain exists for Darwinian survival reasons because every creature must do two incompatible things at once: use a narrow, targeted, focal attention to manipulate the world (grab food, build a nest, catch prey) and simultaneously maintain a broad, sustained, vigilant attention to understand its surroundings and relationships; the left hemisphere is good at helping us manipulate the world but not understand it, while the right hemisphere sustains the broad vigilant attention and a continuous sense of being in the world.