I am curious to find out why the 2 parts of the brain control the opposite sides of the body. This is the best I’ve found so far: “For some as-yet-unknown reason, nearly all of the signals from the brain to the body and vice-versa cross over on their way to and from the brain. This means that the right cerebral hemisphere primarily controls the left side of the body and the left hemisphere primarily controls the right side. When one side of the brain is damaged, the opposite side of the body is affected. For example, a stroke in the right hemisphere of the brain can leave the left arm and leg paralyzed.“ ( from http://www.ninds.nih.gov/disorders/brain_basics/know_your_brain.htm) Does anyone know how far back this goes? I mean, did it get started with the most primitive vertebrate?