In the human brain the right hemisphere controls the left side of the body and the left hemisphere controls the right side of the body. What led to this development? Why doesn't the left side of the brain control the left side and vice-versa?
At a certain point in the phylogeny, decussation is favored for the optical pathways. Since the images are inverted on the retina, the crossing is necessary in order to build a coherent and continuous image on the cortex.
Decussation of the motor pathways appears next, and it is supposed to make the flight response more efficient. If a danger is perceived visually at the left hand side of one's body, it is processed in the right hemisphere and the motor signal is sent to the left hand side muscles, which are the one that can propel the body to the right, thus escaping the danger. The efficiency resides in that the interpretation of the danger and the signal for the reaction happen in the same hemisphere.