The change of the brain as an animal’s behavior control system in the course of evolution is discussed. An animal’s behavior consists from the successive stages: the choice of the goal, the movement, the reaction of environment and so on. The starting point of the evolution of the brain was the emergence of multicellularity, which is considered as the union of unicellular organisms with different ways of perceiving external influences and different ways of responding to them. Evolution has tried different combinations of unicellular organisms, from which various existing animal species have evolved. Based on the structure of the brain of modern vertebrates and hypotheses about the evolution of behavior, it can be assumed that they had two ancestors, one whose leading sensation was mechanoreception, through which movement was assessed as dangerous or neutral; and the second, with a leading chemical reception, on the basis of which a goal selection system developed. It is assumed that in behavior choice the brain use the principles of Reinforcement Learning theory. An approximate scheme of evolution is constructed up to the appearance of the basal brain.