All vertebrate brains have the same general structure, with major modifications corresponding to life-style and intelligence. Embryologically, the vertebrate brain begins as a simple tube, which soon develops into three parts: the hindbrain, midbrain, and forebrain. It is believed that in the earliest vertebrates, these three anatomical divisions were also functional divisions: the hindbrain governed automatic behaviors such as breathing and health rate, the midbrain controlled vision, and the fore brain dealt largely with sense of smell. In nonmammalian vertebrates, these three divisions remained prominent. However, in mammals, and particularly in humans, the brain region is significantly modified. Some have been reduced in size, and others, especially the forebrain, greatly enlarged.