Brain Evolution

During the first few weeks of embryonic life, the bodies and central nervous systems of different vertebrates are remarkably similar. It is difficult to differentiate between the human embryo at the gestational age of one month and the embryo of an amphibian, fish, reptile, or rodent at a comparable stage. All have tails, primordial (primitive) gill clefts, and similar nervous systems, including primordial brains in the form of small bulges in the neural tube. The early embryos of these vertebrates most closely resemble, on a miniature scale, the adult fish, which is the oldest true vertebrate on the phylogenetic, or evolutionary, ladder. The developing human embryo, including the brain, passes through stages that, on a small scale, resemble the evolution of its ancestors. The embryological development of the human brain thus parallels the evolution of the brain. The small bulges in the primitive neutral tube gradually enlarge and form the anatomical divisions of the future adult brain, or encephalon. These division are the hindbrain, midbrain, and forebrain.

In Primitive animals, the forebrain lags in development, but certain parts of the brain that are essential to survival of the species develop much more than others. In fish and amphibians, the olfactory (smell) system, including the ofloctory cortex, the cortex being the outer layer of the brain, is particularly well developed.

In Reptile and birds, both evolutionary descendants of the amphibians, the corpus striatum (a mass of gray matter close to the thalamus) is well developed and serve as a coordination center for reflexes involving the eye and birds is extremely large compared to the rest of the brain, because it controls the motor coordination and balance necessary for flight. The visual systems of reptiles and birds are also well developed.

In the mammals, the olfactory cortex is part of a large neuronal system, the limbic system. The ancient olfactory system has been modified to serve not only smell but also the behavioral responses known as emotion and sexual behavior. Embryologically and according to evolutionary development, the mammalian brain gradually enlarges because of the expansion of the neocortex (composed of the frontal, parietal, occipital, and temporal lobes). The neocortex reaches its fullest development in the primates specifically in humans.