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To be more specific we can say that throughout its evolution, the human brain has acquired three components that progressively appeared and became superimposed, just like in an archeological site: the oldest, located underneath and to the back; the next one, resting on an intermediate position and the most recent, situated on top and to the front. They are, respectively: 1 - The archipallium or primitive (reptilian) brain, comprising the structures of the brain stem - medulla, pons, cerebellum, mesencephalon, the oldest basal nuclei - the globus pallidus and the olfactory bulbs. It corresponds to the reptile brain, also called "R-complex", by the famous neuroscientist Paul MacLean. 2 - The paleopallium or intermediate (old mammalian) brain, comprising the structures of the limbic-system. It corresponds to the brain of the inferior mammals. 3 - The neopallium, also known as the superior or rational (new mammalian) brain, comprises almost the whole of the hemispheres (made up of a more recent type of cortex, called neocortex) and some subcortical neuronal groups. It corresponds to the brain of the superior mammals, thus including the primates and, consequently, the human species. These three cerebral layers appeared, one after the other, during the development of the embryo and the fetus (ontogenesis), recapitulating, chronologically, the evolution of animal species (phylogenesis), from the lizards up to the homo sapiens. According to Maclean, they are three biological computers which, although interconnected, retained, each one, "their peculiar types of intelligence, subjectivity, sense of time and space, memory, mobility and other less specific functions". Actually, we have three cerebral units in a single brain. The primitive one is responsible for self preservation. It is there that the mechanisms of aggression and repetitive behavior are developed. It is there that occur the instinctive reactions of the so-called reflex arcs and the commands which allow some involuntary actions and the control of certain visceral functions (cardiac, pulmonary, intestinal, etc), indispensable to the preservation of life. The development of the olfactory bulbs and their connections made possible an accurate analysis of olfactory stimuli and the improvement of answers oriented by odors, such as approach, attack, flight and mating. Throughout evolution, some of these reptilian functions were lost or minimized (in humans, the amygdala and the entorhinal cortex are the only limbic structures that connect with the olfactory system). It is also in the R-complex that started the first manifestations of the phenomena of ritualism, by means of which the animal tries to define its hierarchic position inside the group and to establish its own space in the ecological niche.
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