Throughout its evolution, the human brain has acquired three components that progressively appeared and became superposed, 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

In 1878, the French neurologist Paul Broca called attention to the fact that, on the medial surface of the mammalian brain, right underneath the cortex, there exits an area containing several nuclei of gray matter (neurons) which he denominated limbic lobe (from the Latin word "limbus" that implies the idea of circle, ring, surrounding, etc) since it forms a kind of border around the brain stem ( in another part of this text we shall write more about these nuclei).

The entirety of these structures, that, years later would receive the name of "limbic system", developed with the emergence of the inferior ( primitive) mammals. This system commands certain behaviors that are necessary for the survival of all mammals. It gives rise and modulates specific functions that allow the animal to distinguish between the agreeable and the disagreeable. Here specific affective functions are developed, such as the one that induces the females to nurse and protect their toddlers, or the one which induces these animals to develop ludic behaviors (playful moods). Emotions and feelings, like wrath, fright, passion, love, hate, joy and sadness, are mammalian inventions, originated in the limbic system. This system is also responsible for some aspects of personal identity and for important functions related to memory. And, when the superior mammals arrived on the Earth, the third cerebral unit was finally developed : the neopallium or rational brain, a highly complex net of neural cells capable of producing a symbolic language, thus enabling man to exercise skillful intellectual tasks such as reading, writing and performing mathematical calculations. The neopallium is the great generator of ideas or, as expressed by Paul MacLean, "it is the mother of invention and the father of abstractive thought".