R-complex

Reptilian complex or R-complex is a part of the triune brain model ('tri', as in 3 part) proposed by Paul D. MacLean. This theory seeks to explain brain function through the evolution of existing structures of the human brain. The triune brain consists of:
 * 1) The R-complex (also known as the "brainstem"),
 * 2) The Limbic system and
 * 3) The neo-cortex.

The theory, observable not only through fossil records and animal phylogeny, but notably during the stages of all mammalian and human prenatal development as well, explains that the evolution of the mammalian brain depended on and was enhanced in both its structure and function by a series of evolutionary plateaus. These evolutionary plateaus correspond closely with the phylogenic grouping of animals throughout the history of life on Earth.

Mammalian brain structure exists in the outer, most recently developed sphere of the brain, where evolution is newer and younger. These areas are the Limbic systems and neo-cortex. The Limbic system, which was first introduced by MacLean in a paper in 1952, is similar to the brain of the more primitive mammals and is the source of emotions, some aspects of personal identity, and some memory functions. The Limbic system is composed of the amygdala and the hippocampus. The neo-cortex, also known as the cerebral cortex, resembles the brain of more recent mammals in that it controls more highly evolved mentation such as reason and speech. Memory; the concepts of culture, art and literature; a prolonged childhood wherein learned behavior, vital to survival, is acquired along with generational recognition and care of family members are some of these more evolutionarily advanced brain activities with which those beings of the Mammalian Order have used to raise their individual species to such global prominence in so short a period of evolutionary time.

The brainstem and older, atavistic areas of the Central Nervous System control normal involuntary behavior that the conscious mind does not, such as cardiopulmonary function. These are found in all vertebrates. In between these two brains, lies the R-complex.

The R-complex is named for the most advanced part of the brain higher mammals share with reptiles. It is responsible for rage, xenophobia , basic survival fight-or-flight responses , territoriality and social hierarchy, along with the desire to submit to stronger (Alpha Type) members of one's own species. Often, the R-Complex can override the more rational function of the brain and result in unpredictable, primitive behavior in even the most sentient of creatures, humans included. A well developed and healthy neo-cortex can monitor R-Complex activity in sentient beings. The Reptilian complex is the most ancient part of a very successful brain scheme, evolutionarily speaking.