The triune brain is a model of the evolution of the vertebrate forebrain and behavior proposed by the American physician and neuroscientist Paul D. MacLean. MacLean originally formulated his model in the 1960’s and propounded it at length in his 1990 book The Triune Brain in Evolution. The triune brain consists of the reptilian complex, the paleomammalian complex (limbic system), and the neomammalian complex (neocortex), viewed as structures sequentially added to the forebrain in the course of evolution. The triune brain hypothesis became familiar to a broad popular audience through Carl Sagan’s Pulitzer prize winning 1977 book The Dragons of Eden. Though embraced by some psychiatrists and at least one leading affective neuroscience researcher, the model never won wide acceptance among comparative neurobiologists. Comparative evolutionary neuroanatomists currently regard its claims about brain evolution to be outdated
The reptilian complex
The reptilian complex, also known as the R-complex or “reptilian brain” was the name MacLean gave to the basal ganglia, structures derived from the floor of the forebrain during development. The term derives from the fact that comparative neuroanatomists once believed that the forebrains of reptiles and birds were dominated by these structures. MacLean contended that the reptilian complex was responsible for species typical instinctual behaviors involved in aggression, dominance, territoriality, and ritual displays.
The paleomammalian complex
The paleomammalian brain consists of the septum, amygdala, hypothalamus, hippocampal complex, and cingulate cortex. MacLean first introduced the term “limbic system” to refer to this set of interconnected brain structures in a paper in 1952. Whatever the merits of the triune brain hypothesis, MacLean’s recognition of the limbic system as a major functional system in the brain has won wide acceptance among neuroscientists, and is generally regarded as his most important contribution to the field. MacLean maintained that the structures of the limbic system arose early in mammalian evolution (hence “paleomammalian”) and were responsible for the motivation and emotion involved in feeding, reproductive behavior, and parental behavior.
The neomammalian complex
Consists of the cerebral neocortex, a structure found uniquely in mammals. MacLean regarded its addition as the most recent step in the evolution of the human brain, conferring the ability for language, abstraction, planning, and perception.
Current status of the model
MacLean originally formulated the triune brain hypothesis in the 1960’s, drawing on comparative neuroanatomical work done by Elizabeth Crosby and C. J. Herrick early in the twentieth century. The 1980’s saw a rebirth of interest in comparative neuroanatomy, motivated in part by the availability of a variety of new neuroanatomical techniques for charting the circuitry of animal brains. Subsequent findings have invalidated the traditional neuroanatomical ideas upon which MacLean based his hypothesis. The basal ganglia, structures derived from the floor of the forebrain and making up MacLean’s reptilian complex, were shown to take up a much smaller portion of the forebrains of reptiles and birds (together called sauropsids) than previously supposed, and to exist in amphibians and fishes as well as mammals and sauropsids. Because the basal ganglia are found in the forebrains of all modern vertebrates, they most likely date to the common evolutionary ancestor of the vertebrates, more than 500 million years ago, rather than to the origin of reptiles. Sauropsids were shown to possess forebrain roof structures similar in connectivity and function to the cerebral cortex (MacLean’s neomammalian complex) in mammals. Because these structures look different from the corresponding forebrain roof structures in mammals, they were originally mistaken for a part of the basal ganglia. They apparently look different because they have been shaped by more than 300 million years of independent evolution that have followed the most recent common ancestor of sauropsids and mammals. Recent behavioral studies likewise do not support the traditional view of sauropsid behavior as stereotyped and ritualistic (as in MacLean’s reptilian complex). Birds have been shown to possess highly sophisticated cognitive abilities, such as the toolmaking of the New Caledonian crow and the language-like categorization abilities of the African Gray Parrot. Structures of the limbic system, which MacLean contended arose in early mammals, have now been shown to exist across the whole range of modern vertebrates, and thus, like the basal ganglia presumably date to the common vertebrate ancestor. The “paleomammalian” trait of parental care of offspring is widespread in birds and occurs in some fishes as well.
Continuing popular interest in the model
While of little current interest to those evolutionary neuroscientists who are focused on exploring the networked, complex interconnection of brain systems, the triune model continues to hold interest for some psychologists and members of the general public because of its focus on the recognizable differences between most reptiles, early mammals, and late mammals. Its explanatory as well as metaphoric power is seen by such persons as broadly representing three major evolutionary periods in the development of the brain that are characterized by three recognizably distinct ways of solving adaptive challenges. Under this model, the “neocortex” represents that cluster of brain structures involved in advanced cognition, including planning, modeling and simulation; the “reptilian brain” refers to those brain structures related to territoriality, ritual behavior and other “reptile” behaviors; and “limbic brain” refers those brain structures, wherever located, associated with social and nurturing behaviors, mutual reciprocity, and other behaviors and affects that arose during the age of the mammals. Or to put it in whimsical and grossly oversimplified terms: reptiles eat their young; mammals feed their young; and creatures with large neocortices take their young on vacations to Disneyworld that they have planned a year in advance.
The value of the model can also be seen in the way it parallels recurring themes in popular culture and the arts. For example, it’s not uncommon to hear people speak of someone coming from their “head,” heart,” or “gut”, or philosophically of the three virtues of “wisdom, benevolence and courage”–or psychologically of “thinking,” “feeling,” and “willing.” In the powerful modern myth The Wizard of Oz the quest for “a brain,” “a heart,” and “courage” play a central role. The three elements of the triune brain map comfortably onto these more abstract conceptions in ways that more complex and networked brain models, even if neuroanatomically more precise, would fail to do. In this sense the triune brain can be seen as a powerful organizing theme that is grounded in the three largely distinct evolutionary layers of the human brain. As the statistician George E.P. Box once quipped: “Essentially, all models are wrong, but some are useful.”
It has been suggested that the grouping of personality traits into three separate and distinct centers in the Enneagram of Personality may be rooted in the three “brains” described in the triune brain model.
Paul D. MacLean (May 1, 1913 – December 26, 2007) was an American physician and neuroscientist who made significant contributions in the fields of physiology, psychiatry, and brain research through his work at Yale Medical School and the National Institute of Mental Health. MacLean’s evolutionary triune brain theory proposed that the human brain was in reality three brains in one: the reptilian complex, the limbic system, and the neocortex.