Triune brain
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. The triune brain consists of the reptilian complex (or lizard brain), the paleomammalian complex (limbic system), and the neomammalian complex (neocortex), viewed each as independently conscious, and as structures sequentially added to the forebrain in the course of evolution. Since the 1970s, in some circles of evolutionary and developmental neuroscience, the concept of the triune brain has been subject to criticism and is regarded as a myth. Due to its longevity, the triune  brain idea has also been called "one of the most successful and widespread errors in all of science." as the hypothesis is no longer espoused by the majority of comparative neuroscientists in the post-2000 era. The theory has been embraced by some psychiatrists and at least one leading affective neuroscience researcher. MacLean originally formulated his model in the 1960s and propounded it at length in his 1990 book "The Triune Brain in 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". Status of the  model. MacLean originally formulated the triune brain hypothesis in the 1960s, drawing on comparative neuroanatomical work done by Ludwig Edinger, Elizabeth C. Crosby and Charles Judson Herrick early in the twentieth century. The 1980s 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 refined the traditional neuroanatomical ideas upon which MacLean based his hypothesis. For example, 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 fish 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. Some recent behavioral studies 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 grey parrot. Structures of the limbic system, which MacLean proposed arose in early mammals, have now been shown to exist across a range of modern vertebrates. The "paleomammalian" trait of parental care of offspring is widespread in birds and occurs in some fishes as well. Thus, like the basal ganglia, the evolution of these systems presumably dates to a common vertebrate ancestor. Finally, recent studies based on paleontological data or comparative anatomical evidence strongly suggest that the neocortex was already present  in the earliest emerging mammals. In addition, although non-mammals do not have a neocortex in the true sense (that is, a structure comprising part of the forebrain roof, or pallium, consisting of six characteristic layers of neurons), they possess pallial regions, and some parts of the pallium are considered homologous to the mammalian neocortex. While these areas lack the characteristic six neocortical layers, birds and reptiles generally possess three layers in the dorsal pallium (the homolog of the mammalian neocortex). The telencephalon of birds and mammals makes neuroanatomical connections with other telecencephalic structures like those made by neocortex. It  mediates similar functions such as perception, learning and memory, decision making, motor control, conceptual thinking. Lay interest. The triune model of the mammalian brain is seen as an oversimplified organizing theme by some in the field of comparative neuroscience. It continues to hold public interest because of its simplicity. While inaccurate in many respects as an explanation for brain activity, structure and evolution, it remains one of very few approximations of the truth we have to work with: the "neocortex" represents that cluster of brain structures involved in advanced cognition, including planning, modeling and simulation; the "limbic brain" refers to  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; and the "reptilian brain" refers to those brain structures related to territoriality, ritual behavior and other "reptile" behaviors. The simple explanatory value makes this approximation engaging and may be a useful level of complexity for high-school students to begin engaging with brain research. Howard Bloom, in his book "The Lucifer Principle", references the concept of the triune brain in his explanations of certain aspects of human behavior. Arthur Koestler made MacLean's concept  of the triune brain the centerpiece of much of his later work, notably "The Ghost in the Machine". English novelist Julian Barnes quotes MacLean on the triune brain in the foreword to his 1982 novel "Before She Met Me". Peter A. Levine uses the triune brain concept in his book "Waking the Tiger" to explain his somatic experiencing approach to healing trauma. Glynda-Lee Hoffmann, in her book "The Secret Dowry of Eve, Women's Role in the Development of Consciousness", references the triune theory explored by MacLean and goes one step further. Her theory about human behavior, and the  problems we create with that behavior, distinguishes the prefrontal cortex as uniquely different from the rest of the neocortex. The prefrontal cortex, with its agenda of integration, is the part of the brain that can get the other parts to work together for the good of the individual. Hoffmann claims that in many humans the reptilian cortex (agenda: territory and reproduction; in humans that translates to power and sex) is out of control, and the amygdala stokes the fear that leads to more bad behavior. The prefrontal cortex, she believes, is the key to our future if we can harness  its power.