“The triune brain” is a term coined by American neuroscientist Paul D. MacLean. MacLean emphasized the importance of an evolutionary approach to the study of the brain, and in his 1990 book The Triune Brain in Evolution, he proposed that the human brain could be divided into three general anatomical sections, each with their own unique structures. These could represent distinctive additions to the brain throughout its evolutionary development. MacLean distinguished between the protoreptilian forebrain (present in all reptiles, birds and mammals), the paleomammalian forebrain (containing the limbic system and representing the evolutionary transition from reptiles to mammals), and the neomammalian forebrain, representing a significant expansion of the cortical areas of the brain (1).

Later findings in comparative neuroanatomy have shown that the neocortex and limbic system are not unique to mammals, as areas corresponding to the mammalian neocortex limbic areas are present in both birds and reptiles. In addition, functional aspects which Maclean proposed as being unique to each of the three areas have been shown not to be limited to each anatomical area, but rather to be distributed throughout the three areas (2).

Sources:

1. MacLean, Paul D. (1990). The triune brain in evolution: role in paleocerebral functions. New York: Plenum Press. 126-127.

2. LeDoux, J. E. (2012). Evolution of human emotion: a view through fear. Progress in Brain Research, 195, 431–442.