The following is a blog post I made over at the All in the Mind blog in response to a recent show on evolutionary psychiatry. I wanted to post it here, because I've been wanting to discuss the "reptilian brain" idea for some time, as it may well be one of the most popular but wrong ideas about the evolution of nervous systems out there.
More posts on reptile brains later, I hope.
The basic premise discussed in this show -- that human behaviour has an evolutionary history -- is not terribly contentious. The specific model discussed in the program, Paul MacLean's "triune brain," is more problematic.
As typically expressed, MacLean's model suggests that entire reptilian brain has been conserved through the evolution of the mammals, with new brain regions essentially added on to the existing core, like suburbs being added to a city.
There are a few problems with this model.
First, MacLean's ideas seem to be highly influenced by old ideas that emphasized the "march of progress" or the "great chain of being." In particular, the MacLean model seems to be based on the notion that reptiles were the ancestors of mammals. It's debatable whether reptiles are the ancestors of mammals, however. It may be that the two groups shared a common ancestor, then diverged. It's also somewhat misleading in that it lumps all reptiles together. Snakes, for instance, appear much later in the fossil record than the earliest mammals.
Second, the suggestion that the entire reptile brain is essentially the mammalian hind brain is not supported by modern neuroanatomy. To give an example, in MacLean's model, the limbic system is characterized as a "lower mammalian" part of the brain. There is evidence, however, that reptiles have a limbic system (Bruce and Neary, 1995; Lanuza et al., 1998).
MacLean's "triune brain" hypothesis may have caught the popular imagination, but it has not proved useful in modern neurobiology.
Bruce LL, Neary TJ. 1995. The limbic system of tetrapods: A comparative analysis of cortical and amygdalar populations. Brain, Behavior and Evolution 46(4-5): 224-234.
Lanuza E, Belekhova M, Martinez-Marcos A, Font C, Martinez-Garcia F. 1998. Identification of the reptilian basolateral amygdala: an anatomical investigation of the afferents to the posterior dorsal ventricular ridge of the lizard Podarcis hispanica. European Journal of Neuroscience 10(11): 3517-3534.