ve tripartite brains.
MacLean described three drivers for what he called the "neural chassis." The most ancient surrounds the midbrain, and probably developed several hundred million years ago. It is shared by reptiles and mammals. Surrounding this is the limbic system, which surrounds the underlying brain, and this is shared with man and mammals, but not in its full development with reptiles. The most recent evolutionary accretion of the brain is the neocortex, which surrounds the other two parts of the brain, and is at its highest development in man. The emergence of the tripartite brain is in accordance with the emergence of primates and a major step in brain evolution. The developing human embryo brain goes through the evolutionary process in developing the hindbrain, midbrain, and finally, the neocortex, mirroring the human progress through evolution.
Sagan states that the essential human quality which distinguishes mankind is its intelligence (p. 197). This is due to the tremendous evolution in the size of the neocortex in man compar
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