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Lying under the cerebral hemispheres, at the top of the brain stem, are the reptile brain and the brain of the old mammal, which contain the basic survival programs related to flight from danger, hunger, thirst, procreation and parental care. These parts of the human brain evolved between 100 million and 300 million years ago.

  
 

Today the cerebral cortex, newest and most important region in the human brain, folds around and smothers the older and more primitive regions. Yet these regions have not been completely overpowered. They remain underneath, no longer in undisputed command of the body, but still active.

 
 
 

These primitive parts of the human brain continue to operate in accordance with a stereotyped and unthinking set of programs that go back to the mammals on the forest floor, and back farther still to the reptiles who spawned the mammal tribe.

  
 
Experiments have shown that much of the human repertoire of behavior originates in deeply buried regions of the brain that once directed the business of life for our ancestors. - Jastrow