The neurological components of the first circuit go back to the oldest parts of the brain; Carl Sagan called these functions “the reptile brain.” These neural structures are at least billions of years old. The second circuit structures appeared with the first amphibians and mammals, somewhere around 1000 million or 500 million years ago; Sagan called them “the mammalian brain.” The semantic circuit appeared perhaps 100 thousand years ago; Sagan called it “the human brain.” It should be no surprise that most people, most of the time, are controlled more by the older reptilian-mammalian circuits
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