The next major mutation on the planet occurred when life created mammals. This mammalian or limbic brain, formed itself over the reptilian brain. Instead of just two responses, now the animal can react with a slight degree of variance. In addition to being just on or off, it can be slightly on, halfway on, three-fourths on, or all the way on.
Millions of years later, life mutated yet again into a third brain called the neocortex around 250,000 years ago when modern homo sapiens suddenly appeared out of nowhere from a common "Eve" grandmother. It wasn't until the formation of the neocortex that human's were able to make a choice whether to react or not.
The current evolutionary stage of the human brain bestows us with the ability to think, reason, and calculate mental operations in a split second that would rival the processing speed of a million desktop computers all working at once. But, just because we've evolved into this "super intelligent species" that has figured out how to split the tiny atom, clone a wooly sheep, propel ourselves to the Moon and back, and watch music videos on our hand-held iPods, it doesn't necessarily follow that we're a success. Contrarily, I would say that as a species, we've allowed our intellectual, material, and physical accomplishments to go straight to our collective head. The current human species currently allows over a billion people to live below the poverty level as well as for millions of innocent civilians to perish in countless wars. The success of a human species should be directly related to its ability to get along with one another in a cooperative state of peace and harmony.
Metaphorically speaking, the ego has imprisoned humanity in its own mind and thrown away the key. It's been able to do this successfully because people so easily over-identify with their ego's emotionally reactive state that is intrinsic to the body's survival mechanism. This fight-or-flight response is rooted in the primal fear of death found in all mammals. But we don't need to feel fear anymore to survive. Our species has already evolved past this point. You don't need to feel fear to know you shouldn't walk into oncoming traffic, just a little common sense.
The fear that we are referring to here is a psychological fear, not a "healthy fear" that keeps us alive in life-threatening situations. But unless you're stationed at the front line of a battlefield every day, you probably don't need to be accessing this physiological fear of survival. Yet, most of humanity continues to be arrested in this biochemical-inducing state of fear that stems from the reptilian brain's "on" or "off" response. Where's the free will in this kind of life? But this is how most humans live out their lives, oblivious to the truth that they're perpetually causing psychic and physiological disharmony to themselves and to others. This physical harm manifests as anger, stress, anxiety, nervous tension, worry, depression, paranoia, phobias, panic attacks, and physical disease. It is this ceaseless psychological state of "impending doom" that ultimately is rooted in the primal fear of death.