The brain stem is the oldest and smallest region in the evolving human brain. It evolved hundreds of millions of years ago and is more like the entire brain of present-day reptiles. For this reason, it is often called the 'reptilian brain'. Various clumps of cells in the brain stem determine the brain's general level of alertness and regulate the vegetative processes of the body such as breathing and heartbeat.

It's similar to the brain possessed by the hardy reptiles that preceded mammals, roughly 200 million years ago. It's 'preverbal', but controls life functions such as autonomic brain, breathing, heart rate and the fight or flight mechanism. Lacking language, its impulses are instinctual and ritualistic. It's concerned with fundamental needs such as survival, physical maintenance, hoarding, dominance, preening and mating. It is also found in lower life forms such as lizards, crocodiles and birds. It is at the base of your skull emerging from your spinal column.

The basic ruling emotions of love, hate, fear, lust, and contentment emanate from this first stage of the brain. Over millions of years of evolution, layers of more sophisticated reasoning have been added upon this foundation.

Our intellectual capacity for complex rational thought which has made us theoretically smarter than the rest of the animal kingdom.

When we are out of control with rage, it is our reptilian brain overriding our rational brain components. If someone says that they reacted with their heart instead of their head. What they really mean is that they conceded to their primative emotions (the reptilian brain based) as opposed to the calculations of the rational part of the brain.