The human brain and human thought is physically and conceptually complex. A useful simplification, tied to organisms that preceded us, allow us to see when certain milestones in thought occurred, the origin of reflexes, instincts, habits, and choices.
Developed in invertebrates about 3.5 billion years ago.
Invariable response to a stimulus is the extent of the behavioral repertoire.
Brainstem and Cerebellum
Developed in fish about 500 million years ago, atop the spinal column (Paul D. MacLean, developer of the Triune Theory, combined these with the spinal column as reptilian).
The response to an exterior stimulus is not invariable. Multiple sensory inputs are considered in a behavioral response. The maintenance of life-supporting conditions (food, water, air, etc.), homeostasis directs actions.
This stage delivers instincts.
Developed by mammals about 150 million years ago, atop the spinal column and the brainstem.
With memory the organism can relate situations, actions, and results. The satisfaction, or not, of its 3S Imperatives (satiety, sex, and safety) result in motives we call emotions.
Behavioral choices result in immediate actions, driven by emotions. Supports habits.
Developed in primates, with especial rapid expansion the last 2-3 million years, atop the above three structures.
Real-time reactions as well as planning to arrange better future situations. Long learning curve.
This stage delivers reasoned choices which sometimes can override emotional choices.
Milestones in Human Thought
As important and far-ranging as the fundamental achievements above are, the abilities developed in the past few hundred thousand years–for instance, language, consciousness, writing–that we revel most in, but the basic skills above must not be overlooked. They lay the ground for our behavior choices.
3S Imperatives, Emotions, and Consciousness Evolutionary details