Nervous systems first appeared in primitive marine life, billions of years ago. These earliest systems were reflexive in nature; stimuli (light, touch, pressure waves, etc.) would trigger a response (movement toward or away from the stimulus). Such mechanisms were essential to survival, playing an important role in feeding, defense and reproductive behavior.
As evolution produced more complex life forms, the neurological system developed in concert. Special sensory organs permitted more sophisticated forms of sight, hearing, smell and coordination; we can easily identify this progression in the modern representatives of animal families, from mollusks to mammals. Up through birds and most mammals, the neurologic system is primarily limited to peripheral nerves, spinal cord, brain stem and cerebellum; the brain stem controls basic functioning (such as respiration) while the cerebellum is critical to balance and coordination. The cerebral cortex, with its highly complex network of neurons, is the latest product of neurologic evolution, adding the capacity to think, reason, interpret, remember, learn, create and communicate in sophisticated ways.
While a primitive cerebral cortex is found in all birds and mammals, it has reached its highest form of development in primates; of course, humans are the most advanced primates and our cerebrum is, by far, the most complex. Indeed, our brain power has developed to the point where many humans conclude that we are divine creatures, superior to and separated from all lower forms of animal life. Yet, anatomic evidence demonstrates a clear evolutionary progression of the nervous system and medical science has shown a convincing correlation between specific brain injuries (via trauma, stroke, tumor, congenital deformity) and the loss of specific neurologic capabilities. Though difficult for many to accept, our thoughts, memories and emotions are the products of neuronal pathways and brain chemistry.
