Understanding the Neurobiological Realm of Existence

This is the third in a series of posts on my book, The Four Realms of Existence. Read parts one and two.

Source: Harvard University Press/Used with permission

Human beings can be characterized as a composite of four fundamental, parallel, entwined realms of existence that reflect our evolutionary past and account for our present ways of being, as I explained in the first post in this series. These are: biological, neurobiological, cognitive, and conscious. The present post concerns the neurobiological realm and its emergence from the biological realm.

All organisms that have ever lived have existed biologically. When animals evolved from single-cell protozoa, they became the first and only organisms with nervous systems and, hence, the only organisms with a neurological realm of existence. Because neurobiological existence is a requirement for cognitive existence and cognitive existence is a requirement for conscious existence (at least what we humans mean by conscious existence), animals are the only organisms that exist in all four ways.

How did nervous systems come to exist?

To answer the question of how nervous systems evolved, we need to revisit the biological realm and the functions that keep all organisms alive. These include homeostatic and metabolic functions within the body and somatic interactions with the external environment.

The evolution of nervous systems by animals did not come out of the blue. As with most major evolutionary events, nervous systems were as much about what already existed as about what was new. In this case, that means that the nervous system, and hence the neurobiological realm, evolved as an extension of visceral and somatic functions of the biological realm, allowing control over the visceral and somatic functions of the complex bodies of multicellular animals.

Of course, animals are not the only multicellular organisms. There are also plants and fungi, both of which can, at best, make slow movements in response to their environment. In contrast, in animals, by virtue of having a nervous system that coordinates somatic and visceral responses, cells in distant parts of the body can respond in a fraction of a second to stimuli, signifying sources of things useful (nutrition) or harmful (predators).

The Neurobiological Realm Evolved from the Biological Realm

Source: Joseph LeDoux, used with permission

Early animals, such as the ancestors of jellyfish, had radial bodies with only a top-bottom axis. With the arrival of bilateral bodies, animals acquired left-right and head-tail orientations. The had-tail axis was particularly important for two reasons. One is that it defined the forward direction of locomotion. With key senses (sight, smell, and taste) located in the head, food detected by vision or smell could be rapidly responded to, with taste as the final arbiter of whether to swallow. The other reason is that a concentration of neurons, a brain, was also located there, allowing quick access to the head-based senses for the coordinated control of body responses. The tail became a sacrificial lamb of the body, protecting the precious components in the head when escaping from predators.

An early bilateral aimal with head-tail, left-right, and top-bottom axes.

Source: Joseph LeDoux, used with permission

Neurobiological Realm Behaviors

The key identifying feature of neurobiological activities is that they are automatic reactions to external stimuli. Included are reflexes, instinctual fixed action patterns, Pavlovian conditioned responses, and instrumentally conditioned habits. These can be referred to as 'mere' neurobiological realm responses since they do not involve cognitive or conscious brain activities, which will be discussed in the next two posts.

Partitioning the Nervous System

Visceral and somatic neural functions are typically treated as subcomponents of the central and peripheral nervous system. But decades ago, the paleontologist Alfred Sherwood Romer recognized that the visceral-somatic distinction predated nervous systems in evolution. Romer, therefore, flipped the relations around, in effect, elevating the visceral and somatic distinction to the level of primary partitions of the nervous system and making the central and peripheral locations of the tissues secondary.

Although Romer's insight was lost in the sands of time, it became a fundamental organizational principle in The Four Realms of Existence. It showed me how the new emerges from the old. Specifically, how the neurobiological realm evolved from the biological realm, how the cognitive realm evolved from the neurobiological, and the conscious realm from the cognitive realm.