Rita Carter asserts that the anatomy and functions of our brain are linked to fish that lived 300 million years ago. Beginning as a bundle of nerve cells incased in a small tube in the fish, Carter describes the layers of the brain as they evolve to the complex systems of the human brain of today. Carter connects evolutionary steps of development linking the primitive functions of a tiny aquatic invertebrate, to an earthworm, and then to a fish, reptile, mammal, and finally to the extraordinary specialized functions of the human brain. The story starts with a tiny aquatic invertebrate called a hydra. The hydra showed evidence of brain function in its loose network of sensory cells that connected to groups of cells called ganglia. In the earthworm, this group of cells began to function as a crude brain with a nerve cord that extended the length of the earthworm's body. Just like our spinal cord, the earthworm's nerve cord extended from a centralized location in the head to the tail, functioning as a primitive nervous system that communicated information by sending and receiving messages to produce movement.
2
The next notable step in evolution is the change from invertebrates to vertebrates. The nerves in the fish, the first vertebrate group, came together according to their sensitivity to smell, forming the smell brain.
3
Additional nerve cells organized according to their sensitivity to light, forming eyes. These groups of nerves connected to another group of nerves that controlled movement in a new unit at the top of the spine, the cerebellum. These three grouping of nerves and their specific sensitivity to smell, light and movement characterized the fish brain. The amphibian brain is similar to the fish brain except for the more fully developed olfactory bulb, marking a significant change in the improved ability to perceive smell. This change came along with the first recognizable limbs. As their environment changed, so did the criteria for survival. An improved, more developed olfactory bulb increases these chances for the amphibian.
These sensory groups took a giant leap in evolutionary terms represented in the development of the thalamus in the anatomy of the reptilian brain. The thalamus added a system for sensory control. This system enabled the sensory information collected through the senses of sight, smell, and hearing to become integrated.
4
Consequently, the reptilian's senses were able to work together causing a more complex interaction with its surrounds. This sensory integration gave rise to a more sophisticated response to the environment. The reptile could gather sensory information in the thalamus and use it to eat and avoid being eaten.
The limbic system and a wrinkled covering called a cortex distinguish the mammalian brain. Unlike the smooth cortex of the reptilian brain, the newly developed wrinkles on the cortex of the mammalian brain allowed the enlarged surface area of the cortex to fit within the skull. In addition, within the limbic system, the hippocampus and amygdala together formed a crude memory system for the first time, encoding experiences. This early limbic system enabled the production of emotions and behaviors that extended beyond primitive survival responses of fight or flight. This more complex group of systems involving memory, emotion, and sensory integration allowed for a more sophisticated response to the environment – for the first time, a step beyond pure instinct.
5
The mammalian brain continued to show improvement in response to the changing environment with the development of the cortex and its expressed ability to think and make planned responses to the environment.
Consciousness, as we understand it, came to life in the next evolutionary stage. Carter describes this as an explosive period of development marked by increased neural connections caused by the sensory units forming a thin sheet of cells that allowed intense connectivity. The integration of these systems involved new nerve cells within the cortex with heightened neural activity, interconnecting and forming an extensive matrix of neural connections. This thin layer of cells with intense connectivity is the cerebral cortex. Consciousness emerged from this connectivity. An explosive enlargement of the brain occurred with the development of our cerebrum. It created our flat forehead, the shape of our skull, and our complex thinking.
6
This was the first glimpse of today's mind and its ability to perceive, communicate, remember, understand and appreciate; distinguishing the
Homo sapiens
.
7
Man's brain is unique among other mammalian brains because of its size and the density of the cortex. The neural density increased the gray matter found in the frontal cortex and is responsible for our complex thought, judgment, and reflection; our fully conscious existence.
The comparative anatomist, Neil Shubin, in his discussion of anatomical evolution, begins with life's building blocks, DNA. Shubin describes specific examples of the DNA and the genetic recipes for organ building and traces them back 300 million years ago. He lays out evidence that connects the way organs develop, revealing a reoccurring theme that has continued for millions of years, suggesting that the genetic elements of our anatomy extend all the way back to the fish. Shubin says, "When you see these deep similarities among different organs and bodies, you begin to recognize that the diverse inhabitants of our world are just variations of the same theme."
8
As we compare species in an evolutionary context, we are able to see the relationship between the species and its environment change as the systems of the brain evolve. Primitive species were controlled by and reacted to their environment, through the process of evolution, species developed, interacting with their environment in more complex ways. We have grown and evolved to fully conscious beings interacting with our environment in a highly sophisticated way; a new global consciousness; connecting as a collective world-wide community, assessing, evaluating, creating, developing innovative approaches to problems with a broad scope and vision.
