Biological Evolution and the Human Brain

Biological evolution is based upon the desire to perpetuate oneself and increase reproductive fitness. In this effort, species adapt to existing structure and order of surroundings to ensure survival and reproduction. The ability and methods of adaption and adaptation have evolved as life became more complex. In simpler organisms, instinct is coded in the DNA.

Hawkins (2004) describes this as a sort of genetic memory. For example, one-celled organisms have the ability to detect the presence of nutrients and move to an area with a higher concentration of food. This is an automatic reaction based upon chemical sensors and signals that were coded in an organism's DNA. As organisms evolved, more complex behavior emerged in the form of communication systems. For example, many plants developed a vascular system, which can detect damage in its structure. In many plants, the communication system can actually detect if the damage is caused by an insect and if so triggers a defense mechanism such as the production of a toxin to combat the predator (Hawkins, 2004).

Hundreds of millions of years ago, simple nervous systems emerged in multicellular creatures as they spread throughout the earth. The development of the brain as an organ and part of the nervous system can be seen in the most primitive animals. Perhaps the simplest example is the worm with its bilaterally symmetric body structure consisting of a head end and a tail end and a left and right side (Ornstein, 1984). Each part of the segmented body contains bundles of nerve fibers that send information from receptor cells in the skin to a group of nerve cells. These groups of nerve cells communicate with one another through larger bundles of nerve fibers that extend up and down the body forming the nerve cord. As the first vertebrates developed from invertebrate ancestors, the nerve cord became encased in a bony covering, the spinal vertebrae and this became the spinal cord. Ornstein explains, “what exists as only a few extra cells in the head of the earthworm, handling information about taste and light, has evolved in us humans into incredibly complex and sophisticated structure of the human brain” (p. 21).

With the brain being the seed of intelligence, evolutionary thinkers initially associated the size of the brain with level of intelligence. Anthropological studies have indicated a tripling in human encephalization, which is a dramatic increase over the last 3 million years (Jerison, 1976). However, simply brain size does not fully explain the explosion of complex behavior and intelligence of humans. Jerison describes that "the evolution of hearing and smell to supplement vision as a distance sense is sufficient reason for the evolution of an enlarged brain in the earliest mammals. Only an enlarged brain would allow a reptilian brain to analyze non-visual information” (pp. 11-12). New neural networks would have to evolve in order to process these new senses. Jerison explains, "the first expansion of the vertebrate brain may have been primarily a packaging problem and that it may only incidentally have resulted in the evolution of intelligence” (p. 98).

Additionally, the emergence of bipedalism is associated with the increase in visual processing. Being on two feet gave our ancestors a greater ability and reliance on vision. Walking upright also freed our arms and hands to make and use tools, as well as, to perform more complex tasks such as writing. The increase in brain size occurred concurrently with these behavior changes.

Holloway (1996) examines the evolution of the hominid brain over the last 3 million years. Similar to Jerison, Holloway explains that brain mass alone is not a sufficient variable to explain the evolution of human behavior and the brain. He proposes a major reorganizational change from a “pongid to hominid pattern” based around the “synthesis of mass with reorganization and hierarchy.” Specifically, his anthropological examination has revealed that there was a major reorganizational change in the posterior parietal, anterior occipital and superior temporal portions of the cerebral cortex that preceded the reorganization of the frontal lobe and Broca’s area which took place 1.8 million years ago. Additionally, the doubling of brain size from roughly 750 cc in H. habilis to 1400 cc in modern H. sapiens occurred after these major reorganizational changes in the brain. Another important organizational change is based around greater hemispheric specialization exhibited by the asymmetric changes in the left and right hemispheres (Holloway). Although it is difficult to fully predict the relationship between brain volume changes integrated with reorganization and body size changes, Holloway proposes a sequence shown in Figure 3. Even though the specifics of how the brain evolved may be questioned, and the assumptions may be challenged, it is undeniable that the brain has been an ever-evolving organ.

Figure 3 Summary of reorganizational and size changes in the evolution of the hominid brain (Holloway, 1996)

Allman (2000) illustrates the development of the brain by comparing it to an old power plant. He explains that "because the plant was needed for continuous power output, it could not be shut down and retrofitted with each new technology." Therefore, old "control systems" stayed in place while new ones were integrated into the existing system. Basically, nature could not afford to throw out an old brain system. Similar to Allman, Ornstein (1984) described the brain as a "ramshackle house." He describes that evolution led to the "remodeling of old rooms to serve new functions." These metaphors offer great insight into the evolution of the brain and stress how old regions of the brain have changed and adapted as new regions were developed while the brain as a whole continued to function by providing power and monitoring all of the inner workings of the body. It is important to keep in mind that the brain's initial function was simply life support and moderation of survival functions such as breathing, hunger, thirst, and circadian rhythm. This is often overlooked when studying the capacities of the brain. The brain, even the "intelligent brain" of humans, is an organ and was never redesigned solely for the specific purpose of being complex.

Human behavior is a manifestation of underlying neural circuitry modified by evolution. Neurons initially evolved as a quicker way of sending and receiving information to various parts of the animal. The electrochemical spikes in a neuron travel much faster than the diffusion of chemicals. However, a behavior that evolved in the past to serve one function may serve an entirely different purpose at a later time. Eventually, nervous systems had elements of both memory and learning. This was a huge evolutionary advantage, which allowed mammals to learn from experience. A plastic nervous system allows for a response to environmental changes as they are happening. This allows an individual to modify behavior to achieve better survival and reproductive rates during its own lifetime.