The human brain is immense. It is comprised of 100 billion neurons with many thousands of interconnections. It is no wonder that definitive statements about its overall operation are few and laden with qualifiers. Nonetheless, it’s useful to observe that the brain has evolved by increments over the Earth’s history. A species masters an ecological niche. It develops behaviors that allow it to thrive and propagate. When the organism mutates or the environment changes, the current species may alter their behaviors to fit the new situation. From existing actions, modified behaviors form the life style of the newly emerging species.
Nature Abhors Waste
New species, adapting to new environmental niches, build their behaviors upon what was working in the parent species. Physical organs do not arise from nothing. Instead of an eye with no predecessors, descendant species gradually develop improvements of photon sensitivity. The genetic inheritance of a new species almost always consist of incremental genetic changes rather than entirely new genetic solutions.
Incremental Expansion of Niches
Evolution is a novel response to environmental challenges as much as it is new genetic material. Once a niche is saturated, variants of the organisms (the proportion of genes in the species shifts) start to compete to open new niches. Genetics is a hardened, generic solution that allows a species to flourish in its environmental niche, yet be flexible enough to adapt to new opportunities. In the next page, Brain Through the Ages this Darwinian idea will be developed with respect to the brain.
Biological Brain Stages
Figure 12.2 lays out the stages which underlie our daily decisions. As you read down the columns, each functional layer in built upon the functions above it. With humans the main focus, only neural developments leading to skills in human lineage are shown.
Most of the layers of decision-making are unconscious. There is no conscious attention nor analytical thought needed to guide their behaviors.
- Single cell. Eat and duplicate. No neurons. The first imperative, satiety. Satisfaction of immediate biological requirements.
- Multicelled organism. Sexual reproduction and a neural net. The second imperative emerges along with neurons which allow communication among the various parts of the organism.
- Worm. An invertebrate with local clusters of neurons. It reacts to a stimulus by a local, reflexive behavior. Our patellar reflex (knee reflex) is an example of this reflex behavior. Without a signal to the brain, this reflex occurs.
- Fish. A vertebrate which uses brainstem structure and cerebellum to support homeostasis. We don’t decide to be hungry. We are hungry and therefore act to satisfy the hunger. It is an invariable response that we experience. Similarly for sex; however, safety is instinctual, not planned since amphibians have limited memory.
- Frog. The amphibian has a forebrain which it uses to combine its various homeostatic demands into a single immediate behavioral response.
- Mammal. The mammal’s brain has cortical lobes and limbic system. Mammals have memory and the capacity for consideration of alternatives. If one of the 3S imperatives (Satiety, Sex, and Safety) does not demand immediate satisfaction, the net satisfaction of all three are weighed in behavioral response.
- Primate. The primate’s expanded cortical mass has switched from odor orientation to visual and tactile modes. Association areas in the cortex, which get input from other cortical neurons, not sensory cells, rise in importance for understanding the environment.
- Australopithecine. This ancestral branch shows that bipedal movement preceded the explosion in humanoid brain volume.
- Homo habilis. With supple hands and larger cranial capacity, this branch has left evidence of their tool making.
- Homo erectus. Standing upright, they used their increased brain capacity to fashion behaviors successful to their niche.
- Homo sapiens. Relying upon all the predecessor life and neural structures, our enlarged prefrontal capacity allows us to also alter our environmental niche, making it more pliable to our needs, desires, and fears.
The Adult Human Brain
Following the Brain through the Ages, which expands on the genetic layering introduced above, the Development of the Adult Human Mind will discuss the cultural molding experienced atop the biological changes that accompany growth from infant to maturity.