Brain through the Ages

Let’s go a bit deeper into the brain structures which mark significant, genetic steps on route to the human brain, while recalling that each change is built atop pervious, working neurological and behavioral adaptions.

Before the cortex emerged in mammals 200–225 million years ago, the brainstem, cerebellum, and primitive forebrain were in charge of behavior. These structures predate consciousness thought. They supported behaviors which solved challenges that we still face. We still have those structures. Their decisions still focus our behavioral decisions about eating, sex, and security, except in specific cases when our conscious decision-making overrules our primitive choices.

There are distinct stages in development of brains that led up to the human brain. Although physiologists and cognitive researchers use diverse terms for stages, here we will deal with three significant stages:

  1. Neural net. Before the brain and after the amoeba, a diffuse neural net provided an intermediate stage between external stimulus and organism response developed.
  2. The fish brain (the vertebrate, hindbrain) controls the pulse of life – the drive to keep the body working well and the drive to have sex which propagates the species.
  3. The mammalian brain adds memory to species skills as well as combining needs from homeostasis, sex, and safety into a unified response to current demands.
  4. The primate brain, with its shift to visual and tactile sensory organization, is responsible for combining experiential memory with sensory information.
  5. The final stage, the vast growth of prefrontal cortex, will be discussed, along with culture, in the section on Development of the Adult Human Brain.

In pre-human mammalian brains, obviously there is no thinking in words, but there is a cogitation—consideration, evaluation, and action. Sense data, patterns, categories, and concepts—the elements of the environment—are subject to immediate notice and evaluation by the organism.


Figure 13.1 Tree of Life with 2 branches (fish and mammals) and leaf (human) outlined in yellow

Figure 13.1 Tree of Life

Animals, starting at the vertebrate branch of life (in Figure 13.1 fish, middle left), have a neural brain that monitors levels of biologically important activities and dictates actions to pull those levels closer to long-ago developed levels that make current life more pleasant. In humans, we call this homeostasis—level of oxygen and sugar in the blood, heart rate, body temperature, and so on.

The external world provides behavioral opportunities to adjust the internal levels to the homeostasis levels. This operation is performed in the oldest portion of the brain and typically does not require our conscious attention. These are preconscious behaviors. For example, if your body temperature is too high, your body has two primary ways to counter it. You can perspire or you can get into shade. Perspiration is a preconscious response to the immediate situation in place. Seeking shade can be preconscious, but it can also be a conscious response. Even when it is conscious, it is a motivated act, not a free-willed act.


Vertebrates, early descendants of the invertebrates, developed multiple senses, allowing them to live in environmental niches that the invertebrates hadn’t saturated. They also had a more sophisticated brain, which combined the results of touch, smell, sound, and sight that allowed behaviors related to all of the senses, rather than each sense individually.

An organism with various bodily and sensory systems requires good performance of numerous biochemical reactions. Those reactions occur best in certain temperature and pressure ranges, ion ratios, etc. The outside environment is rarely in alignment with all the organism’s needs. Thus, the organism must maintain that balance if it is to continue living. It must maintain its own homeostasis.

The next step, a huge step, was the adding of memory in the limbic system occurred in Figure 13.1 probably before the branch in the middle left where the mammals splinter off from the reptiles.

Limbic System

Figure 13.2 Limbic system between old brainstem and new cortex

Figure 13.2 Limbic system between old brainstem and new cortex

With mammals came the limbic system (Figure 13.2), which consists of distinct elements with specific capabilities.

  • Thalamus. Receives input streams from sensory organs. It does some sensory processing itself, but it also acts like a switchboard forwarding each sensory stream to a particular cortical lobe. It also returns cortical movement decisions to the muscles
  • Tectum. Enhances auditory and visual data. More immediate, but more primitive reaction than the cortical enhancements
  • Hypothalamus. Acts as a connection between nervous (behavior of organism in environment) and endocrine (alters internal state by biochemical changes) systems
  • Hippocampus. Allows memories with emotional values to be formed
  • Amygdala. Evaluates current situation against remembered situations along the dimension of 3S imperatives (needs, desires, and fears—experienced as emotions)

The limbic system supports more complex behavior, beyond the simple stimulus-response actions that homeostasis provides.

Once memory formation became possible, with the existence of the hippocampus, current environments and past situations could be compared. With the additional information about the result of prior behavioral choices, we have a guideline for choosing actions in the present. That decision-making process is guided by likely completion resulting in biological satisfaction.


The brainstem results in invariable behaviors to satisfy homeostasis. With the limbic system, the organism evaluates the current situation against prior situations in terms of security, satiation, and sexual opportunities. To perform this task, it assigns a value to each situation according to its capacity for satisfying the 3S Imperatives. These values are emotions. Emotional valences is the term to indicate that each situation and each behavioral choice is likely to partially satisfy many emotions and to entirely satisfy none.


The cortex, twin hemispheric neural structures, each with four lobes and responsibility for half the sensory and movement activities, first emerged in the brain of mammals. The three lobes that receive sensory input extract more information than the limbic system does from the environment. The lobes forward results to a relatively small frontal lobe in the early mammals. In the frontal lobe, behavioral responses to the current situation are triggered and sent to the muscles to activate motion.

The active role the cortex takes in assembling a mental worldview is nicely captured by E.T. Jaynes’s (p 133) caution,

Seeing is not a direct apprehension of reality, as we often like to pretend. Quite the contrary, seeing is inference from incomplete information.

The integration of the limbic system and the cortex’s frontal lobe gives rise to the confusing, dual aspects of emotional reactions. First, the limbic system reacts to the situation, non-verbally but biologically—hairs raised, goosebumps, startled reactions—then the cortex receives the information, enhancing it. Only then can we evaluate it consciously. At that point, we can speak of the emotion, attempting to put into words the reasons for our reaction.

Mammal Social Groups

Social groups appear as a response to environmental and competition pressures. Members of a species acting together have a greater chance of survival than a single organism acting alone. The relative strengths of competition and cooperation vary among species and environmental niches. The net result is that mammalian species support different social group sizes.

The largest social groups in chimpanzees and gorillas are the size of small neighborhoods. Building upon the fundamental thought processes of an individual, these high-level primates alter their behavior range to align with the group norms.

The effect of social norms, culture, and civilization will be more fully discussed in Development of the Adult Brain

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