Very earlier, embryonic vertebrate brains are three small bulges in a row, like a match with three heads in a row. We call the topmost bulge the forebrain, the middle bulge is the midbrain, and the first bulge the hindbrain. We must presume that the very first brain was a single bulge, and this mutated to one extra bulge, then into one more. In a fish the hindbrain handles movement, the midbrain handles vision and the forebrain smell. Movement, vision and smell were possibly the evolutionary sequence by which these needs first evolved. Also it seems the deeper brain has most of its wiring fixed at birth, but the forebrain possibly always had some "loose" wires at birth, which could be modified by early learning. From that point forward different parts of the brain began to evolve at different rates. In humans, the midbrain remains tiny, with about 5% of neural bulk. The hindbrain is the next massive with about 15% neural bulk, while the forebrain completely dominates with some 80% of all neural bulk.
As the brain evolves different parts of the brain take over different functions. Learning neurology is flexible. It can acquire new skills in short evolutionary time plus it can quickly increase brain bulk by allowing the final wiring of the extra circuits after birth by learning. On the other hand reflex circuits are more reliable, and might act faster, as they do in computers. Except each new reflex circuit must be carefully refined by selection, which is a slow, costly build up in evolutionary time. Nature, like the computer engineer, must strike a balance between cost, flexibility, speed and reliability over the functions the brain must perform. For example, in early land vertebrates leaning how to walk the first time was done in the forebrain, which is the most flexible, advanced segment of the neurology with the highest concentration of learning circuits. But as walking became increasingly reflexive its basic coordination has been transferred to the hindbrain, leaving the forebrain free for other important things to learn.
Humans too now have a large hindbrain for complex reflexive tasks like muscle coordination in support of walking upright. Some 11% of human brain bulk is in this outgrowth of the hindbrain called the cerebellum. Possibly, early upright walking was focused first in the forebrain, but later shifted to the hindbrain as upright stance became reflexive in humans. Like in early computers, many novel tasks were first performed by the software. But as tasks became universal to all computer programs, it became more reliable and efficient to move these tasks into hardware. For example, 3-D visualization was first done only in software. But as all programs start to require it, this function is now being moved to hardware. Only the human cerebellum is not totally hardware either. Although the hindbrain is more fixed than the forebrain, the cerebellum expanded too quickly in human evolution to have totally fixed neurology. So most of its wiring is completed shortly after birth, though the embryonic hindbrain is totally wired at birth. On the other hand, only basic reflex such as balance or muscle coordination is concentrated in the hindbrain. Athletic skills such as whether one is a good swimmer, are focused in the forebrain where they can be perfected for many years after birth and stay adaptable to circumstance. Again, nature is leveraging that once the learning circuit type is perfected, it can be expanded in brain bulk more rapidly than it would take to select each individual circuit to be perfectly wired at birth. This can be used to expand brain bulk rapidly, while keeping new behaviors flexible through learning after birth.
The process of shifting skills from the fixed circuits of the lower brain to the learning skills of the higher brain is known as encephalization. Only we see that it is a two-way process. Not only are advanced skills moved into more bountiful and flexible learning neurology, but long established, essential skills are slowly refined into hardwired reflex. There are many evolutionary advantages to encephalization. One is that behavior becomes more versatile when relocated to learning circuits. The next advantage concerns genetic instructions. Because learning circuits have an essentially similar design, they can be multiplied in great quantities from much fewer genetic instructions than would be required to design and produce individual circuits of reflex. This allows a greatly expanded brain, from adding only a few genetic instructions. Only as we emphasize encephalization of functions once controlled by reflex allows increased specialization of the remaining reflex circuits. Although encephalization allows less functional reflex the total number of reflex genetic instructions will not go down. Instead, instructions of how to design reflex circuits will be redirected to refining the design of essential reflex, such as vision, metabolism, balance, reproduction and so on.
So, we should not think of encephalization as just of the higher cortex gaining an increased role while the other brain sections remain static. The brain evolves as a unit, and it will pay nature to shift highly repeatable functions back to the lower brain, or refine the functions of the lower brain to specialize while evolving slowly autonomic functions that it does well. In mammals the hindbrain, especially the cerebellum has evolved to a large size over amphibians and reptiles. In early animals movement was directed from the forebrain. But autonomic controls for complex coordination were better refined as semi-reflex. In domesticated animals such as horses and dogs we cannot teach them muscle-coordinated movements wired into the hindbrain that those animals instinctively perform. Yet, humans can train animals to perform movements like jumping with enhanced coordination, and these enhancements are movements controlled from the higher cortex. Finally, in primates, but notably humans, a broad range of muscle coordinated behaviors such as running, fighting, swimming, throwing, jumping, vocalization and acrobatics are learned to a greater or lesser extent. Humans have a large hindbrain because they have many muscles and require complex autonomic control for muscle coordination in any skill. But volitional athletic movement is skill directed in humans from the higher cortex. Maps of the higher cortex show almost the total of it involved in behaviors we might have at first presumed to be reflex.
Some years ago there was a popular triune brain theory, by Richard MacLean. This proposed that the brain had three basic layers, though these layers did not strictly follow the embryonic forebrain, midbrain and hindbrain layers. Maclean's theory was that the inner brain is reptilian, next was the limbic system that was lower mammal, and the outer layer was the neo-cortex or higher mammal. It was an interesting theory, but the three tiered embryonic brain predates mammals, at least to the earliest vertebrates. Plus the brain evolves at least five or six anatomical layers in detail, and neural function evolves outwards as the brain develops across all the layers. Moreover, the physical emotions humans or higher mammals feel such as loneliness or embarrassment are released as physiology in the deeper brain, though such emotions are hardly reptilian. Rather, it seems that transactions leading to behaviors interact across all the brain layers, which evolved to meet those needs at the time. With encephalization especially, the absolute size of the brain's reflex does not go down but increases through refining autonomic controls such as those for highly coordinated actions. Particularly in humans, the relative size of the learning cortex goes up by a large amount.
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