It is not directly possible to know the exact circumstances, or selection pressures, that favored the development of the human brain. Consideration of its structural evolution, and comparative research, on human and nonhumans (other members of the primate order) have provided insights into the early ‘drafts’ of the modern mind. It is believed that, during the evolution of our mind, the nervous system changed in a number of manners, four to be precise. The arrangement of organs first became centralized in architecture, being the next step of evolution from a loose connection of nerve cells, as in jellyfish, to a spinal column and complex brain with impressive swellings at the hindbrain and forebrain. Centralized architecture led to hierarchy amongst structure and it appears that newer ‘drafts’ of the brain overtook the earlier additions and in effect became the Operator, the master of the domain of evaluating sensations…
Initiation of voluntary behavior, alongside the ability to foresee, plan, and engage in complex thought, along with the usage of language, depends on the neocortical structures. The human neocortex can actually destroy itself, and, this form of death is rare amongst the species of the Earth, as human beings, in the throes of a depression, can subdue the natural Will to Life and end their own existence, a rebellion against their genes.
This absurdism was put eloquently in the philosophy of Albert Camus, especially in The Myth of Sisyphus. And the existentialist view point, though it has its merit, must not always give rise to feelings of powerlessness amid the populated world of sight and sound; it doesn’t have to result in Sartre’s Nausea or Dostoyevsky’s Underground Man. Living in a universe without true purpose is not to say that life is without meaning. It is to say that there can be meaning, you just have to work it out for yourself.
There has for the last few thousand years been a trend towards encephalization; a concentration of sense organs and neurons at one end of the organism. Concentrated neural and sensory input in one location, transmission time from sense organs to the brain was minimized. Third, the size and variety of elements within the brain have increased. There has also been an increase in what is known as plasticity; the brains ability to modify itself as a result of experience. By self modification the brain is storing memories of what has been learned of new perceptual constructs and abilities.
One way to understand the evolution of the human brain is to see it as the elevated ability of control. Function of the animal and human behavior can be understood as the control of perceptions, with perceptions corresponding to aspects of the environment in which the organism was selected for and ostensibly adapted to. The very base drives of the animal is to find food, mate, and sleep. The human being must have had other challenges throughout evolutionary history: avoiding enemies, enduring drought, fighting other tribal chiefs for mating, and, as a result, the mind became more complex due to the increasing number of competing organisms. A considerable advantage, one has to agree, is to be able to perceive and control complex aspects of the environment. Bacterium, such as E. coli, can control its sensing of food and toxins in a primitive manner. However, organisms with more complex brains are able to sense and control much more complex aspects of their environment.
Nowhere in the biological world is environmental control more striking than in our species. We use advanced perceptual and behavioral capacities together with a culturally evolved knowledge of science, and of technology, and it is with this technology that we have expanded our view of space and the inner-workings of the Earth. It is interesting to note, on human character, it is not necessary that these things be done out of pride; it is the microcosm of the human being in that party climbing Everest, and each member in that party can be compared, metaphorically, to those who scale the highest peaks of human understanding.
It is this daring, this consummation of wonder and ability, that marks the human race. It is the impossible odds, it is the challenge, and that innate desire to overcome has, most certainly, played a role in our evolutionary history.
It is all beyond doubt that the role of language has had an evolutionary impact on the function of the human mind. Instead of thinking in abstracts, whose wording would be nonsense, the role of language allowed for the formation of thought, comparison, image association, mathematics, and foresight.
It is an intriguing question to put forth: can the most elaborate and complex of human abilities, art and music and science, and morality–could that be a product of natural selection? Our brain has certainly not changed appreciably over the last couple of hundred years, and yet we can solve mathematical, scientific, technological, and artistic problems that did not even exist a hundred years ago. So how could natural selection be responsible for the striking abilities of today’s scientists, engineers, and artists?
This problem also troubled the independent co-discoverer of natural selection, Mr. Wallace and, it should be remembered, that Wallace, despite being a discover of natural selection, didn’t believe it could be powerful enough to select among certain abilities, namely the ability of African’s to sing and perform European music, since nothing in their environment could have selected such an ability. This, to me, brings about the universal nature of mankind. Disparate parts of the world may engender different races and castes of people, when it comes down to ability, those, save for the mentally impaired, are as capable as the next when it comes to learning and remembering. We now know that in his embrace of this providential explanation, Wallace failed to realize that natural selection can lead to new abilities unrelated to those that were originally selected.
A classic example of this phenomenon of functional shift in biological evolution is the transformation of stubby appendages for thermoregulation in insects and birds into wings for flight. In the same way, selection pressure was undoubtedly exerted on early hominids to become better hunters. The ability to understand the behavior of other animals and organize hunting expeditions must have been very important in the evolution of our species.
And the increasingly complex and adapted brain thus selected would have made other skills possible, such as making tools and using language, traits that in turn could become targets for continued natural selection. This transformation of biological structures and behaviors from one use to another was given the unfortunate name of preadaptation by Darwin, unfortunate since it can too easily be misunderstood to imply that somehow evolution “knows” what structures will be useful for future descendants of the current organisms.
From this perspective, it would be easy to conclude that our brain and all of its complexity are an inherited legacy, a direct transmission of genes that affect the growth of the brain in utero. Once evolved, it, thereafter, is coded by specified inherited parameters in the fine print detail of the genome, immortal cells, as it were, marching through the generations, from body to body until it runs into a dead-end, extinction being the result. During its life, however, any one species can branch off into different evolutionary directions. Such as the common ancestry we share with chimpanzees; the common ancestor, which we share, has gone extinct.
Richard Dawkins, in The Ancestor’s Tale, puts this date around 5 to 6 million years ago. The progeny of this ancestor, though an evolutionary dead end, diverged. One of its descendents was h. sapiens and p. trogrolytes, the chimpanzee. So, in a sense, the replicator, that was once the product of our common ancestor, has lived on through the coded information in the genome of human beings and chimpanzees. Looking at evolution this way, there aren’t as many ‘total’ dead ends. An evolutionary dead-end does not necessarily mean that the information of an ancestor is lost, it simply need to mean that the inheritance is being passed through different species of common descent.
Despite all we’ve learned about the brain so far, the question will not go away. I don’t know if mankind is inclined, by nature, to an infinite regress of why, and this infinite regress is part of the process by which we differentiate ourselves from other animals, but in this, this infinite regress, behind every fact there exists a why, when behind every fact there should instead be how. So, how was the brain assembled? Is it just a miasma of tangled wires from synapse to neuron, from signal to function? Is that what thinking is–bioelectric charges along the neocortex? How does a neuron know which muscle fiber to connect with? How are sensory neurons able to join with the correct cell in the visual cortex in the occipital lobe? If this detailed mass of staggering complexity, this neuron-to-neuron connection system, is not in our genes. Where does it come from?
The first clue to solving this puzzle go back to 1906 when it was first observed, in embryonic nerve tissue, that some neurons didn’t ‘stain’ well and, not only did they degenerate, the neurons died. It had long been assumed that, in a developing embryo, nerve cells should be increasing, not dying off, and this discovery was a bit surprising. In the developing nervous system, nerve cell death has been, since then, thoroughly observed.
Despite his name, which I’m sure the English speaking world would chuckle upon hearing it, Viktor Hamburger found that a certain area of the spinal cord of a chicken embryo, over 20,000 neurons were present. However, the adult chicken, much to his bewilderment, had little over half, or 60% of the remaining cells, as neuronal death occurs in the earliest days of the embryo’s existence. Nerve cells continue to die off later in development, but at a slower pace.
The death of obviously useless brain cells cannot account for the specific connections that are achieved by the remaining neurons. For example, the visual cortex of cats and monkeys has what are called ocular dominance columns within a specific region known as cortical layer 4. In any one column of this brain area in the adult animal we find only axons that are connected to the right eye, while in the neighboring column are located only axons with signals originating from the left eye. So not only must the axons find their way to a specific region of the brain, which can be quite far from where their cell bodies are located, they must also find a specific address within a certain neighborhood.
Axons, and their ability to connect to appropriate regions of the brain during development, has been carefully studied since the beginning of the 21st century. Axons grow in the brain like a stem of a plant. At the end of the growing axon is a growth cone. It has been described as, ‘a sort of club or battering ram, possessing an exquisite chemical sensitivity, rapid amoeboid movements, and a driving force which permits it to push aside, or cross, obstacles in its way–until it reaches its destination.
The exact mechanisms by which this occurs are still speculative, it does appear that the growth cone is sensitive to certain chemicals along its path, to be released by its target region. Visual systems, in function, involve axons which originate in the lateral geniculate nucleus and find their way to ‘cortical layer 4′ in the occipital lobe. The way in which they find their way could be represented by a sleuthhound is able to sniff our an escaped prisoner hiding out in some Americana cornfield.
Although growth cones lead axons to the proper region of the brain, or the muscle in the case of motor neurons, they don’t lead them to the target address. For a particular growth cone, it appears that any type of cell can serve as a target. This has been demonstrated on other animals. In a newborn kitten, ocular columns receive axons from both eyes, not just from each other as it is in the adult brain. This ‘tuning’ is achieves many of the original, though terminal connection of the eliminated axon. This is what affords stereoscopic vision. In vision, axonal connections from the ‘wrong eye’ are eliminated. The axonal connections from the ‘correct eye’ are retained.
When it comes to motor systems, which initially have many connections between motor neurons (the spinal column) and muscle fibers (motor neuron axons connected to muscle fibers). Many muscle fibers are connected to the axon. The mature animal has a more acute ocular receptive capacity as the system is more ‘ordered’ as each muscle fiber is enervated by only one motor neuron.
In mammals, the nervous system changes during embryology and from birth to maturity. From a redundant and disordered system to a more accurate apparatus. This makes complex behavior possible, along with stereoscopic vision. The question still bulks large. How does the nervous system differentiate between the necessary connections to retain and which to eliminate? So now the question naturally arises, how does the nervous system know which connections to retain and which to eliminate? Research conducted amongst newborn kittens. For one week an eyelid was closed. The experiment showed that a week without sight altered the connection of the eyes to layer 4 of the occipital cortex.
This showed that axons carrying nervous signal from the closed eye made fewer connections with the cortex. Axons from the open eye made many more connections than was normal, to compensate, as it were. This showed that axons in the visual system compete for space in the visual cortex.
This suggested that visual system axons compete for space in the visual cortex, and this depends on the amount and type of sensory information carried by the axons. Subsequent research, using drugs to block the firing of visual system neurons, as well as artificial stimulation of these neurons, showed that it is not only neural activity that results in the selective elimination of synapses. Only certain types of neural activity result in the retention of certain synapses. Others lose receptibility and ‘short out’ — that is to say, they are eliminated and no longer send responses to the visual motor cortex.
Cells that fire together wire together. Timing of the action-potential activity is critical as it determines which synaptic connections are strengthened and those, the less fit (a nod to natural selection) are abandoned and are gradually discarded. Vision itself correlates the activity of retinal ganglion cells. This is because the cells receive input from the same parts of the visual world.
Dependence on the development of the visual system, via sensory simulation, indicates that ‘fine-tuning’ of connections take place once the animal has been delivered from its warm and comfortable womb into the cold sterility of artificial light. Recent evidence, however, suggests that this process of visual development is done in utero. Prenatal development depends on firings of retinal cells that don’t require the stimulation of light. Endogenous activity, it is thought, may also exist in the spinal cord and may, turn, refine synaptic connections between motor systems.