How The Evolution Of Human Intelligence
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The level of intelligence of a species and therefore the extent to which behavioral responses can be appropriately selected and modified based on experience, is dependent upon the extent to which experience can be stored and valid associations between stored data can be formed that permit learning and its subsequent application in selecting the most appropriate responses. The differences in intelligence between species are a reflection of the differences in the sophistication, design, and size of the neural circuitry, and also on the ratio of brain size to body mass. Having defined intelligence as the ability to respond based on experience we see that it, too, is based on connections between neurons. At a neural level though, it differs from instincts in that intelligence requires neurons that are free to continually form and modify existing synaptic connections with other neurons after birth, as dictated by experience. Hence, although the creation of every brain cell is genetically controlled, it is clear that not all behavior has a genetic basis.
It is now possible to see that instincts and intelligence produce behavioral responses that re fundamentally different in terms of flexibility. Instincts are relatively inflexible, whereas intelligence by definition makes highly variable responses to environmental stimuli possible. Is it more advantageous to have the required intelligence to respond appropriately to a situation than to be able to do so instinctively? Without much consideration, one might say that it depends on the organism and the environment. One could support this position by pointing out that there are thousands of species of organisms that are largely instinctive, with very little intelligence, that have existed relatively unchanged for millions of years. However, what if the question is put another way? Given that all living environments are dynamic in nature, forever changing – sometimes very slowly, sometimes relatively quickly – is it of greater species survival value to be behaviorally flexible, or inflexible, in the long stretches of evolutionary time?
Evolution relentlessly favors maximum adaptability. The stimuli that trigger instinctive responses are specific to a particular environment. When environments change and the old stimuli are no longer present, the old instincts will no longer be triggered into operation. Thus, while instincts allow an animal to be adapted to environmental conditions that favored their evolution, a change in the environment can render them useless and may even pose a survival risk. If a species lacks sufficient intelligence to exploit the new environment without the aid of instincts, new instincts and/or anatomical traits must evolve that will make a species’ continued survival possible. The problem is that the evolution of new instincts occurs relatively slowly, and mutations creating beneficial anatomical traits are so rare that to depend on their timely emergence is tantamount to betting on a lottery. If environmental conditions change slowly on the geological scale of time, and if they change relatively little, then there is perhaps sufficient time for new instincts and/or anatomical traits to evolve. Species do evidently accomplish this since life forms continue to exist. However, if environments change more quickly and to a great extent, species relying heavily on instincts that do not operate in the changing environment may not acquire new instincts and/or suitable anatomical changes quickly enough, and may consequently face the prospect of extinction. In terms of probability, it should be intuitive that the more often the emergence of such low-probability mutations and slow evolving instincts are relied upon as a response to dramatic changes in environmental conditions, the more likely that eventually the environment will change too quickly and too drastically, and a species will become extinct. Hence, the greater the instinctive makeup of an animal’s behavioral repertoire, the more rigidly it is confined to its environment, and must depend on the environment not changing for it to survive without the emergence of favorable mutations and/or instincts.
Conversely, the ability to respond based on experience allows a flexibility of response that has a high survival value when dealing with changing environmental conditions. Intelligence – the ability to respond flexibly to environmental stimuli based on experience – is therefore synonymous with adaptability and it allows an animal to cope with, and even exploit, changes in its environment. Whereas species with a significant instinctive makeup and relatively low intelligence are pinned to their particular native environments, species with adequate intelligence can opportunistically exploit new or changing environments. Through experience there also exists the possibility to formulate better responses and time them more appropriately than would be permitted by an instinctive response. The most important advantage of intelligence over instincts however, is that increases in the intelligence of a species lower the dependence on the evolution of new instincts or anatomical changes to meet new, changing environmental conditions. This is a critical advantage, for when averaged out over the long scales of evolutionary time it allows species possessing higher intelligence to survive more often under changing environmental conditions, compared to species possessing less intelligence and greater instinctive drives. What we can say overall is this: significantly often, environmental changes occur more quickly than either anatomical changes or appropriate instincts can evolve (29), and consequently, increases in intelligence – with their inherently lower dependence on such low probability mutations – are favored by natural selection.
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Extending this argument, it would seem reasonable to suggest that evolution will always favor the loss of a given instinct if an organism has sufficient intelligence to develop appropriate behavioral responses through experience that can compensate for its loss. The loss of instincts in this manner is necessary to prevent them from operating when sufficient intelligence exists to allow a species to respond flexibly and hence more successfully, based on experience alone. There are however, some instincts and reflex responses that operate at birth or too shortly after birth, before sufficient experience could ever be acquired from which to formulate appropriate responses. No amount of intelligence could ever adequately compensate for the loss of these types of early life instincts and/or simple reflex responses, and hence all animal species (including humans) possess them to some extent or another.
Despite the relative long term advantages of intelligence over instincts however, it should be remembered that adaptability does not necessarily require intelligence. In nature whatever can survive does survive, without needing purpose or justification. When a species produces variants that will be the catalyst for a new species both species may coexist if they exploit different portions of the native ecosystem, or if the offshoot species – by virtue of anatomical changes perhaps – moves into a previously impenetrable environment at the boundary of the parent one. In this way, one species may give rise to several species, potentially with greater intelligence, that do not necessarily compete with it. Owing to this mechanism, even after 2 billion years of evolution significantly many single-celled life forms still exist today that possess no intelligence and are purely reflexive in their responses.
The aforementioned relative advantages of intelligence over instincts have important evolutionary consequences. Because intelligence is in the long run more adaptable than instinct, random increases in intelligence (though they tend to occur very slowly and in small increments) have been cumulative over time. For this reason, given millions and millions of years to evolve freely, every species of living organism on earth would have the potential to eventually give rise to a species with a degree of intelligence comparable to our own. Humans were simply the first to arrive at this pinnacle of intelligence, namely an intelligence level that gave our species the ability to overcome the extremes of environmental challenges our earthly environment had to offer, and which therefore made further increases in intelligence offer no critical competitive survival advantage. Given millions of years to evolve freely, another species with intelligence comparable to our own would in time inevitably arise.
With increasing intelligence natural selection favors the loss of progressively more instincts in order to permit an increased long term flexibility of response to changing environments. Even with increasing intelligence though, those instincts that continue to exist, as well as those that newly evolve in response to changes in environment, will do so only if the behavioral responses they deliver are not only vital to survival, but are also ones which could not, or would not, tend to be performed based on experience and the limitations of the species’ particular level of intelligence. In the case of human beings, for some yet unknown reason there was a tremendous increase in the size of the brain in the course of evolution. At some point in the expansion of the human brain, the corresponding increase in the sophistication and design of the neural circuits was so great that the loss of progressively more instincts, including the sexual instinct, was eventually favored by natural selection. Intelligence has reached such a pinnacle in human beings that we no longer possess any instincts. What humans likely possess are a few roughly etched neural templates – neural masses that are to some extent pre-wired – which facilitate skills such as learning to walk upright, or which give infants a propensity to imitate. Because certain elements of language structure are so remarkably universal, linguists believe that even some of the rules of language may be wired to some extent into our nervous systems (29, p. 79). However, such templates do not constitute instincts – they are not triggered by environmental stimuli to automatically produce complex, stereotyped behavioral responses. Rather, such templates, if they exist, merely facilitate the slow learning processes associated with the acquisition of certain skills – skills that once acquired, remain fully under conscious control.
The only preset, automatic behavioral responses we possess are the simple birth reflex responses in newborns (such as suckling, crying, smiling, grasping, the Moro reflex, etc.), as well as a few simple yet vital reflex responses including those to sudden unidentifiable sources of energy (i.e. sound, heat, touch) – something which will always have a survival value. Based on arguments that have been given, we can say that only humans were able to lose all of their instincts, particularly the sexual instinct, because only humans acquired the level of increased intelligence that made it possible to do so. It should be clear that because the relative advantages of intelligence over instinct with respect to long term species survival have been argued based on statistical considerations alone, the diminution of instincts with increasing intelligence must be viewed as a truly universal phenomenon. Even in the farthest reaches of the universe where life remains to be discovered, evolution will either have proceeded, or be in the process of proceeding to the extent where only the automatic early-life reflexes and other simple reflexes will remain. In terms of intelligence, that is as far as evolution can proceed in any given environment. Life anywhere else in the universe with intelligence comparable to our own will also be found to lack a sexual instinct.
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The advantages of gaining full conscious control of reproduction were great. Could societies having any resemblance to our own present ones exist if human beings possessed a sexual instinct that made individuals unable to resist the urge to engage in heterosexual intercourse whenever sexual pheromones were being given off by individuals of the opposite sex? How much more constantly competitive and aggressive would our societies be? Rape would not be punishable by law because individuals could legitimately claim an inborn lack of sexual control. Would anyone be able to concentrate for long on any task with so much sexuality being communicated through the air? Certainly, monogamous relationships could only exist if couples lived in relative isolation, much like gibbons do. Hence, the loss of the sexual instinct made possible a sexual self-restraint that was important because acceptable codes of sexual conduct were likely beginning to be forged between individuals, and something like uncontrollable rape would have undermined the social cohesion that was vital to human survival. As humans forged their way into new environments during various periods of global expansion, full conscious control of reproduction would also have alleviated the communal burden of supporting children born at inopportune times, before the idiosyncrasies of local environments and their resources became sufficiently predictable. This would have reduced infant mortality and also prevented situations where undernourished females became pregnant during lean times, thereby endangering their own lives. As competing groups of human beings arose, full conscious control of reproduction also empowered each group with a means of increasing their populations in response to ongoing conflict. In all these ways, the loss of the sexual instinct was clearly advantageous.
To this point, it has only been shown why evolution will favor a decline in the instinctive makeup of a species’ behavioral repertoire in response to sufficient increases in intelligence. The brain mechanism by which this occurs has not yet been discussed. It is appropriate to do so now. There is a relationship between instinct and intelligence that has hitherto not been appreciated. This relationship is crucial in understanding how and why instincts are lost, and in particular it will help us account for the loss of sexual instinct in humans. At a most basic level, consciousness may be viewed to exist for the purpose of making decisions, and all our decisions involve the control of our muscles, predominantly skeletal muscles (which enable movement), irrespective of whether movement is actually involved in the response. Consciousness makes possible a centralized discrimination and prioritization of movement options in such a manner as to prevent conflicts of movement, while permitting purposive behavior to be executed. Thus, the product of consciousness is always a movement response. A response, used in this sense, is really the sum of all (non-conflicting) movements being expressed at any given instant. It may involve no movement of any body part, a simple movement involving control of a single body part (i.e. a hand), or a more complex movement such as simultaneously clenching a fist and raising a leg, for instance.
Let us suppose for example, that two different responses are being considered by your brain: one involves clenching your right fist, and the other requires your right hand to be held open. Your brain must choose one of the two options, because both cannot be performed simultaneously by the same set of muscles. It is important to emphasize that it is not being suggested that two or more different movements (involving two or more body parts) cannot be performed simultaneously; clearly multitasking is something consciousness permits. Every conceivable movement is made possible by an associated neural program, whether the program is written based on experience, or whether it has been pre-wired before birth. Depending on their complexity, two or more movements (or tasks) may be initiated and run simultaneously if they do not involve conflicts of movement. If the tasks being performed simultaneously are complex, the seat of consciousness must constantly switch back and forth from one task to the other, directing each successfully by making appropriate changes to motor control. With practice, even complex tasks require less mentoring, operating largely at an unconscious level during certain stages of execution.
We can divide all nervous impulses into two classes: those that compete for control of the seat of consciousness, and those that do not. All nervous impulses generated due to events outside the body, and impulses generated internally due to biological needs (such as food, water, mating, and the elimination of pain, etc.), compete for the singular seat of consciousness because all these events require responses that involve the control of movement, and hence cannot be initiated simultaneously without some prioritization occurring at the level of the seat of consciousness to eliminate conflicts of movement. The remaining nervous impulses that are generated due to changes inside the body can be simultaneously responded to without prioritization because the required responses pertain to parts of the body (such as internal organs, glands, blood vessels, etc.) that are not controlled by skeletal muscles and can therefore be affected relatively independently without conflict with respect to movement controls. It should be clear that these arguments must apply to all organisms where a brain assumes a central coordinating role for the entire nervous system and movement is centrally controlled.
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Given that movement responses prescribed by instinctive drives, as well as those stemming from intelligence-based motivations, both potentially require whole-scale control of movement, some prioritization must take place within the brain to determine which of the two, instinct or intelligence, will predominate at any given instant in time so that possible conflicts of movement can be eliminated. Before we look at the consequences of this constraint, note that this conclusion was reached without needing to know what gives rise to consciousness, or where in the brain it resides. A neurological model to explain consciousness will now be developed to show the interaction between the neural networks responsible for instincts and those that make intelligence possible.
When the electrical activity of neurons in the brain is analyzed through brains scans, what we see is that there are many regions of activity constantly emerging, being amplified, subsiding, and shifting all over the brain. Of course, a region is not necessarily locally confined, but is rather simply a temporary cooperative activity between many groups of neurons that may be distributed in different parts of the brain. The neurons (cooperating temporarily within the domain of a given region) share some prior association and trigger each other to respond in a chain reaction of activity, all of them working in unison toward nominating a single action or response. Some of this activity pertains to the neural programs which regulate the inner working of the body and maintain homeostasis, and which are for the most part outside the realm of conscious control. The remaining neurons are competing for a share of control of consciousness because they may all be potentially involved in control of movement.
What is the basis for selecting which impulses and which areas of the brain will partake in determining the action, decision, or thought in the next instant of time? Said otherwise, what controls what each of us will do next? At first, this may sound like a ridiculous question. We tend to believe that each healthy person fully controls what he or she does next, that there is some volition preceding all our actions – in essence, that we have some level of control, often full control, over the decisions we make. Most brain researchers today however, believe that free will is illusory and that at the most basic level there is no real control over anything. Although this seems so contrary to our living experience, the correctness of this belief can easily be seen through the following simple argument: if there really is full control in any real sense over what one does, then an action must stem from a prior decision to perform the action. You are asked to make a choice between two doors, A and B. You choose door A for reasons that you consider arbitrary, or perhaps you were given information of what was behind each door. It does not matter for our example. The point is that in order to choose door A you had to decide to choose door A. But in order to decide to choose door A, you had to make an earlier decision to decide to do that as well.
This argument, namely that every action requires a decision, and very decision requires a prior decision, can be carried on indefinitely and it logically leads to the following dilemma: how does action arise, given that every action (in accordance with the premise of full control over action) seemingly requires an infinite number of prior decisions? And yet, living organisms do perform actions in a finite span of time. This dilemma of course only arises when we refuse to accept the reflexive nature of all action, of all behavior, something which is a natural consequence of causality – the principle in physics that says that nothing can occur spontaneously, that everything happens due to some physical event which precedes it. In the case of decision making, every decision (no matter how spontaneous or how deliberated,) must be viewed as a reflexive response to prior experience. Reflexivity in response eliminates the dilemma of requiring infinite decisions preceding action, and it allows us to view actions, whether instinctive or intelligence-driven, as being reflexively executed neurological responses to stimuli. The neurological model for consciousness we are after must reflect this lack of ultimate control in behavioral response.
With this goal in mind, we will begin by looking at the pioneering work of Wilder Penfield, who in the 1950’s made important findings regarding the workings of the brain. Penfield worked with epileptics (28, pp. 459-461). Epilepsy usually results from injuries to the cortex. An epileptic attack occurs when spontaneous and uncontrolled discharges spread from the damaged region into the surrounding healthy tissue. If the discharges spread only a short distance, the attack is mild, whereas if they spread over the entire brain, a catastrophically severe seizure results. Penfield and others developed techniques that allowed them to cut away a flap of bone from the roof of the patient’s skull, stimulate the exposed brain surface with electrodes, and thus locate the diseased region, which was to be excised. This procedure also allowed them to determine the areas essential to proper speech, sensory, and motor function – areas that were not to be removed. Such operations were performed while the patient was under only local anesthesia and was fully conscious. This was possible because interestingly enough, there are no pain receptors in the brain itself.
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During the mapping phase of the operation, Penfield stimulated various parts of the brain while the patient, who could not see him and did not know when stimulation was on or off, answered questions that were posed. When visual areas of the brain were stimulated, patients had visual experiences – they saw streaks of color or flickering lights. When stimulated in the auditory brain regions, they heard clicks or buzzes. Stimulation in still other areas produced an involuntary movement of some part of the body. When Penfield stimulated certain regions of the temporal lobes of the cerebral cortex, he was able to cause patients to experience remarkably detailed recollections of past events. Each time such memories were invoked in these experiments, or other physical sensations were experienced, they obviously had to occupy the seat of consciousness for the patient to become consciously aware of them.
The hypothesis that can be made from Penfield’s work is that targeting electrical stimulation to a particular part of the brain triggered a wave of neural activity partly centered on the stimulated neurons (but also involving other associated neurons) that was presumably greater than competing regions of cooperative neural activity elsewhere in the brain, and thus allowed the region associated with the electrically stimulated neurons to enter into the realm of consciousness. For instance, if the strength and intensity of the electrical stimulation were gradually decreased each time, one would intuitively expect a situation to eventually arise where the electrical stimulation would not trigger any conscious memory or sensation. Extending this thought, we may say that consciousness is controlled by those regions or cooperative groups of neurons, however they may be dispersed, having the greatest neural activity/strength at any given instant in time. Consequently, consciousness may be viewed as shifting regions of spatiotemporal patterns of dominant neural activity within the brain, with some interconnectedness (and possibly some overlap) existing between all competing regions, each region suppressing all other competing regions with a strength proportional to its own total neural activity, thereby allowing non-conflicting regions with the highest neural activities to dominate and sufficiently suppress all others. Furthermore, any two or more regions vying for consciousness may mutually reinforce each other, if they share a common behavioral objective and hence require the same movement controls to be exercised. It is known that although there are dozens of different types of neurotransmitters, they can essentially be divided into two classes: those that inhibit the firing of neurons, and those that promote the firing of neurons. With this model of consciousness, we may now infer the role of each: they make the competition between regions of neurons possible.
One may ask how the subject of consciousness changes from one instant to the next. How do dominant regions lose control of the seat of consciousness so that other regions can take over? After every neuron fires, there is a brief refractory period during which it cannot fire. It may be conjectured that it is during this refractory period that a dominant region may lose its share of the seat of consciousness and allow another competing region in the next instant of time to occupy it. Using this model, one may conjecture that the subconscious does not operate ‘in the background’ at the same time that consciousness operates, but rather, competes for the seat of consciousness in the same manner as other neural events. As Benjamin Libet has proposed, a certain duration or “time on” of neuronal activity may be one controlling factor separating conscious and unconscious functioning (30). Hence, the subconscious must be seen as simply the totality of all those thought patterns that sporadically dominate the seat of consciousness for periods too short to be perceived or remembered. In other words, their associated neural regions do not recursively share the seat of consciousness with sufficient frequency to impinge upon conscious awareness. It may be that it is only by assuming the seat of consciousness (even for short instants of time) that the characteristics of the subconscious become modified based on experience.
Using this model, we can conjecture that when an instinct first evolves, evolutionary processes must ensure that its total neural activity during operation is greater than that which could be associated with any intelligence-driven response that would generally be possible based on the existing neural design. It is only in this way that nature could ensure that an instinctive drive, when required, could consistently displace from the seat of consciousness any competing motivational drive based on intelligence. Having defined intelligence as the ability to act based on experience, we can say that increased intelligence must mean not only an enhanced ability to form valid associations between data flowing through the seat of consciousness, but also an improved memory storage system that would potentially allow more and more prior experiences to play a role in determining appropriate responses. Therefore, an increase in intelligence must necessarily mean an increase in the average number of neurons being simultaneously involved in conscious decision making. The consequences of this is that if in the course of evolution the intelligence of a given species increases sufficiently, the number of neurons that become simultaneously involved in intelligence-driven responses, and their combined neural activity, can at times be greater than that of a particular instinct, one which was designed to be able to dominate consciousness at an earlier evolutionary period when the brain was less evolved and the species was less intelligent4.
4 The model for consciousness that has been proposed may explain why the primitive reflex responses seen in human babies (such as the palmar grasp reflex and the Moro reflex) only operate for the first few months after birth: they become eclipsed by larger and more complex neural networks that are created by life experiences as the baby matures. The simple reflexes at extreme infancy are able to dominate the seat of consciousness and hence express themselves as required because, although vastly outnumbered by the ‘uncommitted’ neurons that will in time freely form neural networks based on experience, at this early stage these experience-based neural networks are still forming, they are smaller, and do not have the neural outputs exceeding those associated with the simple early-life reflex responses. In the short span of a few months however, as the infant matures into its 5th or 6th month of life and its experience level rises, the size of the experience-based neural networks and their neural outputs surpass those of the early-life reflex responses, and are therefore subsequently able to prevent them from assuming the seat of consciousness in order to operate. Most interestingly, in advanced Alzheimer’s disease, and also in patients with vascular dementia, it is not uncommon to see the re-emergence of some primitive reflex responses such as the palmar grasp reflex. Their renewed ability to express themselves must rest in the fact that disease has (in such instances) substantially damaged and fragmented the larger and more robust experience-based neural networks that were built over a lifetime, greatly reducing their associated neural outputs and consequently preventing them from continuing to be able to suppress the neural networks associated with the reflex responses. The explanatory power of the proposed Neurological Model of Consciousness to account for the expression, subsequent suppression, and (in some cases of disease) later re-expression of these primitive reflex responses, appears to validate it.
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We may further postulate that increases to the neural output of neural networks associated with instincts are only favored by natural selection under circumstances where increases in the complexity and size of the neural networks associated with intelligence have not resulted in a level of intelligence high enough to eliminate the species’ reliance on a given instinct. Under such circumstances, evolution will simultaneously favor a parallel increase in the neural output of the particular instinct network to the extent that it will continue to be able to operate successfully by assuming the seat of consciousness as required. It follows therefore, that for each instinct there exists a critical level of intelligence above which a parallel increase in the strength of the instinct will not be favored by natural selection because the intelligence level is sufficiently high to allow the species to survive without the particular instinct, and hence become more adaptable in the long run. With this model, we can see how instinctive drives can become gradually diminished over long evolutionary periods if the intelligence of a species increases sufficiently.
The instinct requiring the largest compensating intelligence level before it can be lost is the sexual instinct. The validity of this statement will become evident in a later section when we look at the social infrastructure and all the foresight that was necessary to see a benefit in producing offspring, as well as the intelligence that was needed to be able to link heterosexual intercourse with reproduction. Increases in intelligence are exceedingly rare events. For some yet undetermined reason they occurred relatively frequently and in large increments in the course of human evolution, and allowed us to reach an intelligence level where the loss of all our instincts, including our heterosexual instinct, was favored by natural selection. Thus far, only humans have reached this intelligence level. In the case of the Great Apes, although their intelligence level is considerably higher than that of a mouse, for example, it is still not sufficiently high enough to allow them as a species to lose their sexual instinct. Consequently, the neural output of their heterosexual instinct network grew proportionately with respect to increases in intelligence in the course of evolution, all the while continuing to possess a neural output larger than those of the largest neural networks that could be assembled based on intelligence.
With respect to losing our sexual instinct however, there arose a unique situation because whereas the loss of all other instincts typically pertain to behavioral responses that no longer have a survival value under changed environmental conditions, the sexual instinct performed a critical function – it ensured procreation, something which had to continue for our species to survive. Hence, the complete loss of our sexual instinct was not favoured by natural selection until reproduction was sufficiently assured based on conscious, experience-driven motivations alone. Accordingly, it was only eliminated after human beings started to have vested interests in reproducing, and had also discovered what caused reproduction. For reasons previously outlined, once the knowledge and motivation to reproduce existed the advantages of quickly and completely losing the sexual instinct were great indeed.
It has already been stated that one of the ways in which an instinct can disappear is by it atrophying through disuse - a relatively slow process in evolutionary terms - and it becoming eventually suppressed by larger neural networks associated with intelligence-driven motivations. Concurrent increases in intelligence can accelerate this process of instinct elimination. However, both these processes tend to be erratic and slow, thus making it likely that instincts lost in this manner will disappear gradually over relatively long periods of time. When the advantages of quickly losing a particular instinct are too great however, any mutation or developmental change that can single-handedly either eliminate the instinct's neural networks in the brain, or interfere with the developmental set up of its associated sensory receptor cells, is favoured by natural selection. Did such a mutation and/or developmental change ever arise in the course of human evolution, and all-at-once completely eliminate the sexual instinct in the human line?
A nasal plug is a short-lived mass of epithelial cells that forms and completely seals the nostril openings during fetal development. It appears to be a general
mammalian feature, though its true function remains unknown.
Its functional significance may be conjectured by noting its location, right within the area of olfaction and pheromone detection.
It is known that in mammals, a sexual instinct relies on detecting and responding to sexual pheromones. Typically, a specialized organ (with pheromone receptors) within the
nasal cavity, called a vomeronasal organ (VNO) -
distinct from the area that permits olfaction - transduces chemical signals from pheromones into nervous impulses, and passes them on to areas of the brain involved in
triggering the expression of the sexual instinct. The critical reproductive role played by sexual instincts, and the consequent need to have them reliably functional,
may have early on in mammalian evolution warranted isolation of the developing VNO to ensure its proper development without interference from extraneous chemical agents.
It may be hypothesized that the presence of the nasal plug in humans, from roughly the seventh to the fifteenth week after conception, was a
developmental change compared to its period of existence in all prior hominid species -
and was possibly responsible for single handedly eliminating the sexual in the human line.
This hypothesis is strengthened when one considers that the nasal plug exists at roughly the same time that important changes are occurring in sexual differentiation of the foetus.
By 7 weeks, the ovaries appear in the female embryo. In the male embryo, a gene on the Y-chromosome produces a substance causing the testes to begin to differentiate.
Until the 9th week there is no apparent difference in the external genitals of boys and girls: they both look like boys. Both have a creased bump that is the phallus.
In boys in the 9th week of development, the testis releases a burst of hormones, the crease fuses and disappears, and the phallus stays. In girls, nothing genitally
dramatic happens during week 9, but over the next few weeks the crease stays and the phallus retracts to become a clitoris. It may have been the case that the
differentiating sex organs of the foetus were the source of the chemical messengers in the amniotic fluid that the nasal plug now served to block.
More specifically, for the gender-specific form of the sexual instinct to be set up at a receptor/neural level, it seems reasonable to conjecture that delivery
of chemical signals denoting gender - arising from sexual differentiation of the foetus - to the developing pheromonal receptors, and perhaps the developing foetus's brain, were necessary.
An intriguing question might now be posed: would any genetic manipulation of a human fetus, capable of deffering the timing of the nasal plug to occur after sexual diffeentiation, be consequential? Would such a baby be born with a functioning sexual instinct? Once the sexual instinct ceased to function, the integrity and functionality of both - the chemosensory receptors in the nasal cavity, and that of the neural circuits in the brain encoding for the sexual instinct - were no longer under the scrutiny of natural selection, and this in theory would have permitted random mutations to accumulate over time and render the receptors / neural programs of the sexual instinct permanently damaged and irrecoverable. Moreover, a prolonged evolutionary period of disuse would likely have resulted in an atrophying of the sexual instinct's neural circuits, or its neurons being absorbed by the remainder of the brain. It is therefore doubtful that the nasal plug serves the role it once did. It may however, be the 'smoking gun' of human sexual evolution, a developmental change of monumental consequence that was the decisive last step on the road to modern humans, conferring upon the human line an unprecedented reproductive freedom. The decline of human sexual instinct and the parallel increase in intelligence marked a new era in human history. Human beings overall as a species could no longer ever be truly sexually free like their ape ancestors, freely engaging in any type of sex consciously and relying upon an instinctive heterosexual drive to ensure species survival. Because unregulated sexuality from now on had the power to affect human populations, and hence the vibrancy of societies, sexual freedoms permitted by any given society would be determined by the population requirements fuelled by the motivations of that society at any given time.
All material copyright 1991-2010 / Christopher Gomes