"Aristotle was famous for knowing everything.He taught that the brain exists merely to cool the blood and is not involved in the process of thinking. This is true only of certain people." Will Cuffy.
When a new gene modifies the hardwired neural connections of some brain region (by creating new connections between neurons or by changing the size of synapses of existing connections), the function of the modified brain region is likely to be in conflict with other brain regions. Since the purpose of the brain is to influence behavior on behalf of the genes, brain regions necessarily are in competition with other brain regions for influencing behavior. Rarely is the individual aware of this conflict. When the conflict is extreme, when it affects emotional state, we might refer to the conscious manifestation of the unsettled emotional state as "cognitive dissonance." Almost all competitions for influencing thought and behavior are worked out peacefully below conscious awareness.
The Brain as a Mechanism
The brain is a mechanism, albeit a "wet chemistry" mechanism. Just as all chemical interactions are merely physical interactions at the atomic and molecular level, so are all brain interactions ultimately the working out of physical relationships between atoms and molecules. When we say that current flows along a neuron's axon, we refer to a physical process of the axon's membrane becoming more permeable to sodium atoms, allowing charged atoms to enter the axon from the surrounding fluid, etc. Every motion of every atom is governed by a = F/m and quantum physics (as explained in Chapter 1). It would be cumbersome to try to understand brain function by invoking this basic level of physics since such a task would be incomprehensibly difficult. Wet chemistry is a less cumbersome level, but still too daunting for most brain studies. A more tractable, and hence powerful, level for understanding brain function is to think in terms of neural networks.
A neural network is a partially interconnected group of neurons. One network may also have connections with other neural networks. The term "partially interconnected" is important, for it is the genes that determine the overall pattern of which connections exist. A "fully connected" network is impractical when the number of elements (neurons) exceeds a few hundred, since the number of possible connections between elements grows as "N 1 factorial." Synaptic connections between neurons are either excitatory or inhibitory. In the brain a neuron may have many synaptic connections to a specific target neuron, and absolutely no direct connections to most other neurons.
Consider all neurons in the network that are connected to one individual neuron. At any moment some of them will be in the process of "discharging," causing their synaptic connections to other neurons to become active (releasing neurotransmitters across a synaptic gap). Each target neuron sums the excitatory and inhibitory discharges on its cell body, and if this sum exceeds a threshold it in turn discharges, causing neighboring neurons with which it has output connections to possibly also discharge by the same process that led to its discharge. A neural network can be made to "resonate," which is a way of stating that a pattern of firings within the network continues for many clock cycles (tens of milliseconds in the brain) once triggered by an appropriate stimulation from the connections that the neural network has with neighboring neurons (or neural sub networks). All of this is well understood by neural network specialists, and I provided a brief introduction of it here to give the reader a taste for the mechanistic, or reductionist nature of brain phenomena.
An even more useful level for understanding brain function is to speak of brain regions in terms of their function. When we use such terms as "the reticular activating system" (RAS) we know that the elaborate neural network explanation for the region's function is theoretically possible but at the present state of brain understanding these a = F/m ways of accounting for a regions function are not very feasible or even useful. So we proceed by saying, with blatant anthropomorphism, that a cortical region sends a "request for activation" to the RAS, and if RAS "grants the request" this originating cortical area becomes more active, and this activity enables it to increases it's inhibition of "competing" cortical brain areas, allowing it to succeed in "achieving behavioral expression." Even that way of speaking is cumbersome, but it captures the flavor of the mechanistic competition of one cortical neural network, having gene directed hard wirings, with a neighboring cortical neural network, having other gene directed hard wirings.
In any description that attempts to achieve brevity, such as this one, there are many unmentioned details about which a specialist could complain when they are left out. Sure, I didn't mention neurotransmitters, and their re uptake, or their breakdown, and dozens of other things going on, but they are all mere elaborations of the same basic physical mechanism. Additional details are too numerous to mention, but also too similar in terms of their ultimately physical action to warrant mention for present purposes.
I have risked boring you with some physics of the brain in order to show how in principle brain function can be understood as coming under the influence of the genes. For it is the genes that direct the process of "pre wiring" the brain. Initially, too many connections are created, and for several years after birth approximately half of the neurons and their connections wither and are lost. But the starting point at approximately birth and some years later (depending on the brain region), the overall placement of neuron type and the majority of connections from each neuron to others, is supervised in a general way by the genes. Some genes influence one region (i.e., a neural network) and not others, while other genes influence several different specific regions. Any single neural network is most likely the result of several genes.
This way of viewing brain development, emphasizing as it does the role of evolutionary forces on the architecture and interconnectedness of the brain, leads to a perspective in which overall brain function is the working out of a competition of mental modules, each endeavoring to express itself by maximizing its influence over behavior. The "modularity of mentality" perspective, in which modules compete with certain others, is still controversial (for reasons I don't understand). And the idea of connecting specific modules to specific genes is so amorphous a speculation that it is not yet a sub discipline of the brain sciences. Evolutionary psychologists adopt this view (see Barkow et al, 1992), and it seems inevitable to me that sometime in the 21st century neuropsychologists will also, and maybe late in the 3rd millennium people who call themselves psychologists will come aboard.
Recent Evolutionary Hotspots in the Human Brain
In humans the prefrontal cortex is proportionately larger than the rest of the brain compared with all other animals. Thus, there's an evolutionary trend revealing that the prefrontal cortex has been the focus of recent human evolutionary adaptations. This makes the prefrontal cortex one of the most interesting brain areas to understand.
Comparison with other mammals reveals that the tertiary cortices of the posterior lobes are also proportionately larger in humans, indicating that they also have been undergoing rapid evolution in recent evolutionary time. The most obvious example is Wernicke's Area, located in the temporal lobe's tertiary cortex. So add LB posterior lobe tertiary cortical areas to the list of interesting human evolutionary "hotspots."
Are there any evolutionary hotspots in the human right brain posterior lobes? The short answer is "no." It therefore seems that the left brain has evolved more during human history than the right. It is even tempting to suggest that what distinguishes humans from other animals is their left brain.
Note one qualification that applies to most usages of the terms "left brain" and "right brain": about 2% of the population has laterality reversed. In these people language and other sequential tasks are performed by areas in their right brain, and holistic functions are performed by their left brains. Most of these people are left handed with the unhooked writing position. Neuropsychologists use the terms right brain and left brain to refer to the specializations found in that 98% of the population with "normal" lateralization. So, whenever the terms LB and RB are used, think of the left and right brains of the 98% of people who possess the normal lateralization.
Why is the Left Brain Evolving Faster?
What is it about the left brain that gave it the greater burden for advancing human evolution? One clue comes from the microscope. The left brain isn’t as "white" because fewer neurons are coated with an electrical insulator composed of a whitish, fatty substance called myelin. The greater myelinization of the right side is required by the greater proportion of right side neurons that connect with distant neurons through long axons. In contrast, neurons on the left side are more often connected to nearby neurons, and therefore require less insulating myelin.
But what does this mean? The left brain is characterized by a neural architecture in which isolated neural networks perform their specialized tasks and then communicate their results among themselves through a smaller network of interconnections. Functionally, this is a better architecture for performing sequential tasks. Language is a good example. A sentence consists of a sequence of sounds that have to be in their proper place in order to convey the intended meaning.
Some have speculated that the evolution of lateralization started with our fruit eating ancestors, who would use their left arm and hand to support themselves and maintain balance while the right hand reached out to pick fruit. Fruit picking is somewhat sequential, as the hand must be guided by the eyes to reach for the ripe colored fruit, grasp it with fingers using just the right force, tear it off the branch, and then bring it to the mouth for eating. Recall that the sequentially performing right hand is controlled by the left brain, which would therefore be the one requiring a sequential neuronal architecture.
If embryological development provided for a sequential brain architecture in one part of LB because it evolved in that location by chance for the purpose of picking fruit, then when another sequential task became adaptive the forces of evolution would more often find a favorable mutation of genes that code for the left brain, since fewer mutations would be needed to add to a pre existing architectural capability for the task of producing a new sequential capability. This, according to one speculation, is why the left brain took on most new sequential tasks presented to it by subsequent evolutionary opportunities.
Brain "Dominance"
Damaging brain strokes in LB tend to produce more noticeable deficits than those in RB. This is because LB performs language tasks. For this reason, unfortunately, it has become customary to regard LB as the "dominant" hemisphere. But, to call LB dominant over RB for this superficial reason is misleading!
The limbic system (that drives emotions) influences the RB frontal lobe more strongly than the LB frontal lobe. This makes RB a better candidate for playing a dominant role. RB gives overall shape to behavior, while LB is relegated to a supporting role. When LB began its sequential specializations it must have been a useful "tool" for RB (which in turn was a tool for the limbic system, which in turn was a tool for the genes). "Values" are more likely to originate with RB, and I claim that the genes have put in place more of their “agenda protection circuits” in RB. The natural condition, I suggest, is for RB to be "in control," using LB to help achieve genetic ends. This important thought will come up repeatedly in subsequent chapters, and it is a basis for the individual to design strategies for individual liberation from the genes!
Even though RB has control over decisions that matter to the genes, I believe the seat of consciousness is in LB's tertiary cortex. This may seem to be a curious arrangement, but upon further thought it makes sense. We associate consciousness with planning future activities. Recall that brain structures with a more ancient origin usually have veto power over behaviors, as when RAS handles requests for action and either "authorizes" or "vetos" them. It may happen that RB tasks LB with imagining future scenarios and their likely consequences, while RB, in close consultation with the (emotional) limbic system, then makes a “judgment call” and decides whether or not to proceed with the plan of action that was under consideration. After imagining scenarios, one may be accepted while others are vetoed; all of this may occur at a subconscious level, with RB working in conjunction with the ultimate authority: the limbic system. LB must make sense of the outcome, so it "confabulates" an explanation for the chosen plan of action. Michael Gazzaniga names the left prefrontal module that performs this confabulation the "interpreter" (see Gazzaniga, 1978, p. 146; especially Gazzaniga, 1988, p. 229; and Gazzaniga, 1992, p. 121).
Rational thought has become an ever more important tool for evaluating the consequences of hypothetical actions. This is why LB must have been such a "hot spot" for human evolution for probably the past 130,000 years, and especially the past 12,000 years.
Before LB began to evolve its unique specializations, perhaps 250,000 years ago, the function performed by a damaged area of one side could be easily assumed by the counterpart area of the other side (relying upon the corpus callosum for inter-hemispheric communication). Lateralization brought with it risks of lost redundancy, yet this loss was apparently smaller than the gains from being able to solve problems that were common in the late Pleistocene and Holocene. We must assume that some important need started the selection for LB specializations. It may have been the payoffs for improved tool making, language, or dealing with a more complicated social setting that required logical thinking skills (such as "theory of mind" abilities).
Whatever the original impetus for LB specialization, it seems to have assumed the new duties as if forsaking redundancy with abandon. Just consider the list of important LB skills that are unique to humans: verbal, analytic, logical, rational, time oriented and deductive skills. It seems inescapable that LB has acquired more recently evolved, distinctly human adaptations than RB. When you damage LB (posterior lobes), you get a regressed, more primitive person; but when you damage RB (posterior lobes), you get someone handicapped in mostly long standing, primitive traits.
Since recently-evolved traits are the least entrenched, and are most subject to disruption by the latest mutations, we should expect to encounter a wider variation of ability for the recently evolved traits than long established ones. This view correctly predicts that literacy, being a recent human achievement, should be more variable than other abilities; whereas verbal language ability, having started its evolution much earlier, should be more robust. It also explains why everyone is capable of anger, fear, sexual arousal, and jealousy, while some people are deficient in logical, rational and analytic ability.
Brain Modules and Genes
As with any organ, no single gene codes for the construction and function of an entire organ; many genes contribute. When several genes contribute to the same trait, they constitute a group of "polygenes." For example, one gene may play a major role in forming the heart's left ventricle, with minor support from other genes; another gene may have major responsibility for assemble of the right ventricle, but also contribute to the left ventricle's assembly. Both genes belong to a polygene group for constructing the heart.
The same argument applies to the brain. Many genes are required to assemble the primitive brain stem's reticular activating system, for example. Others assemble various parts of the limbic system. Finally, other genes assemble the surrounding neo cortex, LB and RB, and the interconnecting corpus callosum.
All brain components are interconnected with other components, and they function together as if they were "designed" to work together. When the various components work together it is because they have been present in the genome together long enough to adapt to each other's presence. Initially, when a new brain component is mutating into existence, it is useful to understand that the pre existing components were not meant to work together with the new component. Each new "addition" occurs against a background of pre existing brain components which had worked together successfully prior to the appearance of the new component. As components appear, they, as well as the pre existing components, co evolve to enhance the working relationship.
When our ancestors began to lose their fur, the fur altering allele had to co evolve with the gene(s) that made furry babies irresistibly attractive to mothers. There are many baby features that cue the mother to act like a mother, and the lack of fur amidst all the other baby features must have been disconcerting to mothers during the transition. Today, a cat resembles a primitive baby in size, weight and furriness. The fact that many people find cats irresistible, and sometimes hold them like a baby and speak "motherese" to them, suggests that the ancient collection of cues for eliciting mothering behaviors still exist in some residual form.
As the gene for a new brain module is selected, it evolves to be compatible with pre existing modules, and the genes for the pre existing modules simultaneously undergo modification in response to the new module. The evolution of genes that affect the brain is governed by the consequences each gene allele has on the success of the individuals carrying the genes to survive and reproduce. Or, to be more rigorous, a brain related gene allele's success depends on its ability to produce phenotypic changes that work with the prevailing phenotype in a way that enhances the individual's success in delivering all of its genes to future generations, under a typical range of environments.
Since there are many ways to construct any organ, there will be many potential competitions between genes. An allele that produces a larger heart ventricle is in competition with alleles that produce smaller ventricles. Natural selection achieves a better heart by rewarding individuals having the better heart, and thereby rewarding those gene alleles responsible for producing the "better" ventricle size.
Whereas it may be easy to comprehend how a gene that codes for anatomy, such as heart ventricle size, can be in competition with another gene, it is more difficult to imagine the competition between genes that assemble brain circuits governing behavior; nevertheless, it happens. A brain gene may be in competition with another allele, even while it is "cooperating" with a different set of brain genes. (Excuse the anthropomorphizing; if it bothers you just convert my brief descriptions to a rigorous lengthy one).
Genes compete for phenotypic expression at impressively high conceptual levels. Language ability evolved by creating proto Wernicke's Area circuits and proto Broca's Area circuits within LB (plus other cortical areas, interconnections and anatomy modifications). This was a major accomplishment, involving many small incremental steps. Other frontal lobe traits, such as assertiveness, aggressiveness, nurturance, empathy and altruism, are under significant genetic control, accounting for approximately 50% of observed variance (Rushton, 1997).
It has once again become fashionable to think of brain function as being "localized." Although "phrenology" deserved to be discredited, it's ultimate theme was correct: namely, that most attributes of brain function are determined by activity in specific brain regions. They were wrong to place "combativeness" where the temporal and occipital lobes join, for example, but it is localized, and belongs in the prefrontal lobes (probably in RB). Many functions require the participation of several specific areas. Productive language is a well studied example, exhibiting involvement of specific parts of the left frontal, temporal, and parietal lobes. Physical damage to each region produces specific, predictable language deficits. This means that something as complicated as language requires the cooperation of regions with specialized capabilities, and the fact that these regions aren't next to each other, but are located in different lobes, does not undermine the view that brain functions are localized.
This reductionist way of viewing brain function is supported by the notion that a finite number of genes assemble the brain. Polygenes create brain modules, consisting of specific physical networks of interconnected neurons along with an approximate set of synapse sizes.
Mental function, like brain architecture, appears to be modular (Fodor, 1983, Gardner, 1983, Gazzaniga, 1985, Restak, 1986, Cosmides, 1989, Cosmides and Tooby, 1992, Restak, 1994). Granted, the modules interact with each other, but they can be usefully considered as modules with functional specifications. Consider the analogy of a system's analyst parceling out the task of writing a large computer program to several teams of programmers. Each team is charged with delivering a module of code that meets functional specifications. A programming team is like a gene, their code is like a hard wired brain module, the function performed by the module is like a mental module, the joining of modules is performed by the systems analyst, and the running of the completed program code is like a brain performing mental tasks.
Modules compete with each other for "expression." For example, one area in the occipital lobe may be able to correctly perceive and identify an object from its visual appearance, a cup for example, while another area in the parietal lobe may be able to identify the same object by its tactile feel, and another area in the temporal lobe may be able to infer the same object's identity from the sound it makes when set down upon a table top. Each will produce a signal of recognition when the necessary stimuli are presented, and somewhere in the adjacent tertiary cortical regions (where the three posterior lobes merge) the object identification of "coffee cup" is made.
It would be ridiculous to conclude from this capability that there's a "coffee cup recognition" gene. Rather, there's a polygene created module for recognizing curved shapes, another for shadings that contain surface topography information, etc. These modules are interconnected so that experience with the real world, or at least one that contains coffee cups, allows synapse strengths to be modified such that when a coffee cup with arbitrary orientation is viewed the various percepts are joined together to trigger the perception "coffee cup."
The brain's experience with the real world adjusts synapse strengths so that no other region will be triggered to "resonant" activity when a cup is presented to the posterior lobes' primary cortex (sensory input) areas. If a totally unfamiliar object is presented, there will be a competition to identify it. When shown a German beer stein for the first time, the occipital lobe (sight) may report "something like a vase," the parietal lobe (feeling) may report "something like a large handled cup" and the temporal lobe (hearing) might report "like a brick." The discrepant reports would compete, as the frontal lobe might want to engage in further exploration to resolve the discrepancy (which is a job for “consciousness).
Figure 8.01. Reversible Goblet, illustrating competition between brain regions vying for "acceptance" of their respective interpretations. Look for the two dark face profiles, facing each other.
The many examples of images with "figure/ground reversal" conveys in the most dramatic way how competing modules strive to prevail in having their "interpretation" accepted. Escher drawings (Escher, 1961) exhibit a wealth of figure/ground perceptual competition.
Identifying situations, such as a "social situation," is subject to the same perceptual competition, although the frontal lobes will play a more active role in generating competing hypotheses. Context may be an important "input." "Do I know the person? Does he have a hostile stance? Does he have comrades?"
Consider the metaphor of a school classroom for understanding brain module competitions. The teacher poses a question, and the students try to understand the question and come up with an answer. Some students will both understand the question and have a possible answer, and they will raise their hand. The teacher calls upon a student to present an answer, quite often it's the student whose hand is waving most excitedly (or maybe the student with the best past performance), and after hearing the answer she passes judgment. If it is incorrect or inadequate, the teacher calls upon another student.
This classroom example is a good metaphor for how the brain works. When a person is presented with an unusual situation, some modules in the brain "recognize" something, and they request activation by the RAS (reticular activating system). The RAS, working in coordination with a higher level cortical system that keeps score of previous successes and failures, tentatively authorizes a module to "present its case" for evaluation. The module that wins the first round for presenting its interpretation may be the one that most strongly felt it understood the situation and had the correct interpretation (like the student who waved his hand most excitedly); its request to RAS may have been the strongest among the competing modules. When the first module presents its interpretation, some type of evaluation occurs (perhaps involving the reaction of other modules), and this interpretation may be accepted, or it may be tentatively rejected. If it is rejected, or set aside, another round of RAS requests for activation is performed, and another judgment is made. At some point, a winner is declared, and the winning module's interpretation is what serves as the basis for any required action. The losing modules do not simply stop trying to compete for RAS attention, however. As more perceptions occur, or as behaviors either validate the accepted interpretation or invalidate it, the other modules are ever ready to renew their claim for being heard. The example of the "reversible goblet" shows how this process "feels" for the domain of visual interpretations.
The process of generating behavior is also a competition between competing frontal lobe modules. "Shall I turn and run? Or scream? Or attack?" Imagine that one person may inherit a propensity to "attack" in ambiguous social encounters, while another may be genetically inclined to "run away." Just as animals have inborn temperaments, so do humans. And the mental process that precedes an action consists of a competition between brain areas. To the extent that one brain area is assembled by a different polygene group than another brain area, which is inclined to a different type of behavioral response, the genes primarily responsible for wiring the competing brain modules are competing with each other for behavioral expression. The same classroom metaphor described above can be used to understand this situation. After a situation is understood, and when action is necessary, the frontal lobe modules will compete for expression (i.e., control of behavior) in the same manner that the "understanding and interpretation of the situation" modules competed. The RAS (another part of RAS than used for “adjudicating” perceptions) receives requests for action, and eventually one module's proposed action is "accepted" (given “authorization” for initiating a behavior).
Thus, both perceptions and behaviors exhibit the quality of involving several mental modules in competition for acceptance and expression.
Does it matter whether the brain accepts, and acts upon, the perception that the sky is angry and the wind god and sun god are arguing, versus the competing perception that the wind is bringing clouds from somewhere which cover the sun and may cause rain? In the contemporary world it can matter more than it did in the ancestral world. For this reason, it matters whether RB interpretations versus LB interpretations tend to gain acceptance in an individual's brain. To the extent that genes wire brains to be predisposed to some "interpretation styles" over others, the respective genes are in competition. The next chapter will deal with this subject in greater detail.
Intelligence and IQ
“IQ is what IQ tests measure!” It should be emphasized that IQ, as measured this way, is just one of many components of what most people refer to by the term "intelligence." Ironically, IQ is not a prefrontal function. Prefrontal lesions do not reduce IQ; indeed, in some cases frontal lesions have enhanced IQ. This enhanced performance could be explained by a theory that views the frontal lobes as being prone to "interfere" with posterior lobe performance (such as a tennis player "thinking" too much); by injuring a prefrontal lobe the posterior lobes are freer to perform unhindered, boosting measured IQ. The WAIS (Wechsler Adult Intelligence Scale) IQ test has two parts: the "verbal" part and "performance" part, and these parts probe left and right (posterior) brain function, respectively. The Woodcock Johnson has two parts, also probing left and right (posterior) lobes. The WAIS verbal and performance IQ scores differ by 3 points, on average. A difference of 10 points should occur in only 5% of cases; and differences larger than this are usually caused by a lesion to one side of a posterior lobe.
This concordance of IQ scores that separately probe LB- and RB function invite speculation on the number of genes that affect posterior lobe capability on both sides. However, it is possible that a small number of genes contribute to "general" intelligence, and the rest contribute to specific abilities. This is consistent with the finding that a person's profile of subtests will have a pattern, with some parts of the "verbal" being low, and others high, while the average of the verbal parts average about the same as the average of the performance subtest parts. Psychometricians continue to find it useful to make a distinction between specific sub test performances and Spearman's (1927) "g factor" of mental ability. Tests that identify g loaded performance afford better correlations with genetic relatedness (i.e., identical versus paternal twins), and g loaded test scores are better predictors of academic performance than standard IQ tests.
Tests have been developed for assessing frontal lobe performance. The Halstead Reitan Battery includes tests of frontal lobe assessment. Components of the Montreal Neurological Institute Battery, and also Luria's Neuropsychological Investigation, also test for frontal lobe function. The frontal lobes are so complex that no single test can capture all significant features. For example, effective business executives have especially capable frontal lobes, and they excel in the development, evaluation and implementation of "big picture" strategies. The business "world of hard knocks" reveals who some of these especially well endowed frontal lobe "executive function" people are. Bill Gates, Steven Spielberg and Lee Iacoca are examples. It would be interesting to know if they would have been identified in childhood as having especially talented frontal lobes using existing tests purporting to probe frontal lobe function. Some day, tests for executive function may capture this elusive capability.
Whereas someone like Bill Gates must have superior scores for both frontal and posterior lobe function, it must occasionally occur that people are born with disparities. For example, president Jack Kennedy is supposed to have scored a mere 125 on IQ tests. He obviously would have scored higher on any executive frontal lobe test. It may be more common for people to be born with the opposite disparity, in which posterior lobe IQ is higher than frontal lobe executive ability. Indeed, this could be the more common disparity because frontal lobe function is a more recent focus in human evolution. The ability to create culture, and to absorb and use cultural elements that other people are observed to use, must have been an important pressure for human evolutionary selection during the past 60,000 years. This idea will be taken up in a later chapter.
Number of Brain Genes
It has been estimated that as many as half of the entire set of human genes have some influence upon intelligence (Weschler, 1974, as cited by Seligman, 1992). For the calculations that follow, I will assume that 30% of human genes affect the brain. In theory, every aspect of brain function can be associated with a gene that has the most control over it. (To call such a gene the "whatever trait gene" overlooks another fact, that the same gene probably affects several other phenotypic traits which are sometimes unrelated to the main trait; this is referred to as "pleiotropy.")
If the human genome consists of 35,000 genes (“pick a number!” it keeps changing), and if humans share 98.6% of genes with living chimpanzees, then humans differ from chimpanzees at approximately 490 gene locations (1.4% of 35,000). Of these 490 genes, probably more than 30% have some influence over the brain's development. Let us assume that 250 genes are responsible for making the human brain different from the chimpanzee brain.
If the common ancestor for modern humans lived 200,000 years ago, and if the human/chimpanzee evolutionary split occurred 6 million years ago, then it is possible to estimate the number of brain genes that are more recent than 200,000 years to be 250*0.2/6 = 8 genes. This calculation assumes that the pace of evolutionary change has been constant during the past 6 million years. Human evolution may have proceeded faster during the past 200,000 years than before this time, and the brain is likely to have been the focus of more than 30% of this evolutionary change, considering that major human brain expansions occurred at about 1.8 and 0.5 million years ago (Aielo and Dunbar, 1993). Hence, modern humans are likely to have more than 8 multi allelic gene sites that mainly affect the brain. If during the past 200,000 years 80% of the genes that were actively evolving were brain related, then the 8 multi allelic number increases to 21. If the evolutionary pace for recent times (the past 200,000 years) versus before (6 million years to 200,000 years) is greater by the factor of 10, then there could be 200 multi allelic gene sites that affect the brain. This number is compatible with the estimate that humans and chimpanzees differ at 490 gene sites. Since each site may have many more than 2 alleles per locus, there could be possibly 300 alleles whose main effect is on the brain and which are still vying for a presence in the human genome. This may seem like a small number of gene sites, but there are 2N combinations of configurations when each site has two possible states, and when N = 300, there are 1090 such states. That's an incredibly large number, being larger than the human population by the factor 1080 (a one followed by 80 zeros)!
The point of these calculations is to prepare the case for stating that perhaps half of the present human genetic diversity, and genetic competition (among perhaps several hundred alleles), pertains mainly to the brain. The brain is a major focus for ongoing evolution for Homo sapiens.
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