Evolution of human intelligence

Evolution of the brain and the origin of language

The evolution of human intelligence is closely tied to the evolution of the human brain and to the origin of language. The timeline of human evolution spans approximately seven million years, from the separation of the genus Pan until the emergence of behavioral modernity by 50,000 years ago. The first three million years of this timeline concern Sahelanthropus, the following two million concern Australopithecus and the final two million span the history of the genus Homo in the Paleolithic era. Many traits of human intelligence, such as empathy, theory of mind, mourning, ritual, and the use of symbols and tools, are somewhat apparent in great apes, although they are in much less sophisticated forms than what is found in humans like the great ape language. The cognitive tradeoff hypothesis proposes that there was an evolutionary tradeoff between short term working memory and complex language skills over the course of human evolution.

The great apes (hominidae) show some cognitive and empathic abilities. Chimpanzees can make tools and use them to acquire foods and for social displays; they have mildly complex hunting strategies requiring cooperation, influence and rank; they are status conscious, manipulative and capable of deception; they can learn to use symbols and understand aspects of human language including some relational syntax, concepts of number and numerical sequence. One common characteristic that is present in species of "high degree intelligence" (i. e. dolphins, great apes, and humans Homo sapiens) is a brain of enlarged size. Along with this, there is a more developed neocortex, a folding of the cerebral cortex, and von Economo neurons. Said neurons are linked to social intelligence and the ability to gauge what another is thinking or feeling and are also present in bottlenose dolphins.

Around 10 million years ago, the Earth's climate entered a cooler and drier phase, which led eventually to the Quaternary glaciation beginning some 2.6 million years ago. One consequence of this was that the north African tropical forest began to retreat, being replaced first by open grasslands and eventually by desert (the modern Sahara). As their environment changed from continuous forest to patches of forest separated by expanses of grassland, some primates adapted to a partly or fully ground dwelling life. Here they were exposed to predators, such as the big cats, from whom they had previously been safe. These environmental pressures caused selection to favor bipedalism walking on hind legs. This gave the Homininae's eyes greater elevation, the ability to see approaching danger further off, and a more efficient means of locomotion. It also freed the arms from the task of walking and made the hands available for tasks such as gathering food. At some point the bipedal primates developed handedness, giving them the ability to pick up sticks, bones and stones and use them as weapons, or as tools for tasks such as killing smaller animals, cracking nuts, or cutting up carcasses. In other words, these primates developed the use of primitive technology. Bipedal tool using primates from the subtribe Hominina date back to as far as about 5 to 7 million years ago, such as one of the earliest species, Sahelanthropus tchadensis. From about 5 million years ago, the hominin brain began to develop rapidly in both size and differentiation of function. There has been a gradual increase in brain volume as humans progressed along the timeline of evolution (see Homininae), starting from about 600 cm3 in Homo habilis up to 1500 cm3 in Homo neanderthalensis. Thus, in general there's a positive correlation between brain volume and intelligence. However, modern Homo sapiens have a brain volume slightly smaller (1250 cm3) than neanderthals, and the Flores hominids (Homo floresiensis), nicknamed hobbits, had a cranial capacity of about 380 cm3 (considered small for a chimpanzee) about a third of that of Homo erectus. It is proposed that they evolved from H. erectus as a case of insular dwarfism. With their three times smaller brain, the Flores hominids apparently used fire and made tools as sophisticated as those of their ancestor H. erectus.

Roughly 2.4 million years ago Homo habilis had appeared in East Africa: the first known human species, and the first known to make stone tools, yet the disputed findings of signs of tool use from even earlier ages and from the same vicinity as multiple Australopithecus fossils may put to question how much more intelligent than its predecessors H. habilis was. The use of tools conferred a crucial evolutionary advantage, and required a larger and more sophisticated brain to co ordinate the fine hand movements required for this task. Our knowledge of the complexity of behaviour of Homo habilis is not limited to stone culture; they also had habitual therapeutic use of toothpicks. A larger brain requires a larger skull, and thus is accompanied by other morphological and biological evolutionary changes. One such change required for the female to have a wider birth canal for the newborn's larger skull to pass through. The solution to this was to give birth at an early stage of fetal development, before the skull grew too large to pass through the birth canal. Other accompanying adaptations were the smaller maxillary and mandibular bones, smaller and weaker facial muscles, and shortening and flattening of the face resulting in modern human's complex cognitive and linguistic capabilities as well as the ability to create facial expressions and smile. Regardless, humans' increasingly sedentary lifestyle to protect their more vulnerable offspring led them to grow even more dependent on tool making to compete with other animals and other humans, and rely less on body size and strength. Characteristics such as a supraorbital torus, or prominent eyebrow ridge, and flat face also makes Homo erectus distinguishable. Their brain size substantially sets them apart from closely related species, such as H. habilis, as we can see an increase in average cranial capacity of 1000 cc. Compared to earlier species, H. erectus developed keels and small crests in the skull showing morphological changes of the skull to support increased brain capacity. It is believed that Homo erectus were, anatomically, modern humans as they are very similar in size, weight, bone structure, and nutritional habits. Over time, however, human intelligence developed in phases that is interrelated with brain physiology, cranial anatomy and morphology, and rapidly changing climate and environments. The 4E cognition approach describes cognition as embodied, embedded, enactive, and extended, to understand the interconnected nature between the mind, body, and environment. Acheulean culture, associated with Homo erectus, is composed of bifacial, or double sided, hand axes, that "requires more planning and skill on the part of the toolmaker; he or she would need to be aware of principles of symmetry". Fossils of Homo sapiens were found in East Africa which are c. 200,000 years old. It is unclear to what extent these early modern humans had developed language, music, religion, etc. The cognitive tradeoff hypothesis proposes that there was an evolutionary tradeoff between short term working memory and complex language skills during the span of human evolution. According to proponents of the Toba catastrophe theory, the climate in non tropical regions of the earth experienced a sudden freezing about 70,000 years ago, because of a huge explosion of the Toba volcano that filled the atmosphere with volcanic ash for several years. This reduced the human population to less than 10,000 breeding pairs in equatorial Africa, from which all modern humans are descended. Being unprepared for the sudden change in climate, the survivors were those intelligent enough to invent new tools and ways of keeping warm and finding new sources of food (for example, adapting to ocean fishing based on prior fishing skills used in lakes and streams that became frozen). Around 80,000–100,000 years ago, three main lines of Homo sapiens diverged, bearers of mitochondrial haplogroup L1 (mtDNA) / A (Y DNA) colonizing Southern Africa (the ancestors of the Khoisan/Capoid peoples), bearers of haplogroup L2 (mtDNA) / B (Y DNA) settling Central and West Africa (the ancestors of Niger–Congo and Nilo Saharan speaking peoples), while the bearers of haplogroup L3 remained in East Africa. The "Great Leap Forward" leading to full behavioral modernity sets in only after this separation. Rapidly increasing sophistication in tool making and behaviour is apparent from about 80,000 years ago, and the migration out of Africa follows towards the very end of the Middle Paleolithic, some 60,000 years ago. Fully modern behaviour, including figurative art, music, self ornamentation, trade, burial rites etc. is evident by 30,000 years ago. The oldest unequivocal examples of prehistoric art date to this period, the Aurignacian and the Gravettian periods of prehistoric Europe, such as the Venus figurines and cave painting (Chauvet Cave) and the earliest musical instruments (the bone pipe of Geissenklösterle, Germany, dated to about 36,000 years ago). The human brain has evolved gradually over the passage of time; a series of incremental changes occurring as a result of external stimuli and conditions. It is crucial to keep in mind that evolution operates within a limited framework at a given point in time. In other words, the adaptations that a species can develop are not infinite and are defined by what has already taken place in the evolutionary timeline of a species. Given the immense anatomical and structural complexity of the brain, its evolution (and the congruent evolution of human intelligence), can only be reorganized in a finite number of ways. The majority of said changes occur either in terms of size or in terms of developmental timeframes. The cerebral cortex is divided into four lobes (frontal, parietal, occipital, and temporal) each with specific functions. The cerebral cortex is significantly larger in humans than in any other animal and is responsible for higher thought processes such as reasoning, abstract thinking, and decision making. Another characteristic that makes humans special and sets them apart from any other species is our ability to produce and understand complex, syntactic language. The cerebral cortex, particularly in the temporal, parietal, and frontal lobes, are populated with neural circuits dedicated to language. There are two main areas of the brain commonly associated with language, namely: Wernicke's area and Broca's area. The former is responsible for the understanding of speech and the latter for the production of speech. Homologous regions have been found in other species (i. e. Area 44 and 45 have been studied in chimpanzees) but they are not as strongly related to or involved in linguistic activities as in humans.

The social brain hypothesis was proposed by British anthropologist Robin Dunbar, who argues that human intelligence did not evolve primarily as a means to solve ecological problems, but rather as a means of surviving and reproducing in large and complex social groups. Some of the behaviors associated with living in large groups include reciprocal altruism, deception, and coalition formation. These group dynamics relate to Theory of Mind or the ability to understand the thoughts and emotions of others, though Dunbar himself admits in the same book that it is not the flocking itself that causes intelligence to evolve (as shown by ruminants). Dunbar argues that when the size of a social group increases, the number of different relationships in the group may increase by orders of magnitude. Chimpanzees live in groups of about 50 individuals whereas humans typically have a social circle of about 150 people, which is also the typical size of social communities in small societies and personal social networks; this number is now referred to as Dunbar's number. In addition, there is evidence to suggest that the success of groups is dependent on their size at foundation, with groupings of around 150 being particularly successful, potentially reflecting the fact that communities of this size strike a balance between the minimum size of effective functionality and the maximum size for creating a sense of commitment to the community. One potential explanation for this finding is that frugivory requires "extractive foraging", or the process of locating and preparing hard shelled foods, such as nuts, insects, and fruit. Extractive foraging requires higher cognitive processing, which could help explain larger brain size. There are also studies that show that Dunbar's number is not the upper limit of the number of social relationships in humans either. The hypothesis that it is brain capacity that sets the upper limit for the number of social relationships is also contradicted by computer simulations that show simple unintelligent reactions to be sufficient to emulate "ape politics" and by the fact that some social insects such as the paper wasp do have hierarchies in which each individual has its place (as opposed to herding without social structure) and maintains their hierarchies in groups of approximately 80 individuals with their brains smaller than that of any mammal. Insects provide an opportunity to explore this since they exhibit an unparalleled diversity of social forms to permanent colonies containing many individuals working together as a collective organism and have evolved an impressive range of cognitive skills despite their small nervous systems. Social insects are shaped by ecology, including their social environment. Studies aimed to correlating brain volume to complexity have failed to identify clear correlations between sociality and cognition because of cases like social insects. In humans, societies are usually held together by the ability of individuals to recognize features indicating group membership. Social insects, likewise, often recognize members of their colony allowing them to defend against competitors. Ants do this by comparing odors which require fine discrimination of multicomponent variable cues. Studies suggest this recognition is achieved through simple cognitive operations that do not involve long term memory but through sensory adaptation or habituation. In honeybees, their symbolic 'dance' is a form of communication that they use to convey information with the rest of their colony. In an even more impressive social use of their dance language, bees indicate suitable nest locations to a swarm in search of a new home. The swarm builds a consensus from multiple 'opinions' expressed by scouts with different information, to finally agree on a single destination to which the swarm relocates.

Similar to, but distinct from the social brain hypothesis, is the cultural intelligence or cultural brain hypothesis, which dictates that human brain size, cognitive ability, and intelligence have increased over generations due to cultural information from a mechanism known as social learning. The hypothesis also predicts a positive correlation between species with a higher dependency and more frequent opportunities for social learning and overall cognitive ability. This is because social learning allows species to develop cultural skills and strategies for survival; in this way it can be said that heavily cultural species should in theory be more intelligent. Humans have been widely acknowledged as the most intelligent species on the planet, with big brains with ample cognitive abilities and processing power which outcompete all other species. In fact, humans have shown an enormous increase in brain size and intelligence over millions of years of evolution. This is because humans have been referred to as an 'evolved cultural species'; one that has an unrivalled reliance on culturally transmitted knowledge due to the social environment around us. This is down to social transmission of information which spreads significantly faster in human populations relative to changes in genetics. Put simply, we are the most cultural species there is, and are therefore the most intelligent species there is. The key point when concerning evolution of intelligence is that this cultural information has been consistently transmitted across generations to build vast amounts of cultural skills and knowledge throughout the human race. Dunbar's social brain hypothesis on the other hand dictates that our brains evolved primarily due to complex social interactions in groups, so in this way the two hypotheses are distinct from each other in that the cultural intelligence hypothesis focuses more on an in increase in intelligence from socially transmitted information. We see a shift in focus from 'social' interactions to learning strategies. In 2018, Muthukrishna and researchers constructed a model based on the cultural intelligence hypothesis which revealed relationships between brain size, group size, social learning and mating structures. What sets humans apart from other species is that, due to our emphasis on culturally acquired information, we have evolved to already possess significant social learning abilities from infancy. Neurological studies on nine month old infants were conducted by researchers in 2012 to demonstrate this phenomenon. The study involved infants observing a caregiver making a sound with a rattle over a period of one week. The brains of the infants were monitored throughout the study. Researchers found that the infants were able to activate neural pathways associated with making a sound with the rattle without actually doing the action themselves. Here we can see human social learning in action infants were able to understand the effects of a particular action simply by observing the performance of the action by someone else. Not only does this study demonstrate the neural mechanisms of social learning, but it also demonstrates our inherent ability to acquire cultural skills from those around us from the very start of our lives it therefore shows strong support for the cultural intelligent hypothesis. Various studies have been conducted to show the cultural intelligence hypothesis in action on a wider scale. One particular study in 2016 investigated two orangutan species, including the more social Sumatran species and the less sociable Bornean species. The aim was to test the notion that species with a higher frequency of opportunities for social learning should evolve to be more intelligent. Results showed that the Sumatrans consistently performed better in cognitive tests compared to the less sociable Borneans. The Sumatrans also showed greater inhibition and more cautious behaviour within their habitat. This was one of the first studies to show evidence for the cultural intelligence hypothesis in a non human species frequency of learning opportunities had gradually produced differences in cognitive abilities between the two species.

A study in 2018 proposed an altered variant of the original version of the hypothesis called the 'transformative cultural intelligence hypothesis'. The research involved investigating four year old's problem solving skills in different social contexts. The children were asked to extract a floating object from a tube using water. Nearly were all unsuccessful without cues, however most children succeeded after being shown a pedagogical solution suggesting video. When the same video was shown in a non pedagogical manner however, the children's success in the task did not improve. Crucially, this meant that the children's physical cognition and problem solving ability was therefore affected by how the task was socially presented to them. Researchers thus formulated the transformative cultural intelligence hypothesis, which stresses that our physical cognition is developed and affected by the social environment around us. This challenges the traditional cultural intelligence hypothesis which states that it is humans social cognition and not physical cognition which is superior to our nearest primate relatives; This theory has received strong support from studies of animal domestication where selective breeding for tameness has, in only a few generations, led to the emergence of impressive "humanlike" abilities. Tamed foxes, for example, exhibit advanced forms of social communication (following pointing gestures), pedomorphic physical features (childlike faces, floppy ears) and even rudimentary forms of theory of mind (eye contact seeking, gaze following). Evidence also comes from the field of ethology (which is the study of animal behavior, focused on observing species in their natural habitat rather than in controlled laboratory settings) where it has been found that animals with a gentle and relaxed manner of interacting with each other – for example stumptailed macaques, orangutans and bonobos – have more advanced socio cognitive abilities than those found among the more aggressive chimpanzees and baboons. It is hypothesized that these abilities derive from a selection against aggression. On a mechanistic level, these changes are believed to be the result of a systemic downregulation of the sympathetic nervous system (the fight or flight reflex). Hence, tamed foxes show a reduced adrenal gland size and have an up to fivefold reduction in both basal and stress induced blood cortisol levels. Similarly, domesticated rats and guinea pigs have both reduced adrenal gland size and reduced blood corticosterone levels. It seems as though the neoteny of domesticated animals significantly prolongs the immaturity of their hypothalamic pituitary adrenal system (which is otherwise only immature for a short period when they are pups/kittens) and this opens up a larger "socialization window" during which they can learn to interact with their caretakers in a more relaxed way. This downregulation of sympathetic nervous system reactivity is also believed to be accompanied by a compensatory increase in a number of opposing organs and systems. Although these are not as well specified, various candidates for such "organs" have been proposed: the parasympathetic system as a whole, the septal area over the amygdala, the endogenous opioids and various forms of quiescent immobilization which antagonize the fight or flight reflex.

One study hypothesizes that reasoning about social exchange between individuals is an adaptation to the human brain. This adaption is predicted to evolve when two parties are both better off than they were before by mutually exchanging things they value less for things they value more. However, selection will only favor social exchange when both parties benefit. In 2004, psychologist Satoshi Kanazawa argued that g was a domain specific, species typical, information processing psychological adaptation, and in 2010, Kanazawa argued that g correlated only with performance on evolutionarily unfamiliar rather than evolutionarily familiar problems, proposing what he termed the "Savanna IQ interaction hypothesis". In 2006, Psychological Review published a comment reviewing Kanazawa's 2004 article by psychologists Denny Borsboom and Conor Dolan that argued that Kanazawa's conception of g was empirically unsupported and purely hypothetical and that an evolutionary account of g must address it as a source of individual differences. In response to Kanazawa's 2010 article, psychologists Scott Barry Kaufman, Colin G. DeYoung, Deirdre Reis, and Jeremy R. Gray gave 112 subjects a 70 item computerized version of the Wason selection task (a logic puzzle) in a social relations context as proposed by Leda Cosmides and John Tooby in The Adapted Mind, and found instead that "performance on non arbitrary, evolutionarily familiar problems is more strongly related to general intelligence than performance on arbitrary, evolutionarily novel problems". Peter Cathcart Wason originally demonstrated that not even 10% of subjects found the correct solution and his finding was replicated. Psychologists Patricia Cheng, Keith Holyoak, Richard E. Nisbett, and Lindsay M. Oliver demonstrated experimentally that subjects who have completed semester long college courses in propositional calculus do not perform better on the Wason selection task than subjects who do not complete such college courses. Tooby and Cosmides originally proposed a social relations context for the Wason selection task as part of a larger computational theory of social exchange after they began reviewing the previous experiments about the task beginning in 1983. Additionally, economist Thomas Sowell has noted that numerous studies finding disparities between the mean test scores of ethnic groups on intelligence tests have found that ethnic groups with lower mean test scores have tended to perform worst on non verbal, non informational, or abstract reasoning test items. Writing after the completion of the Human Genome Project in 2003, psychologist Earl B. Hunt noted in 2011 that no genes related to differences in cognitive skills across various racial and ethnic groups had ever been discovered, and in 2012, American Psychologist published a review of new findings by psychologists Richard E. Nisbett, Joshua Aronson, Clancy Blair, Diane F. Halpern, and Eric Turkheimer, economist William Dickens, and philosopher James R. Flynn that concluded that almost no single nucleotide genetic polymorphisms that have been discovered are consistently associated with variation in IQ in the normal range.

This model, which invokes sexual selection, is proposed by Geoffrey Miller who argues that human intelligence is unnecessarily sophisticated for the needs of hunter gatherers to survive. He argues that the manifestations of intelligence such as language, music and art did not evolve because of their utilitarian value to the survival of ancient hominids. Rather, intelligence may have been a fitness indicator. Hominids would have been chosen for greater intelligence as an indicator of healthy genes and a Fisherian runaway positive feedback loop of sexual selection would have led to the evolution of human intelligence in a relatively short period. Philosopher Denis Dutton also argued that the human capacity for aesthetics evolved by sexual selection. Evolutionary biologist George C. Williams and evolutionary medicine researcher Randolph M. Nesse cite evolutionary psychologists John Tooby and Leda Cosmides as referring to the emotions as "Darwinian algorithms of the mind", while social psychologist David Buss has argued that the sex specialized differences in the emotion of jealousy are adaptive strategies for detecting infidelity by a mating partner and anthropologists Donald E. Brown and Ward Goodenough have argued that marriage is a cultural universal that evolved to regulate sexual access to fertile women within a particular culture in response to male intrasexual competition and dominance. Citing cross cultural research conducted by Buss, Miller has argued that if humans prefer altruistic mating partners that would select by mate choice for altruism directly. Additionally, Nesse and theoretical biologist Mary Jane West Eberhard view sexual selection as a subcategory of social selection, with Nesse and anthropologist Christopher Boehm arguing further that altruism in humans held fitness advantages that enabled evolutionarily extraordinary cooperativeness and the human capability of creating culture, as well as capital punishment by band societies against bullies, thieves, free riders, and psychopaths. In many species, only males have impressive secondary sexual characteristics such as ornaments and show off behavior, but sexual selection is also thought to be able to act on females as well in at least partially monogamous species. With complete monogamy, there is assortative mating for sexually selected traits. This means that less attractive individuals will find other less attractive individuals to mate with. If attractive traits are good fitness indicators, this means that sexual selection increases the genetic load of the offspring of unattractive individuals. Without sexual selection, an unattractive individual might find a superior mate with few deleterious mutations, and have healthy children that are likely to survive. With sexual selection, an unattractive individual is more likely to have access only to an inferior mate who is likely to pass on many deleterious mutations to their joint offspring, who are then less likely to survive. are found sexually attractive by many men is in agreement with sexual selection acting on human females secondary sexual characteristics. Sexual selection for intelligence and judging ability can act on indicators of success, such as highly visible displays of wealth. Growing human brains require more nutrition than brains of related species of ape. It is possible that for females to successfully judge male intelligence, they must be intelligent themselves. This could explain why despite the absence of clear differences in intelligence between males and females on average, there are clear differences between male and female propensities to display their intelligence in ostentatious forms.

The number of people with severe cognitive impairment caused by childhood viral infections like meningitis, protists like Toxoplasma and Plasmodium, and animal parasites like intestinal worms and schistosomes is estimated to be in the hundreds of millions. Even more people with moderate mental damages, such as an inability to complete difficult tasks, that are not classified as 'diseases' by medical standards, may still be considered as inferior mates by potential sexual partners. Thus, widespread, virulent, and archaic infections are greatly involved in natural selection for cognitive abilities. People infected with parasites may have brain damage and obvious maladaptive behavior in addition to visible signs of disease. Smarter people can more skillfully learn to distinguish safe non polluted water and food from unsafe kinds and learn to distinguish mosquito infested areas from safe areas. Additionally, they can more skillfully find and develop safe food sources and living environments. Given this situation, preference for smarter child bearing/rearing partners increases the chance that their descendants will inherit the best resistance alleles, not only for immune system resistance to disease, but also smarter brains for learning skills in avoiding disease and selecting nutritious food. When people search for mates based on their success, wealth, reputation, disease free body appearance, or psychological traits such as benevolence or confidence; the effect is to select for superior intelligence that results in superior disease resistance.

Another model describing the evolution of human intelligence is ecological dominance social competition (EDSC), explained by Mark V. Flinn, David C. Geary and Carol V. Ward based mainly on work by Richard D. Alexander. According to the model, human intelligence was able to evolve to significant levels because of the combination of increasing domination over habitat and increasing importance of social interactions. As a result, the primary selective pressure for increasing human intelligence shifted from learning to master the natural world to competition for dominance among members or groups of its own species. As advancement, survival and reproduction within an increasing complex social structure favored ever more advanced social skills, communication of concepts through increasingly complex language patterns ensued. Since competition had shifted bit by bit from controlling "nature" to influencing other humans, it became of relevance to outmaneuver other members of the group seeking leadership or acceptance, by means of more advanced social skills. A more social and communicative person would be more easily selected.

Human intelligence is developed to an extreme level that is not necessarily adaptive in an evolutionary sense. Firstly, larger headed babies are more difficult to give birth to and large brains are costly in terms of nutrient and oxygen requirements. Thus the direct adaptive benefit of human intelligence is questionable at least in modern societies, while it is difficult to study in prehistoric societies. Since 2005, scientists have been evaluating genomic data on gene variants thought to influence head size, and have found no evidence that those genes are under strong selective pressure in current human populations. The trait of head size has become generally fixed in modern human beings. While decreased brain size has strong correlation with lower intelligence in humans, some modern humans have brain sizes as small as with Homo erectus but normal intelligence (based on IQ tests) for modern humans. Increased brain size in humans may allow for greater capacity for specialized expertise.

The two major perspectives on primate brain evolution are the concerted and mosaic approaches. In the concerted evolution approach, cortical expansions in the brain are considered to be a by product of a larger brain, rather than adaptive potential. Researchers have attributed hominin evolution to mosaic evolution. The LPFC is commonly associated with planning and working memory functions. Studies on cortical expansions in the brain have been used to examine the evolutionary basis of neurological disorders, such as Alzheimer's disease.

Human brain evolution involves cellular, genetic, and circuitry changes. On a genetic level, humans have a modified FOXP2 gene, which is associated with speech and language development. The human variant of the gene SRGAP2, SRGAP2C, enables greater dendritic spine density which fosters greater neural connections. On a cellular level, studies demonstrate von Economo neurons (VENs) are more prevalent in humans than other primates. Studies show that VENs are associated with empathy, social awareness and self control. On a circuitry level, humans exhibit a more complex mirror neuron system, greater connection between the two major language processing areas (Wernicke's area and Broca's area), and a vocal control circuit that connects the motor cortex and brain stem. Studies have demonstrated the presence of the mirror neuron system in both macaques in humans; However, the mirror neuron system is only activated in macaques when observing transitive movements. Between group selection can be used to explain the changes and adaptations that arise within a group of individuals. Group related adaptations and changes are a byproduct of between group selection as traits or characteristics that prove to be advantageous in relation to another group will become increasingly popular and disseminated within a group. In the end, increasing its overall chance of surviving a competing group. However, this explanation cannot be applied to humans (and other species, predominantly other mammals) that live in stable, established social groupings. This is because of the social intelligence that functioning within these groups requires from the individual. Humans, while they are not the only ones, possess the cognitive and mental capacity to form systems of personal relationships and ties that extend well beyond those of the nucleus of family. The continuous process of creating, interacting, and adjusting to other individuals is a key component of many species' ecology. These concepts can be tied to the social brain hypothesis, mentioned above. This hypothesis posits that human cognitive complexity arose as a result of the higher level of social complexity required from living in enlarged groups. These bigger groups entail a greater amount of social relations and interactions thus leading to an expanded quantity of intelligence in humans. Diets deficient in iron, zinc, protein, iodine, B vitamins, omega 3 fatty acids, magnesium and other nutrients can result in lower intelligence either in the mother during pregnancy or in the child during development. While these inputs did not have an effect on the evolution of intelligence they do govern its expression. A higher intelligence could be a signal that an individual comes from and lives in a physical and social environment where nutrition levels are high, whereas a lower intelligence could imply a child, its mother, or both, come from a physical and social environment where nutritional levels are low. Previc emphasizes the contribution of nutritional factors to elevations of dopaminergic activity in the brain, which may have been responsible for the evolution of human intelligence since dopamine is crucial to working memory, cognitive shifting, abstract, distant concepts, and other hallmarks of advanced intelligence.