Evolution of human intelligence
|Abilities, traits and constructs|
|Models and theories|
|Fields of study|
The evolution of human intelligence refers to a set of theories that attempt to explain how human intelligence has evolved. These theories are closely tied to the evolution of the human brain and to the emergence of human language.
The timeline of human evolution spans approximately 7 million years,citation needed from the separation of the Pan genus until the emergence of behavioral modernity by 50,000 years ago. The first 3 million years of this timeline concern Sahelanthropus, the following 2 million concern Australopithecus and the final 2 million span the history of actual human species (the Paleolithic).
Many traits of human intelligence, such as empathy, theory of mind, mourning, ritual, and the use of symbols and tools, are already apparent in great apes although in lesser sophistication than in humans.
Chimpanzees make tools and use them to acquire foods and for social displays; they have sophisticated 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.1
In one study, young chimpanzees outperformed human college students in tasks requiring remembering numbers.2 This claim was refuted in a later study after it was noted that the chimpanzees had received extensive practice with the task while the students were evaluated on their first attempt. When human subjects were given time to practice, they substantially outperformed the young chimps.3 Chimpanzees are capable of empathy, having been observed to feed turtles in the wild, and show curiosity in wildlife (such as pythons).
Around 10 million years ago, the Earth's climate entered a cooler and drier phase, which led eventually to the ice ages 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). This forced tree-dwelling animals to adapt to their new environment or die out. 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.
Some Homininae (australopithecines) adapted to this challenge by adopting bipedalism: walking on their hind legs. This gave their eyes greater elevation, the ability to see approaching danger further off, and a more efficient means of locomotion (see main article for details).citation needed It also freed the forelimbs (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 form the Hominina subtribe, of which the earliest species, such as Sahelanthropus tchadensis, date to about 7 to 5 million years ago.
From about 5 million years ago, the Hominin brain began to develop rapidly in both size and differentiation of function.
It has been shown that Great Ape cooperation and communication is severely impeded by their competitiveness, and thus that the apes would revolutionize their culture-bearing ability if they could just shrug off their competitiveness.4 It is also well known that even early hominins lacked the size and sharpness of their canine teeth that apes use as a threat signal, suggesting prehumans simply had no use for threat signals. That means they had already transcended ape competitiveness and thus developed superior cooperation and communication.
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 sapiens neanderthalensis. Thus, in general there's a 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 H. 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. In this case, it seems that for intelligence, the structure of the brain is more important than its volume.
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. The evolution of a larger brain created a problem for early humans, however. A larger brain requires a larger skull, and thus requires the female to have a wider birth canal for the newborn's larger skull to pass through. But if the female's birth canal grew too wide, her pelvis would be so wide that she would lose the ability to run: still a necessary skill in the dangerous world of 2 million years ago.
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. This adaptation enabled the human brain to continue to grow, but it imposed a new discipline. The need to care for helpless infants for long periods of time forced humans to become less mobilecitation needed. Human bands increasingly stayed in one place for long periods, so that females could care for infants, while males hunted food and fought with other bands that competed for food sourcescitation needed. As a result, humans became even more dependent on tool-making to compete with other animals and other humans, and relied less on body size and strengthcitation needed.
Around 200,000 years ago, Homo sapiens first appears in East Africa. It is unclear to what extent these early modern humans had developed language, music, religion etc. They spread throughout Africa over the following 50,000 years or so.
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 80–100,000 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).5
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 intelligence evolved 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).6
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 now referred to as Dunbar's number. According to the social brain hypothesis, when hominids started living in large groups, selection favored greater intelligence. As evidence, Dunbar cites a relationship between neocortex size and group size of various mammals.6 However, meerkats have far more social relationships than their small brain capacity would suggest.7 Another hypothesis is that it is actually intelligence that causes social relationships to become more complex, because intelligent individuals are more difficult to learn to know.8
There are also studies that show that Dunbar's number is not the upper limit of the number of social relationships in humans either.9
This model 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 selected for greater intelligence as a proxy for healthy genes and a positive feedback loop of sexual selection would have led to the evolution of human intelligence in a relatively short period.10
A sexual selection theory must explain why both sexes are intelligent. In many species, only males have impressive ornaments and show-off behavior.11 Sexual selection is also thought to be able to act on both males and females in species that are at least partially monogamous.12 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.10
Sexual selection is often thought to be a likely explanation for other female-specific human traits, for example breasts and buttocks far larger in proportion to total body size than those found in related species of ape.10 It is often assumed that if breasts and buttocks of such large size were necessary for functions such as suckling infants, they would be found in other species. Growing human brains require more nutrition than brains of related species of ape. Human males find human female breasts attractive, in agreement with sexual selection acting on human females.
Sexual selection for intelligence and judging ability can act on indicators of success, such as highly visible displays of wealth (cattle, farmland, servants, etc.). 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.10
A predominant model describing the evolution of human intelligence is ecological dominance-social competition (EDSC) 13 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.
Studies of chimpanzees show that they cooperate well in conflictless situations but not in situations with conflict, so human cooperativeness cannot have evolved to form coalitions against each other.4
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.14 Thus the direct adaptive benefit of human intelligence is questionable at least in modern societies, while it is difficult to study in prehistoric societies. However, alleles coding for even larger human brains are spreading continuously even in modern societies 1516 This suggests that humans of higher intellect may gain indirect selective benefits.
A recent study17 argues that human cleverness is simply selected within the context of sexual selection as an honest signal of genetic resistance against parasites and pathogens. The number of people living with cognitive abilities seriously damaged by childhood infections is high; estimated in hundreds of millions. Even more people live with moderate mental damages, such as 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. Pathogens currently playing a major role in this global challenge against human cognitive capabilities include viral infections like meningitis, protists like Toxoplasma and Plasmodium, and animal parasites like intestinal worms and schistosomes.18
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. Smarter people 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.
Group selection theory contends that organism characteristics that provide benefits to a group (clan, tribe, or larger population) can evolve despite individual disadvantages such as those cited above. The group benefits of intelligence (including language, the ability to communicate between individuals, the ability to teach others, and other cooperative aspects) have apparent utility in increasing the survival potential of a group.
Higher cognitive functioning develops better in an environment with adequate nutrition,19 and diets deficient in iron, zinc, protein, iodine, B vitamins, omega 3 fatty acids, magnesium and other nutrients can result in lower intelligence2021 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 (and/or the child's mother) comes from a physical and social environment where nutritional levels are low. Previc22 emphasizes the contribution of nutritional factors, especially meat and shellfish consumption, 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.
The statement that such high intelligence "lack survival value", which is used by believers in social intelligence and sexual selection, invariably assumes a stable environment. If climate change is factored in, however, the evolution of human intelligence can be perfectly explained by flexible problem solving during those climate changes.2324 For example, solving the problem of catching fish when freshwater streams freeze and the only available fish are in nearby unfrozen seawater. However, such a theory must also account for the apparent uniqueness of human levels of intelligence, which can be explained by free hands that allows for efficient manipulation of the environment such as carrying things and inventing and using handmade tools, which gives intelligence a practical survival value that can be selected by evolution.25
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- Rowan Hooper (2007-12-03). "Chimps outperform humans at memory task". New Scientist. Retrieved 2008-03-24.
- "Do young chimpanzees have extraordinary working memory?". PSYCHONOMIC BULLETIN & REVIEW. 2010-04-17. Retrieved 14 October 2011.
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- Miller. The Mating Mind. ISBN 0-8058-5749-4.
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- Ian L. Jones and Fiona M. Hunter) (1993). "Mutual sexual selection in a monogamous seabird". Nature 362: 238–239. doi:10.1038/362238a0.
- "Flinn, M. V., Geary, D. C., & Ward, C. V. (2005). Ecological dominance, social competition, and coalitionary arms races: Why humans evolved extraordinary intelligence". Retrieved 2007-05-05.
- Isler K, van Schaik CP 2006. Metabolic costs of brain size evolution. Biology Letters, 2, 557–60.
- Evans PD, Gilbert SL, Mekel-Bobrov N, et al. 2005. Microcephalin, a gene regulating brain size, continues to evolve adaptively in humans. Science, 309, 1717-1720.
- Mekel-Bobrov N, Gilbert SL, Evans PD, et al. 2005. Ongoing adaptive evolution of ASPM, a brain size determinant in Homo sapiens. Science 2005;309:1720–1722.
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- Previc, F. H. (2009). The dopaminergic mind in human evolution and history. Cambridge, England: Cambridge University Press
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- Byrne, Richard W. (1995) The Thinking Ape: Evolutionary origins of intelligence Oxford University Press, Oxford, England, ISBN 0-19-852188-X
- Greenspan, Stanley I. and Shanker, Stuart (2004) The First Idea: How symbols, language, and intelligence evolved from our early primate ancestors to modern humans Da Capo Press, Cambridge, Mass., ISBN 0-7382-0680-6
- Itzkoff, Seymour W. (1983) The Form of Man: The evolutionary origins of human intelligence Paideia Publishers, Ashfield, Mass., ISBN 0-913993-00-X
- Lynch, Gary and Granger, Richard (2008) Big Brain: The Origins and Future of Human Intelligence Palgrave Macmillan, New York, ISBN 1-4039-7978-2
- Skoyles, John R. and Sagan, Dorion (2002) Up from Dragons: The evolution of human intelligence McGraw-Hill, New York, ISBN 0-07-137825-1
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