Evolution of human intelligence

The evolution of human intelligence refers to a set of theories that attempt to explain how human intelligence has evolved. The question is closely tied to the evolution of the human brain, and to the emergence of human language.

The timeline of human evolution spans some 7 million years, from the separation of the Pan genus until the emergence of behavioral modernity by 50,000 years ago. Of this timeline, the first 3 million years concern Sahelanthropus, the following 2 million concern Australopithecus, while 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.

History

Hominidae

Chimpanzee mother and baby

Further information: Primate empathy, Great ape language, and Chimpanzee intelligence

The great apes show considerable abilities for cognition and empathy.

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.

In one study, young chimpanzees outperformed human college students in tasks requiring remembering numbers. Chimpanzees are capable of empathy, having been observed to feed turtles in the wild, and show curiosity in wildlife (such as pythons).

Hominina

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 Hominina (Australopithecines) adapted to this challenge by adopting bipedalism: walking on their hind legs. This gave their eyes greater elevation and the ability to see approaching danger further off. 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 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.

Homo

Further information: Homo (genus)

By 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.

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 mobile. 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 sources. As a result, humans became even more dependent on tool-making to compete with other animals and each other, and relied less on size and strength.

About 200,000 years ago Europe and the Near East were colonized by Neanderthal man, extinct by 20,000 following the appearance of modern humans in the region from 40,000 years ago.

Homo sapiens

Further information: Intelligence and Archaic Homo sapiens

Further information: Behavioral modernity and Race and intelligence

File:BBC-artefacts.jpg

Middle Stone Age bifacial points, engraved ochre and bone tools from the c. 75,000 year old M1 & M2 phases at Blombos cave.

"The Lion Man," found in the Hohlenstein-Stadel cave of Germany's Swabian Alb and dated to 32,000 years ago, is associated with the Aurignacian culture and is the oldest known anthropomorphic animal figurine in the world.

Between 170,000 to 120,000 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: around 100-80,000 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 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).

Models

Social brain hypothesis

The model was proposed by 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 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.

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.

Howewer, the more intelligent the being is, the more difficult it should be to learn to know that being and therefore intelligence should be unrelated to group size. And there is empirical evidence for that too. Chickens, meerkats and Jack Jumpers all have far more social relations than their small brain capacity could account for if Dunbar were right.

Sexual selection

See also: sexual selection in human evolution

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 are of no utilitarian value to the survival of ancient hominids. Rather, intelligence may have been a fitness indicator. Hominids would have selected for intelligence as a proxy for healthy genes and a positive feedback loop leading runaway sexual selection would have led to the for evolution of human intelligence in a relatively short period.

In species where females pay the most expensive biological price for reproduction, which includes humans and other mammals, only males have traits that are selected through sexual selection. So the fact that women can think disproves the hypothesis that sexual selection were the driving force behind the evolution of human intelligence.

Ecological dominance-social competition model

A predominant model describing human intelligence's evolution 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 due to human domination over its habitat. As a result its primary competition shifted from nature to members or groups of its own species.

Only then humans were "free" to develop more advanced social skills such as communication of concepts through complex language patterns. Since competition had shifted from 'nature' to their own species, 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 naturally selected.

Modern research shows that the "genetic bottleneck" in human evolution were not a true bottleneck at all, but that hominins were rare all the time. This aligns with the conspicious rarity of hominin fossils compared to fossils of other animals in areas with similar conditions for fossilization. Such rarity changes the sociobiological evolutionary pressure by removing the need to compete against each other, the need for dominance hierarchies and the need to deceive each other. The result were that hominins evolved to be willing to share things (including knowledge), to not lie, and to not suspect others for lying. That was likely crucial to the evolution of human language and culture. Great apes and other smart animals can learn by imitating, but they do not deliberately educate and only imitate the parts of the process that gives positive results,the former most likely because they are too competitive to willingly share knowledge(or anything), the latter because their biological adaptation to political life makes them instinctively suspect all communication to be manipulative deception.

Intelligence as a resistance signal

Human intelligence developed to an extreme level that is not necessarily adaptive in an evolutionary sense. Firstly, larger-headed babies are more difficult to give birth 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. However, alleles coding for even larger human brains are spreading continuously even in modern societies This suggests that cleverer humans may gain indirect selective benefits.

A recent study 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, that are not classified as ‘diseases’ by medical standards, who 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.

Thus, widespread, virulent, and archaic infections are greatly involved. Given this situation, our sexual preferences for clever partners increase the chance that our descendants will inherit the best resistance alleles. Like some people search for mates based on their (perceived) bodily beauty, height, or social position (e.g. wealth or fame), or psychological traits such as benevolence or confidence; people are just searching for signals of good resistance genes. Intelligence appears to be one of these signals.

Group Selection and Evolvability

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.

Intelligence is one of a class of inherited characteristics that depend for their utility on the acquisition of something (in this case, experience or information concerning the outside world) that can be retained indefinitely by an individual but not genetically transmitted to descendents. The ability of an organism to acquire such information and then non-genetically transmit it to descendents that could then benefit from the experience of their parent without having to acquire the experience themselves appears to be a major group advantage and essentially multiplies the intelligence of an individual by allowing progressive group accumulation of experience.

Evolvability, another proposed modification to classical evolution theory suggests a connection between a purposely limited organism life span and the evolution of intelligence. The suggestion is that without a limited life span, the acquired characteristic (experience) would tend to override the inherited characteristic (intelligence). An older and more experienced animal would tend to have an advantage over a younger more intelligent but less experienced animal thus interfering with the evolution of intelligence. This factor is ameliorated by an organism design that limits life span. See Evolution of ageing.

Nutritional Status

Higher cognitive functioning develop better in an environment with adequate nutrition, and 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 IQ 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 IQ could imply a child (and/or the child's mother) comes from a physical and social environment where nutritional levels are low.

See also

• Behavioral modernity
• Heritability of IQ
• Human evolution
• Fisherian runaway
• Primate cognition

Further reading

• 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
• Tobias, Phillip V. (1971) The Brain in Hominid Evolution Columbia University Press, New York, ISBN 0-231-03518-7
• Roth and Dicke (2005). "Evolution of the brain and intelligence" (PDF). {{cite journal}}: Cite journal requires |journal= (help)