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, though Dunbar himself admits in the same book that it is not the flocking itself that causes intelligence to evolve (as shown by ruminants).[4]
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.[4] Howewer, meerkats have far more social relationships than their small brain capacity would suggest.[5] 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.[6]
[edit] 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 evolution of human intelligence in a relatively short period.[7]
[edit] Ecological dominance-social competition model
A predominant model describing human intelligence’s evolution is ecological dominance-social competition (EDSC) [8] 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.
Howewer, modern archaeological research shows that transitional early humans with gradually evolving brains (such as Homo erectus) were not ecologically dominant (indeed, big cats ate them), and they were cooperative rather than competetive, as shown by their care for their sick and disabled, eg. the Dmanisi jaw from Georgia.[9]
[edit] 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.[10] 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 [11][12] This suggests that cleverer humans may gain indirect selective benefits.
A recent study [13] 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.[14]
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.
[edit] 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[15] 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.
[edit] Nutritional Status
Higher cognitive functioning develops better in an environment with adequate nutrition,[16] and diets deficient in iron, zinc, protein, iodine, B vitamins, omega 3 fatty acids, magnesium and other nutrients can result in lower intelligence[17][18] 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.