- Hominid 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 evolutionspans some 7 million years. from the separation of the "Pan" genus until the emergence of behavioral modernityby 30,000 years ago. Of this timeline, the first 3 million 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, use of symbols and tools are already apparent in great apes, specifically Chimpanzees, although in lesser sophistication than in humans.
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 huntingstrategies 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. [cite web | url = http://www.indiana.edu/~origins/teach/A105/lectures/A105L12.html | title = Chimpanzee intelligence | publisher = Indiana University| accessdate = 2008-03-24 | date = 2000-02-23] Young chimpanzees have outperformed human college students in tasks requiring remembering numbers. [cite web | url = http://www.newscientist.com/article/dn12993-chimps-outperform-humans-at-memory-task.html | title = Chimps outperform humans at memory task | publisher = New Scientist| date = 2007-12-03| accessdate = 2008-03-24 |author = Rowan Hooper] Chimpanzees are capable of empathy, having been observed to feed turtles in the wild, and to appreciate natural beauty (such as sunsets) 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.
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, an adaptation no other animals have attained. Bipedal tool-using primates form the Homininasubtribe, 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.
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 canalfor the newborn's larger skull to pass through. But if the female's birth canal grew too wide, her pelviswould 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 colonised by Neanderthal man, extinct by 20,000 following the appearance of modern humans in the region from 40,000 years ago.
They spread throughout Africa over the following 50,000 years or so: around 100-80,000 years ago, three main lines of "Homo sapiens sapiens" diverged, bearers of mitochondrial haplogroup L1 (mtDNA) / A (Y-DNA) colonizing
Southern Africa(the ancestors of the Khoisan/ Capoidpeoples), bearers of haplogroup L2 (mtDNA) / B (Y-DNA) settling Central and West Africa(the ancestors of Niger-Congoand Nilo-Saharanspeaking peoples), while the bearers of haplogroup L3 remained in East Africa.
The "Great Leap Forward" leading to full
behavioral modernitysets 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 artdate to this period, the Aurignacianand the Gravettianperiods of prehistoric Europe, such the Venus figurinesand cave painting( Chauvet Cave) and the earliest musical instruments (the bone pipe of Geissenklösterle, Germany, dated to about 36,000 years ago [http://www.mus.cam.ac.uk/~ic108/lithoacoustics/BAR2002/BARpreprint.pdf] ).
Ecological dominance-social competition model
A predominant modern model explaining human intelligence's evolution is ecological dominance-social competition (EDSC) cite web| url=http://web.missouri.edu/~gearyd/Flinnetal2005.pdf| title=Flinn, M. V., Geary, D. C., & Ward, C. V. (2005). Ecological dominance, social competition, and coalitionary arms races: Why humans evolved extraordinary intelligence| accessdate=2007-05-05] explained by Mark V. Flinn, David C. Geary and Carol V. Ward based mainly on work by
Richard D. Alexander. The model proposes that human intelligence was able to evolve due to domination of their habitat. Only then were they 'free' to develop more advanced social skills such as communication through language. Their main competition after habitat's dominance, shifted from 'nature' to their own species, making it of vital importance to 'outmanoeuvre' other members of the group through more advanced social skills. A more social, communicative person would be more easily naturally selected.
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 need Isler K, van Schaik CP 2006. Metabolic costs of brain size evolution. "Biology Letters," 2, 557–60.] . Secondly, cleverer people apparently do not survive or reproduce better. Thus the direct adaptive benefit of human intelligence is questionable at least in modern societies, while it is impossible to study in prehistoric societies. However, alleles coding for even larger human brains are spreading continuously even in modern societies 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. ] This suggests that cleverer humans may gain indirect selective benefits.
A recent study cite web| url=http://www.zoologia.hu/list/clever.pdf|title=Rozsa L 2008. The rise of non-adaptive intelligence in humans under pathogen pressure. "Medical Hypotheses," 70, 685-690. |accessdate=2008-05-26] argues that human cleverness is simply selected within the context of
sexual selectionas an honest signal of genetic resistance against parasitesand 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 Toxoplasmaand Plasmodium, and animal parasites like intestinal worms and SchistosomesOlness K 2003. Effects on brain development leading to cognitive impairment: a worldwide epidemic. "J Dev Behav Pediatr," 24, 120–130] . 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 mate-searching men’s or women’s preference for female bodily beauty or male body height; people are just searching for signals of good resistance genes. Intelligence appears to be one of these signals. And, since the human brain is particularly vulnerable to infectious diseases, cleverness is a particularly reliable signal. There are several reasons to presume that early humans were more heavily affected by pathogens than any other primates (longer life-span, more sedentary, more carnivorous etc.), thus this hypothesis can explain why humans became cleverer while other primates did not. It also explains why humans apparently continue to increase brain size and intelligence even nowadays.
Heritability of IQ
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