Hominid intelligence
From Wikipedia, the free encyclopedia
The evolution of human intelligence refers to a set of theories that attempt to explain how human intelligence has evolved. The incomplete nature of scientific knowledge of the human brain is a significant impediment to progress in this field. Another difficulty is that there is no universally accepted definition of intelligence; one definition is "the ability to reason, plan, solve problems, think abstractly, comprehend ideas and language, and learn."
Contents |
[edit] Ice Age
Around 10 million years ago, the earth's climate entered a cooler and drier phase, which led eventually to the ice ages. 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 primates 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 became hominids, of which the earliest species, such as Sahelanthropus tchadensis, are dated to about 7 million years ago.
[edit] Development of the brain
Up to this point the hominid brain was no larger or more sophisticated than the brains of other mammals of comparable size. But from about 5 million years ago, the hominid brain began to develop rapidly (in evolutionary terms) in both size and differentiation of function. The use of tools conferred a crucial evolutionary advantage on those hominids which had this skill - in other words, they were more likely than other species to survive long enough to breed, and thus gradually drove out those species which did not use tools. The use of tools required a larger and more sophisticated brain to co-ordinate the fine hand movements required for this task. Larger-brained species therefore survived and prospered while others did not, and brains got steadily larger. By 2 million years ago Homo habilis had appeared in east Africa: the first hominid to make tools rather than merely use them.
The evolution of a larger brain created a problem for the hominids, 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 is why hominid babies, and especially human babies, are so helpless compared with the babies of other mammals.
This adaptation enabled the hominid brain to continue to grow, but it imposed a new discipline on the hominids. The need to care for helpless infants for long periods of time forced hominids to become less mobile. Hominid bands increasingly stayed in one place for long periods, so that females could care for infants, while males hunted food and fought with other hominid bands that competed for food sources. As a result, hominids became even more dependent on tool-making to compete with other animals and each other, and relied less on size and strength. Each successive hominid species had a larger brain relative to its body, and was more gracile (fine-boned)[citation needed] and less physically powerful,[citation needed] than its predecessor.
[edit] Homo erectus, Homo sapiens, and Neanderthal man
These hominids develop language, and the range of activities we call culture, including art and religion. Homo sapiens, like homo erectus, is a migratory animal. He reached Australia 70,000 years ago, Europe 40,000 years ago (the later date is explained by the Ice Age, which made Europe unsuitable for settlement until the ice retreated), and the Americas 30,000 years ago. Homo sapiens is also an aggressive animal. In the course of his expansion, earlier hominids were exterminated or were out competed for resources. As Colin McEvedy wrote: "If the evidence for man's descent is scanty, we can thank our ancestors, who probably ate most of it."
About 200,000 years ago Europe and the Near East were colonised by hominids known to us as Neanderthal man. Once thought to be a race of homo sapiens, the Neanderthals are now thought to have been a closely related but distinct species. They were larger and stronger than modern humans, and their brains were advanced enough for them to have language and culture. They decorated their tools for aesthetic pleasure and buried their dead in ways which suggest spiritual beliefs. Despite these modern characteristics, the Neanderthals were no match for the more numerous homini sapiens when he entered the region about 40,000 years ago, and by 25,000 years ago they were extinct.
[edit] Human culture and civilization
Between 120,000 to 165,000 years ago Homo sapiens had reached his modern physical form in Africa. This dominance allowed a rapid rise in population, which soon reached a point where in some places the human population could no longer be fed by hunting and food gathering. In the fertile river valleys of the Nile, the Euphrates, the Indus and the great rivers of China, this problem was solved by agriculture, which first appeared about 10,000 years ago. The accumulation of food surpluses permitted by agriculture led to the beginnings of urban life and the establishment of the first states, in Sumer in about 3500 BC.
[edit] Consciousness
It may be asked whether earlier hominids possessed consciousness as it is experienced and understood by modern humans. Consciousness is an even harder term to define than intelligence. Consciousness is often understood as a quality of the mind comprising subjectivity, self-awareness, sentience, sapience, and the ability to perceive the relationship between oneself and one's environment. Thus defined, consciousness can be seen as a by-product of the increasing size and sophistication of the brain, a process driven by the demands of natural selection. The ability to think of oneself as distinct from other humans, to recall the past and speculate about the future, to wonder about things which are not known or which cannot be directly experienced, must all have evolved over the course of hominid history as the brain became larger and its ability to think abstractly more advanced.
[edit] Ecological dominance-social competition model
A predominant modern model explaining human intelligence's evolution is ecological dominance-social competition (EDSC) [1] 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.
[edit] Alternative hypothesis: 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 [2]. 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. In spite of that, genetic alleles coding for even larger human brains are spreading continuously even in modern societies [3] [4] This suggests that cleverer humans may gain indirect selective benefits.
A recent study [5] 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 be still 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 [6]. Thus, widespread, virulent, and archaic infections of our species are greatly involved either directly or indirectly. 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 hominids were more heavily affected by pathogens than any other primates, thus this hypothesis can explain why humans became cleverer than other primates. It also explains why humans apparently continue to increase brain size and intelligence even nowadays.
[edit] See also
[edit] References
- ^ Flinn, M. V., Geary, D. C., & Ward, C. V. (2005). Ecological dominance, social competition, and coalitionary arms races: Why humans evolved extraordinary intelligence. Retrieved on 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.
- ^ Rozsa L 2008. The rise of non-adaptive intelligence in humans under pathogen pressure. Medical Hypotheses, 70, 685-690.. Retrieved on 2008-05-26.
- ^ Olness K 2003. Effects on brain development leading to cognitive impairment: a worldwide epidemic. J Dev Behav Pediatr, 24, 120–130
|
|||||||||||||||||||||||||||||
