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High-altitude adaptation in humans is an instance of evolutionary modification in human populations in Tibet, the Andes and Ethiopia, who have acquired the ability to survive at extremely high altitudes. The phrase is used to signify irreversible, long-term physiological responses to high-altitude environments, associated with heritable behavioural and genetic changes. While the rest of human population would suffer serious health consequences, these native inhabitants thrive well in the highest parts of the world. These people have undergone extensive physiological and genetic changes, particularly in the regulatory systems of respiration and circulation, when compared to the general lowland population. This special adaptation is now recognised as a clear example of natural selection in action. In fact, the adaptation account of the Tibetans has become the fastest case of human evolution in the scientific record, as it is estimated to have occurred in less than 3,000 years. ==Origin and basis== Human species are naturally adapted to lowland environment where oxygen is generally abundant. When people from the general lowlands go to altitudes above , they experience mountain sickness, which is a type of hypoxia, a clinical syndrome of severe lack of oxygen. Complications include fatigue, dizziness, breathlessness, headaches, insomnia, malaise, nausea, vomiting, body pain, loss of appetite, ear-ringing, blistering and purpling of the hands and feet, and dilated veins. The sickness is compounded by related symptoms such as cerebral oedema (swelling of brain) and pulmonary oedema (fluid accumulation in lungs). For several days, they breathe excessively and burn extra energy even when the body is relaxed. The heart rate then gradually decreases. Hypoxia, in fact, is one of the principal causes of death among mountaineers. In women, pregnancy can be severely affected, such as development of high blood pressure, called preeclampsia, which causes premature labour, low birth weight of babies, and often complicated with profuse bleeding, seizures, and death of the mother.〔 More than 140 million people worldwide are estimated to live at an elevation higher than above sea level, of which 13 million are in Ethiopia, 1.7 million in Tibet (total of 78 million in Asia), 35 million in the South American Andes, and 0.3 million in Colorado Rocky Mountains. Certain natives of Tibet, Ethiopia, and the Andes have been living at these high altitudes for generations and are protected from hypoxia as a consequence of genetic adaptation.〔 It is estimated that at , every lungful of air only has 60% of the oxygen molecules that people at sea level have.〔 At elevations above , lack of oxygen becomes seriously lethal. That is, these highlanders are constantly exposed to an intolerably low oxygen environment, yet they live without any debilitating problems in these adverse environments. Basically, the shared adaptation is the ability to maintain relatively low levels of haemoglobin, which is the chemical complex for transporting oxygen in the blood.〔 One of the best documented effects of high altitude is a progressive reduction in birth weight. It has been known that women of long-resident high-altitude population are not affected. These women are known to give birth to heavier-weight infants than women of lowland inhabitants. This is particularly true among Tibetan babies, whose average birth weight is 294-650 (~470) g heavier than the surrounding Chinese population; and their blood-oxygen level is considerably higher. The first scientific investigations of high-altitude adaptation was done by A. Roberto Frisancho of the University of Michigan in the late 1960s among the Quechua people of Peru. However, the best scientific studies were started among the Tibetans in the early 1980s by an anthropologist Cynthia Beall at the Case Western Reserve University. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「High-altitude adaptation in humans」の詳細全文を読む スポンサード リンク
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