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High-altitude adaptation
High-altitude adaptation is an evolutionary modification in animals, most notably in birds and mammals, by which species are subjected to considerable physiological changes to survive in extremely high mountainous environments. As opposed to short-term adaptation, or more properly acclimatisation (which is basically an immediate physiological response to changing environment), the term "high-altitude adaptation" has strictly developed into the description of an irreversible, long-term physiological responses to high-altitude environments, associated with heritable behavioural and genetic changes. Perhaps, the phenomenon is most conspicuous, at least best documented, in human populations such as the Tibetans, the South Americans and the Ethiopians, who live in the otherwise uninhabitable high mountains of the Himalayas, Andes and Ethiopia respectively; and this represents one of the finest examples of natural selection in action.
Oxygen, essential for animal life, is proportionally abundant in the atmosphere with height from the sea level; hence, the highest mountain ranges of the world are considered unsuitable for habitation. Surprisingly, some 140 million people live permanently at high altitudes (>2,500 m) in North, Central and South America, East Africa, and Asia, and flourish very well for millennia in the exceptionally high mountains, without any apparent complications. This has become a recognised instance of the process of Darwinian evolution in humans acting on favourable characters such as enhanced respiratory mechanisms. As a matter of fact, this adaptation is so far the fastest case of evolution in humans that is scientifically documented. Among animals only few mammals (such as yak, ibex, Tibetan gazelle, vicunas, llamas, mountain goats, etc.) and certain birds are known to have completely adapted to high-altitude environments.
These adaptations are an example of convergent evolution, with adaptations occurring simultaneously on three continents. Tibetan humans and Tibetan domestic dogs found the genetic mutation in both species, EPAS1. This mutation has not been seen in Andean humans, showing the effect of a shared environment on evolution〔 High-altitude adaptation in humansを参照)
At elevation higher than , which is called the "death zone" in mountaineering, the available oxygen in the air is so low that it is considered insufficient to support life. Altitudes higher than 7,600 m (slightly less than 25,000 feet) are seriously lethal.〔 Yet, there are Tibetans, Ethiopians and Americans who habitually live at places higher than 2,500 m from the sea level. For normal human population, even a brief stay at these places means mountain sickness, which is a syndrome of hypoxia or severe lack of oxygen, with complications such as fatigue, dizziness, breathlessness, headaches, insomnia, malaise, nausea, vomiting, body pain, loss of appetite, ear-ringing, blistering and purpling and of the hands and feet, and dilated veins. Amazingly for the native highlanders, there are no adverse effects; in fact, they are perfectly normal in all respects. Basically, the physiological and genetic adaptations in these people involve massive modification in the oxygen transport system of the blood, especially molecular changes in the structure and functions hemoglobin, a protein for carrying oxygen in the body. This is to compensate for perpetual low oxygen environment. This adaptation is associated with better developmental patterns such as high birth weight, increased lung volumes, increased breathing, and higher resting metabolism.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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