Tibetan Plateau

From Wikipedia, the free encyclopedia

Tibetan Plateau
青藏高原 (Qīng–Zàng Gāoyuán, Qinghai–Tibet Plateau)
The Tibetan Plateau lies between the Himalayan range to the south and the Taklamakan Desert to the north.
Dimensions
Length	2,500 km (1,600 mi)
Width	1,000 km (620 mi)
Area	2,500,000 km2 (970,000 sq mi)
Geography
Tibetan Plateau and surrounding areas above 1600 m
Location	China (Tibet, Qinghai, Western Sichuan, Northern Yunnan)  India (Ladakh, Lahaul & Spiti)  Pakistan (Gilgit Baltistan)    Nepal (Northern Nepal)
Range coordinates	33°N 88°ECoordinates: 33°N 88°E

The Tibetan Plateau (Tibetan: བོད་ས་མཐོ།, Wylie: bod sa mtho), also known in China as the Qinghai–Tibet Plateau or the Qing–Zang Plateau (Chinese: 青藏高原; pinyin: Qīng–Zàng Gāoyuán) or Himalayan Plateau, is a vast elevated plateau in Central Asia and East Asia, covering most of the Tibet Autonomous Region and Qinghai in western China, as well as Ladakh (Jammu and Kashmir) and Lahaul & Spiti (Himachal Pradesh) in India. It stretches approximately 1,000 kilometres (620 mi) north to south and 2,500 kilometres (1,600 mi) east to west. With an average elevation exceeding 4,500 metres (14,800 ft), the Tibetan Plateau is sometimes called "the Roof of the World" because it stands over 3 miles (4.8 km) above sea level and is surrounded by imposing mountain ranges that harbor the world's two highest summits, Mount Everest and K2, and is the world's highest and largest plateau, with an area of 2,500,000 square kilometres (970,000 sq mi) (about five times the size of Metropolitan France). Sometimes termed the Third Pole, the Tibetan Plateau contains the headwaters of the drainage basins of most of the streams in surrounding regions. Its tens of thousands of glaciers and other geographical and ecological features serve as a "water tower" storing water and maintaining flow. The impact of global warming on the Tibetan Plateau is of intense scientific interest.

Description

The Tibetan Plateau is surrounded by the massive mountain ranges of High-mountain Asia. The plateau is bordered to the south by the inner Himalayan range, to the north by the Kunlun Mountains, which separate it from the Tarim Basin, and to the northeast by the Qilian Mountains, which separate the plateau from the Hexi Corridor and Gobi Desert. To the east and southeast the plateau gives way to the forested gorge and ridge geography of the mountainous headwaters of the Salween, Mekong, and Yangtze rivers in northwest Yunnan and western Sichuan (the Hengduan Mountains). In the west the curve of the rugged Karakoram range of northern Kashmir embraces the plateau. The Indus River originates in the western Tibetan Plateau in the vicinity of Lake Manasarovar. 

Tibetan Buddhist stupa and houses outside the town of Ngawa, on the Tibetan Plateau.

 

The Tibetan Plateau is bounded in the north by a broad escarpment where the altitude drops from around 5,000 metres (16,000 ft) to 1,500 metres (4,900 ft) over a horizontal distance of less than 150 kilometres (93 mi). Along the escarpment is a range of mountains. In the west the Kunlun Mountains separate the plateau from the Tarim Basin. About halfway across the Tarim the bounding range becomes the Altyn-Tagh and the Kunluns, by convention, continue somewhat to the south. In the 'V' formed by this split is the western part of the Qaidam Basin. The Altyn-Tagh ends near the Dangjin pass on the Dunhuang-Golmud road. To the west are short ranges called the Danghe, Yema, Shule, and Tulai Nanshans. The easternmost range is the Qilian Mountains. The line of mountains continues east of the plateau as the Qinling, which separates the Ordos Plateau from Sichuan. North of the mountains runs the Gansu or Hexi Corridor which was the main silk-road route from China proper to the West. 

The plateau is a high-altitude arid steppe interspersed with mountain ranges and large brackish lakes. Annual precipitation ranges from 100 to 300 millimetres (3.9 to 11.8 in) and falls mainly as hail. The southern and eastern edges of the steppe have grasslands which can sustainably support populations of nomadic herdsmen, although frost occurs for six months of the year. Permafrost occurs over extensive parts of the plateau. Proceeding to the north and northwest, the plateau becomes progressively higher, colder and drier, until reaching the remote Changtang region in the northwestern part of the plateau. Here the average altitude exceeds 5,000 metres (16,000 ft) and winter temperatures can drop to −40 °C (−40 °F). As a result of this extremely inhospitable environment, the Changthang region (together with the adjoining Kekexili region) is the least populous region in Asia, and the third least populous area in the world after Antarctica and northern Greenland. 

NASA satellite image of the south-eastern area of Tibetan Plateau. Brahmaputra River is in the lower right.

Geology and geological history

Yamdrok Lake is one of the three largest sacred lakes in Tibet.

 

The geological history of the Tibetan Plateau is closely related to that of the Himalayas. The Himalayas are among the youngest mountain ranges on the planet and consist mostly of uplifted sedimentary and metamorphic rock. Their formation is a result of a continental collision or orogeny along the convergent boundary between the Indo-Australian Plate and the Eurasian Plate.

The collision began in the Upper Cretaceous period about 70 million years ago, when the north-moving Indo-Australian Plate, moving at about 15 cm (6 in) per year, collided with the Eurasian Plate. About 50 million years ago, this fast moving Indo-Australian plate had completely closed the Tethys Ocean, the existence of which has been determined by sedimentary rocks settled on the ocean floor, and the volcanoes that fringed its edges. Since these sediments were light, they crumpled into mountain ranges rather than sinking to the floor. The Indo-Australian plate continues to be driven horizontally below the Tibetan Plateau, which forces the plateau to move upwards; the plateau is still rising at a rate of approximately 5 mm (0.2 in) per year.

Much of the Tibetan Plateau is of relatively low relief. The cause of this is debated among geologists. Some argue that the Tibetan Plateau is an uplifted peneplain formed at low altitude, while others argue that the low relief stems from erosion and infill of topographic depressions that occurred at already high elevations.

Environment

Typical landscape

 

The Tibetan Plateau supports a variety of ecosystems, most of them classified as montane grasslands. While parts of the plateau feature an alpine tundra-like environment, other areas feature monsoon-influenced shrublands and forests. Species diversity is generally reduced on the plateau due to the elevation and low precipitation. The Tibetan Plateau hosts the Tibetan wolf, and species of snow leopard, wild yak, wild donkey, cranes, vultures, hawks, geese, snakes, and water buffalo. One notable animal is the high-altitude jumping spider, that can live at elevations of over 6,500 metres (21,300 ft).

Ecoregions found on the Tibetan Plateau, as defined by the World Wide Fund for Nature, are as follows:

• The Pamir alpine desert and tundra covers the western end of the Tibetan Plateau where it transitions to the Pamir Mountains
• The North Tibetan Plateau-Kunlun Mountains alpine desert covers the northwestern limits of the Tibetan Plateau along the Kunlun Mountains
• The Karakoram-West Tibetan Plateau alpine steppe covers the westernmost parts of the Tibetan Plateau and Ladakh
• The Northwestern Himalayan alpine shrub and meadows on the edges mountains bordering the extreme west of the Tibetan Plateau
• The Central Tibetan Plateau alpine steppe covers most of the central portions of the Tibetan Plateau and the eastern Changtang
• The Western Himalayan alpine shrub and meadows covers the southwestern plateau in the Garuda Valley region
• The Qaidam Basin semi-desert located in the Qaidam Basin on the northern Tibetan Plateau
• The Qilian Mountains subalpine meadows covering the Qilian Mountains in the northernmost portions of the plateau
• The Qilian Mountains conifer forests covering parts of the mountain ranges in the northeastern Tibetan Plateau
• The Tibetan Plateau alpine shrub and meadows covering a swath of the central and northeastern Tibetan Plateau
• The Yarlung Tsangpo arid steppe in the Yarlung Tsangpo River Valley, where most of the permanent human population on the Tibetan Plateau lives
• The Eastern Himalayan alpine shrub and meadows cover the southern Tibetan Plateau on the north side of the Himalayas
• The Southeast Tibet shrub and meadows cover the southeastern and eastern parts of the plateau and are generally rainier than the other high-altitude Tibetan Plateau regions
• The Northeastern Himalayan subalpine conifer forests reach up mountain valleys in the southern plateau and contain some of the highest altitude forests in the world
• The Nujiang Langcang Gorge alpine conifer and mixed forests cover the mountain valleys that reach 500 km (310 mi) into the southeastern Tibetan Plateau
• The Hengduan Mountains subalpine conifer forests cover the southeasternmost mountain valleys on the plateau
• The Qionglai-Minshan conifer forests cover the eastern edges of the plateau and are the densest forests to be found anywhere on the Tibetan Plateau

Human history

Pastoral nomads camping near Namtso.

Nomads on the Tibetan Plateau and in the Himalayas are the remainders of nomadic practices historically once widespread in Asia and Africa. Pastoral nomads constitute about 40% of the ethnic Tibetan population. The presence of nomadic peoples on the plateau is predicated on their adaptation to survival on the world's grassland by raising livestock rather than crops, which are unsuitable to the terrain. Archaeological evidence suggests that the colonization leading to the full-time occupation of the plateau occurred much later than the previously thought 30,000 years ago. Since colonization of the Tibetan Plateau, Tibetan culture has adapted and flourished in the western, southern, and eastern regions of the plateau. The northern portion, the Changtang, is generally too high and cold to support permanent population. One of the most notable civilizations to have developed on the Tibetan Plateau is the Tibetan Empire from the 7th century to the 9th century AD.

Impact on other regions

Role in monsoons

Natural-colour satellite image of the Tibetan Plateau

 

Monsoons are caused by the different amplitudes of surface temperature seasonal cycles between land and oceans. This differential warming happens because heating rates differ between land and water. Ocean heating is distributed vertically through a "mixed layer" that may be fifty meters deep through the action of wind and buoyancy-generated turbulence, whereas the land surface conducts heat slowly, with the seasonal signal penetrating only a meter or so. Additionally, the specific heat capacity of liquid water is significantly greater than that of most materials that make up land. Together, these factors mean that the heat capacity of the layer participating in the seasonal cycle is much larger over the oceans than over land, with the consequence that the land warms and cools faster than the ocean. In turn, air over the land warms faster and reaches a higher temperature than does air over the ocean. The warmer air over land tends to rise, creating an area of low pressure. The pressure anomaly then causes a steady wind to blow toward the land, which brings the moist air over the ocean surface with it. Rainfall is then increased by the presence of the moist ocean air. The rainfall is stimulated by a variety of mechanisms, such as low-level air being lifted upwards by mountains, surface heating, convergence at the surface, divergence aloft, or from storm-produced outflows near the surface. When such lifting occurs, the air cools due expansion in lower pressure, which in turn produces condensation and precipitation. 

In winter, the land cools off quickly, but the ocean maintains the heat longer. The hot air over the ocean rises, creating a low-pressure area and a breeze from land to ocean while a large area of drying high pressure is formed over the land, increased by wintertime cooling. Monsoons are similar to sea and land breezes, a term usually referring to the localized, diurnal cycle of circulation near coastlines everywhere, but they are much larger in scale, stronger and seasonal. The seasonal monsoon wind shift and weather associated with the heating and cooling of the Tibetan plateau is the strongest such monsoon on Earth.

Glaciology: the Ice Age and at present

The Himalayas as seen from space looking south from over the Tibetan Plateau.

 

Today, Tibet is an important heating surface of the atmosphere. However, during the Last Glacial Maximum, an approximately 2,400,000 square kilometres (930,000 sq mi) ice sheet covered the plateau. Due to its great extent, this glaciation in the subtropics was an important element of radiative forcing. With a much lower latitude, the ice in Tibet reflected at least four times more radiation energy per unit area into space than ice at higher latitudes. Thus, while the modern plateau heats the overlying atmosphere, during the Last Ice Age it helped to cool it.

This cooling had multiple effects on regional climate. Without the thermal low pressure caused by the heating, there was no monsoon over the Indian subcontinent. This lack of monsoon caused extensive rainfall over the Sahara, expansion of the Thar Desert, more dust deposited into the Arabian Sea, and a lowering of the biotic life zones on the Indian subcontinent. Animals responded to this shift in climate, with the Javan rusa migrating into India.

In addition, the glaciers in Tibet created meltwater lakes in the Qaidam Basin, the Tarim Basin, and the Gobi Desert, despite the strong evaporation caused by the low latitude. Silt and clay from the glaciers accumulated in these lakes; when the lakes dried at the end of the ice age, the silt and clay were blown by the downslope wind off the Plateau. These airborne fine grains produced the enormous amount of loess in the Chinese lowlands.

Effect of climate change

The Tibetan Plateau contains the world's third-largest store of ice. Qin Dahe, the former head of the China Meteorological Administration, issued the following assessment in 2009:

Temperatures are rising four times faster than elsewhere in China, and the Tibetan glaciers are retreating at a higher speed than in any other part of the world. [...] In the short term, this will cause lakes to expand and bring floods and mudflows. [...] In the long run, the glaciers are vital lifelines for Asian rivers, including the Indus and the Ganges. Once they vanish, water supplies in those regions will be in peril.

High-altitude adaptation in humans

From Wikipedia, the free encyclopedia

High-altitude adaptation in humans is an instance of evolutionary modification in certain human populations, including those of Tibet in Asia, the Andes of the Americas, and Ethiopia in Africa, who have acquired the ability to survive at extremely high altitudes. This adaptation means irreversible, long-term physiological responses to high-altitude environments, associated with heritable behavioural and genetic changes.

 

While the rest of the human population would suffer serious health consequences, the indigenous inhabitants of these regions thrive well in the highest parts of the world. These people have undergone extensive physiological and genetic changes, particularly in the regulatory systems of oxygen respiration and blood circulation, when compared to the general lowland population.

This special adaptation is now recognised as an example of natural selection in action. 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.

Modern humans dispersed from Africa less than 100,000 years ago, and eventually colonized the rest of the world, including the harshest environments of extreme cold and high mountains. The abundance of oxygen in the atmosphere is inversely related to altitude from the sea level; hence, the highest mountain ranges of the world are considered unsuitable for human habitation.

Nevertheless, around 140 million people, just under 2% of the world's human population, live permanently at high altitudes, that is, at heights above 2,500 metres (8,200 ft) in South America, East Africa, and South Asia. These populations have done so for millennia without apparent complications. The overwhelming majority, over 98% of humans from other parts of the world, normally suffer symptoms of altitude sickness in these regions, often resulting in life-threatening trauma and even death.

Studies on the detail biological mechanism have revealed that adaptation of the Tibetans, Andeans and Ethiopians is indeed an observable instance of the process of natural selection in acting on favourable characters such as enhanced respiratory mechanisms in humans.

Origin and basis

Himalayas, on the southern rim of the Tibetan Plateau

 

Humans are naturally adapted to lowland environment where oxygen is abundant. When people from the general lowlands go to altitudes above 2,500 metres (8,200 ft), with atmospheric pressure 74% of normal 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 2,500 metres (8,200 ft) 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 4,000 metres (13,000 ft), every lungful of air only has 60% of the oxygen molecules that people at sea level have. At elevations above 7,600 metres (24,900 ft), 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. 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. Paul T. Baker, Penn State University, (in the Department of Anthropology) also conducted a considerable amount of research into human adaptation to high altitudes, and mentored students who continued this research. One of these students went on to conduct research on high altitude adaptation among the Tibetans in the early 1980s through to today, anthropologist Cynthia Beall of the Case Western Reserve University.

Physiological basis

Tibetans

A Sherpa family

 

Scientists started to notice the extraordinary physical performance of Tibetans since the beginning of Himalayan climbing era in the early 20th century. The hypothesis of a possible evolutionary genetic adaptation makes sense. The Tibetan plateau has an average elevation of 4,000 metres (13,000 ft) above sea level, and covering more than 2.5 million km, it is the highest and largest plateau in the world. In 1990, it was estimated that 4,594,188 Tibetans live on the plateau, with 53% living at an altitude over 3,500 metres (11,500 ft). Fairly large numbers (about 600,000) live at an altitude exceeding 4,500 metres (14,800 ft) in the Chantong-Qingnan area. Where the Tibetan highlanders live, the oxygen level is only about 60% of that at sea level. The Tibetans, who have been living in this region for 3,000 years, do not exhibit the elevated haemoglobin concentrations to cope with oxygen deficiency as observed in other populations who have moved temporarily or permanently at high altitudes. Instead, the Tibetans inhale more air with each breath and breathe more rapidly than either sea-level populations or Andeans. Tibetans have better oxygenation at birth, enlarged lung volumes throughout life, and a higher capacity for exercise. They show a sustained increase in cerebral blood flow, lower haemoglobin concentration, and less susceptibility to chronic mountain sickness than other populations, due to their longer history of high-altitude habitation.

General people can develop short-term tolerance with careful physical preparation and systematic monitoring of movements, but the biological changes are quite temporary and reversible when they return to lowlands. Moreover, unlike lowland people who only experience increased breathing for a few days after entering high altitudes, Tibetans retain this rapid breathing and elevated lung-capacity throughout their lifetime. This enables them to inhale larger amounts of air per unit of time to compensate for low oxygen levels. In addition, they have high levels (mostly double) of nitric oxide in their blood, when compared to lowlanders, and this probably helps their blood vessels dilate for enhanced blood circulation. Further, their haemoglobin level is not significantly different (average 15.6 g/dl in males and 14.2 g/dl in females), from those of people living at low altitude. (Normally, mountaineers experience >2 g/dl increase in Hb level at Mt. Everest base camp in two weeks.) In this way they are able to evade both the effects of hypoxia and mountain sickness throughout life. Even when they climbed the highest summits like Mt. Everest, they showed regular oxygen uptake, greater ventilation, more brisk hypoxic ventilatory responses, larger lung volumes, greater diffusing capacities, constant body weight and a better quality of sleep, compared to people from the lowland.

Andeans

In contrast to the Tibetans, the Andean highlanders, who have been living at high-altitudes for no more than 11,000 years, show different pattern of haemoglobin adaptation. Their haemoglobin concentration is higher compared to those of lowlander population, which also happens to lowlanders moving to high altitude. When they spend some weeks in the lowland their haemoglobin drops to average of other people. This shows only temporary and reversible acclimatisation. However, in contrast to lowland people, they do have increased oxygen level in their haemoglobin, that is, more oxygen per blood volume than other people. This confers an ability to carry more oxygen in each red blood cell, making a more effective transport of oxygen in their body, while their breathing is essentially at the same rate. This enables them to overcome hypoxia and normally reproduce without risk of death for the mother or baby. The Andean highlanders are known from the 16th-century missionaries that their reproduction had always been normal, without any effect in the giving birth or the risk for early pregnancy loss, which are common to hypoxic stress. They have developmentally acquired enlarged residual lung volume and its associated increase in alveolar area, which are supplemented with increased tissue thickness and moderate increase in red blood cells. Though the physical growth in body size is delayed, growth in lung volumes is accelerated. An incomplete adaptation such as elevated haemoglobin levels still leaves them at risk for mountain sickness with old age. 

Quechua woman with llamas

 

Among the Quechua people of the Altiplano, there is a significant variation in NOS3 (the gene encoding endothelial nitric oxide synthase, eNOS), which is associated with higher levels of nitric oxide in high altitude. Nuñoa children of Quechua ancestry exhibit higher blood-oxygen content (91.3) and lower heart rate (84.8) than their counterpart school children of different ethnicity, who have an average of 89.9 blood-oxygen and 88-91 heart rate. High-altitude born and bred females of Quechua origins have comparatively enlarged lung volume for increased respiration.

Aymara ceremony

 

Blood profile comparisons show that among the Andeans, Aymaran highlanders are better adapted to highlands than the Quechuas. Among the Bolivian Aymara people, the resting ventilation and hypoxic ventilatory response were quite low (roughly 1.5 times lower), in contrast to those of the Tibetans. The intrapopulation genetic variation was relatively less among the Aymara people. Moreover, unlike the Tibetans, the blood haemoglobin level is quite normal among Aymarans, with an average of 19.2 g/dl for males and 17.8 g/dl for females. Among the different native highlander populations, the underlying physiological responses to adaptation are quite different. For example, among four quantitative features, such as are resting ventilation, hypoxic ventilatory response, oxygen saturation, and haemoglobin concentration, the levels of variations are significantly different between the Tibetans and the Aymaras.

Ethiopians

The peoples of the Ethiopian highlands also live at extremely high altitudes, around 3,000 metres (9,800 ft) to 3,500 metres (11,500 ft). Highland Ethiopians exhibit elevated haemoglobin levels, like Andeans and lowlander peoples at high altitudes, but do not exhibit the Andean’s increased in oxygen-content of haemoglobin. Among healthy individuals, the average haemoglobin concentrations are 15.9 and 15.0 g/dl for males and females respectively (which is lower than normal, almost similar to the Tibetans), and an average oxygen saturation of haemoglobin is 95.3% (which is higher than average, like the Andeans). Additionally, Ethiopian highlanders do not exhibit any significant change in blood circulation of the brain, which has been observed among the Peruvian highlanders (and attributed to their frequent altitude-related illnesses). Yet, similar to the Andeans and Tibetans, the Ethiopian highlanders are immune to the extreme dangers posed by high-altitude environment, and their pattern of adaptation is definitely unique from that of other highland peoples.

Genetic basis

The underlying molecular evolution of high-altitude adaptation has been explored and understood fairly recently. Depending on the geographical and environmental pressures, high-altitude adaptation involves different genetic patterns, some of which have evolved quite recently. For example, Tibetan adaptations became prevalent in the past 3,000 years, a rapid example of recent human evolution. At the turn of the 21st century, it was reported that the genetic make-up of the respiratory components of the Tibetan and the Ethiopian populations are significantly different.

Tibetans

Substantial evidence in Tibetan highlanders suggests that variation in haemoglobin and blood-oxygen levels are adaptive as Darwinian fitness. It has been documented that Tibetan women with a high likelihood of possessing one to two alleles for high blood-oxygen content (which is odd for normal women) had more surviving children; the higher the oxygen capacity, the lower the infant mortality. In 2010, for the first time, the genes responsible for the unique adaptive traits were identified following genome sequences of 50 Tibetans and 40 Han Chinese from Beijing. Initially, the strongest signal of natural selection detected was a transcription factor involved in response to hypoxia, called endothelial Per-Arnt-Sim (PAS) domain protein 1 (EPAS1). It was found that one single-nucleotide polymorphism (SNP) at EPAS1 shows a 78% frequency difference between Tibetan and mainland Chinese samples, representing the fastest genetic change observed in any human gene to date. Hence, Tibetan adaptation to high altitude becomes the fastest process of phenotypically observable evolution in humans, which is estimated to occur in less than 3,000 years ago, when the Tibetans split up from the mainland Chinese population. Mutations in EPAS1, at higher frequency in Tibetans than their Han neighbours, correlate with decreased haemoglobin concentrations among the Tibetans, which is the hallmark of their adaptation to hypoxia. Simultaneously, two genes, egl nine homolog 1 (EGLN1) (which inhibits haemoglobin production under high oxygen concentration) and peroxisome proliferator-activated receptor alpha (PPARA), were also identified to be positively selected in relation to decreased haemoglobin nature in the Tibetans.

Similarly, the Sherpas, known for their Himalayan hardiness, exhibit similar patterns in the EPAS1 gene, which further fortifies that the gene is under selection for adaptation to the high-altitude life of Tibetans. A study in 2014 indicates that the mutant EPAS1 gene could have been inherited from archaic hominins, the Denisovans. EPAS1 and EGLN1 are definitely the major genes for unique adaptive traits when compared with those of the Chinese and Japanese. Comparative genome analysis in 2014 revealed that the Tibetans inherited an equal mixture of genomes from the Nepalese-Sherpas and Hans, and they acquired the adaptive genes from the sherpa-lineage. Further, the population split was estimated to occur around 20,000 to 40,000 years ago, a range of which support archaeological, mitochondria DNA and Y chromosome evidence for an initial colonisation of the Tibetan plateau around 30,000 years ago.

The genes (EPAS1, EGLN1, and PPARA) function in concert with another gene named hypoxia inducible factors (HIF), which in turn is a principal regulator of red blood cell production (erythropoiesis) in response to oxygen metabolism. The genes are associated not only with decreased haemoglobin levels, but also in regulating energy metabolism. EPAS1 is significantly associated with increased lactate concentration (the product of anaerobic glycolysis), and PPARA is correlated with decrease in the activity of fatty acid oxidation. EGLN1 codes for an enzyme, prolyl hydroxylase 2 (PHD2), involved in erythropoiesis. Among the Tibetans, mutation in EGLN1 (specifically at position 12, where cytosine is replaced with guanine; and at 380, where G is replaced with C) results in mutant PHD2 (aspartic acid at position 4 becomes glutamine, and cysteine at 127 becomes serine) and this mutation inhibits erythropoiesis. The mutation is estimated to occur about 8,000 years ago. Further, the Tibetans are enriched for genes in the disease class of human reproduction (such as genes from the DAZ, BPY2, CDY, and HLA-DQ and HLA-DR gene clusters) and biological process categories of response to DNA damage stimulus and DNA repair (such as RAD51, RAD52, and MRE11A), which are related to the adaptive traits of high infant birth weight and darker skin tone and are most likely due to recent local adaptation.

Andeans

The patterns of genetic adaptation among the Andeans are largely distinct from those of the Tibetan, with both populations showing evidence of positive natural selection in different genes or gene regions. However, EGLN1 appears to be the principal signature of evolution, as it shows evidence of positive selection in both Tibetans and Andeans. Even then, the pattern of variation for this gene differs between the two populations. Among the Andeans, there are no significant associations between EPAS1 or EGLN1 SNP genotypes and haemoglobin concentration, which has been the characteristic of the Tibetans. The whole genome sequences of 20 Andeans (half of them having chronic mountain sickness) revealed that two genes, SENP1 (an erythropoiesis regulator) and ANP32D (an oncogene) play vital roles in their weak adaptation to hypoxia.

Ethiopians

The adaptive mechanism of Ethiopian highlanders is quite different. This is probably because their migration to the highland was relatively early; for example, the Amhara have inhabited altitudes above 2,500 metres (8,200 ft) for at least 5,000 years and altitudes around 2,000 metres (6,600 ft) to 2,400 metres (7,900 ft) for more than 70,000 years. Genomic analysis of two ethnic groups, Amhara and Oromo, revealed that gene variations associated with haemoglobin difference among Tibetans or other variants at the same gene location do not influence the adaptation in Ethiopians. Identification of specific genes further reveals that several candidate genes are involved in Ethiopians, including CBARA1, VAV3, ARNT2 and THRB. Two of these genes (THRB and ARNT2) are known to play a role in the HIF-1 pathway, a pathway implicated in previous work reported in Tibetan and Andean studies. This supports the concept that adaptation to high altitude arose independently among different highlanders as a result of convergent evolution.

Human taxonomy

From Wikipedia, the free encyclopedia

Homo ("humans") Temporal range: Piacenzian-Present, 2.865–0 Ma PreЄ Є O S D C P T J K Pg N ↓
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Primates
Suborder:	Haplorhini
Infraorder:	Simiiformes
Family:	Hominidae
Subfamily:	Homininae
Tribe:	Hominini
Genus:	Homo Linnaeus, 1758
Type species
Homo sapiens Linnaeus, 1758
Species
Homo sapiens †Homo erectus other species or subspecies suggested

The taxonomic classification of humans following John Edward Gray (1825).

 

Overview of speciation and hybridization within the genus Homo over the last two million years (vertical axis). The rapid "Out of Africa" expansion of H. sapiens is indicated at the top of the diagram, with admixture indicated with Neanderthals, Denisovans, and unspecified archaic African hominins.

Human taxonomy is the classification of the human species (systematic name Homo sapiens, Latin: "knowing man") within zoological taxonomy. The systematic genus, Homo, is designed to include both anatomically modern humans and extinct varieties of archaic humans. Current humans have been designated as subspecies Homo sapiens sapiens, differentiated from the direct ancestor, Homo sapiens idaltu. 

Since the introduction of systematic names in the 18th century, knowledge of human evolution has increased drastically, and a number of intermediate taxa have been proposed in the 20th to early 21st century. The most widely accepted taxonomy groups takes the genus Homo as originating between two and three million years ago, divided into at least two species, archaic Homo erectus and modern Homo sapiens, with about a dozen further suggestions for species without universal recognition. 

The genus Homo is placed in the tribe Hominini alongside Pan (chimpanzees). The two genera are estimated to have diverged over an extended time of hybridization spanning roughly 10 to 6 million years ago, with possible admixture as late as 4 million years ago. A subtribe of uncertain validity, grouping archaic "pre-human" or "para-human" species younger than the Homo-Pan split is Australopithecina (proposed in 1939). 

A proposal by Wood and Richmond (2000) would introduce Hominina as a subtribe alongside Australopithecina, with Homo the only known genus within Hominina. Alternatively, following Cela-Conde and Ayala (2003), the "pre-human" or "proto-human" genera of Australopithecus, Ardipithecus, Praeanthropus, and possibly Sahelanthropus may be placed on equal footing alongside the genus Homo. An even more radical view rejects the division of Pan and Homo as separate genera, which based on the Principle of Priority would imply the re-classification of chimpanzees as Homo paniscus (or similar).

Prior to the current scientific classification of humans, philosophers and scientists have made various attempts to classify humans. They offered definitions of the human being and schemes for classifying types of humans. Biologists once classified races as subspecies, but today anthropologists reject the concept of race and view humanity as an interrelated genetic continuum. Taxonomy of the hominins continues to evolve.

History

Human taxonomy on one hand involves the placement of humans within the Taxonomy of the hominids (great apes), and on the other the division of archaic and modern humans into species and, if applicable, subspecies. Modern zoological taxonomy was developed by Carl Linnaeus during the 1730s to 1750s. He named the human species as Homo sapiens in 1758, as the only member species of the genus Homo, divided into several subspecies corresponding to the great races. The Latin noun homō (genitive hominis) means "human being". The systematic name Hominidae for the family of the great apes was introduced by John Edward Gray (1825). Gray also supplied Hominini as the name of the tribe including both chimpanzees (genus Pan) and humans (genus Homo).

The discovery of the first extinct archaic human species from the fossil record dates to the mid 19th century, Homo neanderthalensis, classified in 1864. Since then, a number of other archaic species have been named, but there is no universal consensus as to their exact number. After the discovery of H. neanderthalensis, which even if "archaic" is recognizable as clearly human, late 19th to early 20th century anthropology for a time was occupied with finding the supposedly "missing link" between Homo and Pan. The "Piltdown Man" hoax of 1912 was the (fraudulent) presentation of such a transitional species. Since the mid-20th century, knowledge of the development of Hominini has become much more detailed, and taxonomical terminology has been altered a number of times to reflect this. 

The introduction of Australopithecus as a third genus, alongside Homo and Pan, in the Hominini tribe is due to Raymond Dart (1925). Australopithecina as a subtribe containing Australopithecus as well as Paranthropus (Broom 1938) is a proposal by Gregory & Hellman (1939). More recently proposed additions to the Australopithecina subtribe include Ardipithecus (1995) and Kenyanthropus (2001). The position of Sahelanthropus (2002) relative to Australopithecina within Hominini is unclear. Cela-Conde and Ayala (2003) propose the recognition of Australopithecus, Ardipithecus, Praeanthropus, and Sahelanthropus (the latter incertae sedis) as separate genera.

Other proposed genera, now mostly considered part of Homo, include: Pithecanthropus (Dubois, 1894), Protanthropus (Haeckel, 1895), Sinanthropus (Black, 1927), Cyphanthropus (Pycraft, 1928) Africanthropus (Dreyer, 1935), Telanthropus (Broom & Anderson 1949), Atlanthropus (Arambourg, 1954), Tchadanthropus (Coppens, 1965).

The genus Homo has been taken to originate some two million years ago since the discovery of stone tools in Olduvai Gorge, Tanzania, in the 1960s. Homo habilis (Leakey et al., 1964) would be the first "human" species (member of genus Homo) by definition, its type specimen being the OH 7 fossils. However, the discovery of more fossils of this type has opened up the debate on the delineation of H. habilis from Australopithecus. Especially, the LD 350-1 jawbone fossil discovered in 2013, dated to 2.8 Mya, has been argued as being transitional between the two. It is also disputed whether H. habilis was the first hominin to use stone tools, as Australopithecus garhi, dated to c. 2.5 Mya, has been found along with stone tool implements. Fossil KNM-ER 1470 (discovered in 1972, designated Pithecanthropus rudolfensis by Alekseyev 1978) is now seen as either a third early species of Homo (alongside H. habilis and H. erectus) at about 2 million years ago, or alternatively as transitional between Australopithecus and Homo.

Wood and Richmond (2000) proposed that Gray's tribe Hominini ("hominins") be designated as comprising all species after the chimpanzee-human last common ancestor by definition, to the inclusion of Australopithecines and other possible pre-human or para-human species (such as Ardipithecus and Sahelanthropus) not known in Gray's time. In this suggestion, the new subtribe of Hominina was to be designated as including the genus Homo exclusively, so that Hominini would have two subtribes, Australopithecina and Hominina, with the only known genus in Hominina being Homo. Orrorin (2001) has been proposed as a possible ancestor of Hominina but not Australopithecina.

Designations alternative to Hominina have been proposed: Australopithecinae (Gregory & Hellman 1939) and Preanthropinae (Cela-Conde & Altaba 2002).

Species

At least a dozen species of Homo other than Homo sapiens have been proposed, with varying degrees of consensus. Homo erectus is widely recognized as the species directly ancestral to Homo sapiens. Most other proposed species are proposed as alternatively belonging to either Homo erectus or Homo sapiens as a subspecies. This concerns Homo ergaster in particular. One proposal divides Homo erectus into an African and an Asian variety; the African is Homo ergaster, and the Asian is Homo erectus sensu stricto. (Inclusion of Homo ergaster with Asian Homo erectus is Homo erectus sensu lato.) There appears to be a recent trend, with the availability of ever more difficult-to-classify fossils such as the Dmanisi skulls (2013) or Homo naledi fossils (2015) to subsume all archaic varieties under Homo erectus.

Subspecies

Homo sapiens subspecies

1737 painting of Carl von Linné wearing a traditional Sami costume, sometimes named as the lectotype of both H. sapiens and H. s. sapiens.

 

The recognition or non-recognition of subspecies of Homo sapiens has a complicated history. The rank of subspecies in zoology is introduced for convenience, and not by objective criteria, based on pragmatic consideration of factors such as geographic isolation and sexual selection. The informal taxonomic rank of race is variously considered equivalent or subordinate to the rank of subspecies, and the division of anatomically modern humans (H. sapiens) into subspecies is closely tied to the recognition of major racial groupings based on human genetic variation.

A subspecies cannot be recognized independently: a species will either be recognized as having no subspecies at all or at least two (including any that are extinct). Therefore, the designation of an extant subspecies Homo sapiens sapiens only makes sense if at least one other subspecies is recognized. H. s. sapiens is attributed to "Linnaeus (1758)" by the taxonomic Principle of Coordination. William Stearn (1959) in a "passing remark" argued that Linnaeus "must stand as the type of his Homo sapiens". Since Linnaeus describes H. s. europaeus as having blue/green (caerulus) eyes but himself had brown eyes, he cannot have included himself in H. s. europaeus, Linnaeus would therefore have to be classified as H. sapiens sapiens, as not matching any of the descriptions of his five subspecies, and so would stand as the lectotype both for H. sapiens, and for H. s. sapiens within his own subspecies nomenclature.

During the 19th to mid-20th century, it was common practice to classify the major divisions of extant H. sapiens as subspecies, following Linnaeus (1758), who had recognized H. s. americanus, H. s. europaeus, H. s. asiaticus and H. s. afer as grouping the native populations of the Americas, West Eurasia, East Asia and Sub-Saharan Africa, respectively, besides H. s. ferus (for the "wild" form which he identified with feral children) and two further "wild" forms for reported specimens now considered part of cryptozoology, H. s. monstrosus and H. s. troglodytes.

There were variations and additions to the categories of Linnaeus, such as H. s. tasmanianus for the native population of Australia. Bory de St. Vincent in his Essai sur l'Homme (1825) extended Linné's "racial" categories to as many as fifteen: Leiotrichi ("smooth-haired"): japeticus (with subraces), arabicus, indicus, scythicus, sinicus, hyperboreus, neptunianus, australasicus, columbicus, americanus, patagonicus; Oulotrichi ("crisp-haired"): aethiopicus, cafer, hottentotus, melaninus. Similarly, Georges Vacher de Lapouge (1899) also had categories based on race, such as priscus, spelaeus (etc.).

Homo sapiens neanderthalensis was proposed by King (1864) as an alternative to Homo neanderthalensis. There have been "taxonomic wars" over whether Neanderthals were a separate species since their discovery in the 1860s. Pääbo (2014) frames this as a debate that is unresolvable in principle, "since there is no definition of species perfectly describing the case." Louis Lartet (1869) proposed Homo sapiens fossilis based on the Cro-Magnon fossils. 

There are a number of proposals of extinct varieties of Homo sapiens made in the 20th century. Many of the original proposals were not using explicit trinomial nomenclature, even though they are still cited as valid synonyms of H. sapiens by Wilson & Reeder (2005). These include: Homo grimaldii (Lapouge, 1906), Homo aurignacensis hauseri (Klaatsch & Hauser, 1910), Notanthropus eurafricanus (Sergi, 1911), Homo fossilis infrasp. proto-aethiopicus (Giuffrida-Ruggeri, 1915), Telanthropus capensis (Broom, 1917), Homo wadjakensis (Dubois, 1921), Homo sapiens cro-magnonensis, Homo sapiens grimaldiensis (Gregory, 1921), Homo drennani (Kleinschmidt, 1931), Homo galilensis (Joleaud, 1931) = Paleanthropus palestinus (McCown & Keith, 1932). Rightmire (1983) proposed Homo sapiens rhodesiensis.

By the 1980s, the practice of dividing extant populations of Homo sapiens into subspecies declined. An early authority explicitly avoiding the division of H. sapiens into subspecies was Grzimeks Tierleben, published 1967–1972. A late example of an academic authority proposing that the human racial groups should be considered taxonomical subspecies is John Baker (1974). The trinomial nomenclature Homo sapiens sapiens became popular for "modern humans" in the context of Neanderthals being considered a subspecies of H. sapiens in the second half of the 20th century. Derived from the convention, widespread in the 1980s, of considering two subspecies, H. s. neanderthalensis and H. s. sapiens, the explicit claim that "H. s. sapiens is the only extant human subspecies" appears in the early 1990s. This is only true if the nomenclature derived from Linnaeus is rejected. Based on Linnaeus (1758), there are at least six subspecies, with H. s. sapiens catching those specimens not included in any other. 

Since the 2000s, the extinct Homo sapiens idaltu (White et al., 2003) has gained wide recognition as a subspecies of Homo sapiens, but even in this case there is a dissenting view arguing that "the skulls may not be distinctive enough to warrant a new subspecies name". H. s. neanderthalensis and H. s. rhodesiensis continue to be considered separate species by some authorities, but the genetic evidence of archaic human admixture with modern humans discovered in the 2010s has re-opened the details of taxonomy of archaic humans.

Homo erectus subspecies

Homo erectus since its introduction in 1892 has been divided into numerous subspecies, many of them formerly considered individual species of Homo. None of these subspecies have universal consensus among paleontologists.

• Homo erectus erectus (Java Man) (1970s)
• Homo erectus yuanmouensis (Yuanmou Man) (Li et al., 1977)
• Homo erectus lantianensis (Lantian Man) (Woo Ju-Kang, 1964)
• Homo erectus nankinensis (Nanjing Man) (1993)
• Homo erectus pekinensis (Peking Man) (1970s)
• Homo erectus palaeojavanicus (Meganthropus) (Tyler, 2001)
• Homo erectus soloensis (Solo Man) (Oppenoorth, 1932)
• Homo erectus tautavelensis (Tautavel Man) (de Lumley and de Lumley, 1971)
• Homo erectus georgicus (1991)
• Homo erectus bilzingslebenensis (Vlček, 2002)