Himalaya Page 4
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For humans encountering the Himalaya, the very greatest of its impacts is climatic. Stand on the summit of a high mountain along the chain and, if it’s clear, you can look north towards the arid brown of the Tibetan plateau. Then, having turned ninety degrees in either direction, you take in an apparently endless sequence of ridgelines, each of them marking the limit of another river that has carved it out. On one side a desert, on the other some of the wettest places on earth. The contrast is startling, the explanation even more so. As the Tibetan plateau was lifted into the atmosphere, there was less air to interrupt the heat of the sun. As a consequence, the plateau gets hotter than land at sea level; it acts like a vast hot plate that through convection pumps the air above it into the upper troposphere. This in turn draws in a warm, moist wind from the Indian Ocean: the South Asian monsoon. As this air reaches the Himalaya, it rises and cools and its moisture precipitates. It is the action of all that water falling as rain and snow on the south side of the mountains, unable to cross the barrier of the mountains to the north, that has created landscapes so vastly different.
At times the monsoon falters: there is evidence that the population centre around the fortress of Tsaparang in western Tibet failed in the early seventeenth century as the monsoon weakened, drying up fields that were once fertile. The reasons for this are as yet poorly understood but monsoons also weakened in the 1980s and scientists were able to correlate this to a cooling of the Tibetan plateau. The impact of all that hot air rising from the plateau on the jet stream and the global climate is not yet fully understood. There has until recently been a vacuum of data from the region but in recent years Chinese researchers have placed sensors even the plateau’s remoter corners to measure heat rising from all types of land surface. It’s hoped that more and better data will improve monsoon climate models, and with better forecasting comes the possibility of saving lives.
Not only does the monsoon intimately affect the hundreds of millions of people who live in its shadow, the rise of Tibet may have impacted the earth’s climate as a whole. When the monsoon began isn’t certain, but it strengthened markedly seven million years ago, again connected to the rise in altitude of the Tibetan plateau. The palaeoclimatologists Maureen Raymo and William Ruddiman and oceanographer Philip Froelich have linked this increase in rainfall to an increase in chemical weathering, which occurs when carbon dioxide dissolved in rainwater reacts with minerals in the rock, locking it out of the atmosphere. Their theory suggests this process increases during periods of mountain building, because more material is being eroded into rivers. The higher the Tibetan plateau rose, the more rain fell, increasing erosion rates and speeding up the process of chemical weathering. As more carbon dioxide was locked away, the planet cooled, leading to the Pleistocene ice ages that so markedly shaped human history. It is a controversial theory, but there is tantalising evidence supporting it. Climate scientists use the ratio between two oxygen isotopes captured in marine limestone as a proxy for temperature: the greater the ratio the cooler the planet. There is a marked correlation between the timing of this global cooling and the rise of the Himalaya. The Tibetan plateau continues to impact global weather systems; Xiangde Xu from the Chinese Academy of Meteorological Sciences has reported that not only does the Tibetan plateau affect rainfall in China but there is a correlation between greater snowfall on the plateau and warmer Canadian winters.
The combination of rainfall and elevation in the Himalaya is reflected in the immense river systems that drain the mountains. It is not the mountains that frame human activity in the Himalaya: it is the rivers. The three great rivers of East Asia – the Salween, the Mekong and Asia’s longest, the Yangtze – all rise close together on the eastern end of the Tibetan plateau. No wonder bridge-builders have been so admired here. Some of the greatest rivers predate the Himalaya’s rise, including the Indus and the Yarlung Tsangpo–Brahmaputra. Others began with the mountains, including the Ganges and the great river systems of Nepal. These have eroded deep into the mountains, in places right across their axis. The head of the Arun river in Nepal has cut through east of Makalu, the world’s fifth highest mountain, to within just ten kilometres of the Yarlung Tsangpo. As the process continues, the Arun will ‘capture’ the Yarlung Tsangpo and the headwaters of this great river will subsequently flow through Nepal.
It is the combination of altitude and climate that makes the Himalaya so formidable and so formidably diverse. At its narrowest the distance between the plains, or terai, to the south of the mountains and the Tibetan plateau is around a hundred and fifty kilometres. Within that distance the gain in altitude is as much as eight kilometres. Remember that with every kilometre gained in altitude there is a drop in temperature of more than six degrees Celsius. Altitude acts in a similar way to latitude, meaning that in the briefest distance you can move through a wide range of ecosystems: subtropical broadleaf forests in the Siwalik foothills, mixed temperate forests of oak and rhododendron in the middle hills, firs and pines at higher altitudes. Juniper has been found growing in Bhutan at over 4,700 metres, but for the most part the trees thin much lower. Pastureland above the tree line can extend well over five thousand metres. Above that, you are in an ecosystem more akin to the Arctic.
Thanks to altitude, the biodiversity of the Himalaya is astonishing, especially in the eastern part of the range where the monsoon is so strong: eastern Nepal, Sikkim, Bhutan and the Indian state of Arunachal Pradesh. Sikkim, only a little bigger than Delaware, or the English county of Cumbria, has six hundred and fifty different species of orchid. Nepal has a similar number of butterfly species, roughly the same as the whole of the United States, a country more than sixty times its size. At high altitude across the Himalaya are blue sheep, musk deer, red pandas, wolves and snow leopards. In the middle hills I’ve seen leopards, Himalayan bears and langurs, black faces fringed with white fur. In the southern subtropical foothills are tigers, Asian rhinos and wild elephants. In the rivers are dolphins and gharial, a species of crocodile. There are poisonous snakes too, vipers and cobras, another significant risk to life and a major cultural trope on either side and at either end of the mountains in the form of nagas, serpent spirits. Hunting and foraging have been a fact of life for many Himalayan ethnic groups for millennia and even now there is still one group, the Raute, who remain hunter-gatherers. The forests and mountainsides are not just a larder or a place to find building materials: they are a treasure trove of medicinal plants. One of the best-known trades in the Himalaya is in yartsa gunbu, meaning ‘winter worm, summer grass’ in Tibetan, a caterpillar infected with a fungus that grows like a plant and is worth its weight in silver on the Chinese market.
The natural hazards of such a rapid rise in elevation are complex and unpredictable: floods, earthquakes and landslides, but also less obvious dangers, such as glacial lakes draining almost instantly and catastrophically. The most famous example of this was a lake of ten square kilometres near Mount Machhapuchhre in the Annapurna region, which collapsed in the mid sixteenth century, sending a wall of water and five cubic kilometres of debris into the Pokhara valley. These events, called Glacial Lake Outburst Floods, are of great concern today as climate change prompts glacial retreat. Landslides also cause flooding, as they did most notably in early 1841, when a mountain spur on the west side of Nanga Parbat detached and fell into the Indus, creating a dam. A lake quickly formed, and the king of Gilgit, Karim Khan, sent notes written on birch bark and floated downstream, warning that a flood was imminent. When the dam broke in June, a huge wall of water swept down the Indus destroying hundreds of villages and killing thousands of people and their animals. A Sikh army camped near the river outside Attock in northern Punjab was engulfed and five hundred men died in an instant. A survivor, a zamindar or commander, described it thus: ‘As a woman with a wet towel sweeps away a legion of ants, so the river blotted out the army of the Raja.’ The waters at Attock rose fourteen metres above the normal summer flood level. And while exceptional in
its scale, the 1841 disaster was far from unique: Henry Strachey, older brother of John, heard of a similar event in 1835. Floods caused in this way continue to kill along the length of the range.
Earthquakes have been a regular disruptive force throughout Himalayan history. They are mentioned in the Mahabharata and later Buddhist texts. We know a major earthquake in 1255 rocked Nepal’s Kathmandu valley. Even so, despite an extensive literature of historical annals, there is a dearth of accounts. We know there was a series of earthquakes in the sixteenth century, including one in Kumaon in 1505, but it was only in the colonial period that systematic records began to be kept. One of the most important accounts, for its detail and insights, is from 1897, when Richard Dixon Oldham of the Geological Survey of India witnessed an earthquake of magnitude 8.7 in Assam. A fault on the northern side of the Shillong plateau was displaced by as much as sixteen metres and the northern part of the plateau was lifted instantly into the air by eleven metres. Loss of life was surprisingly low, but houses were destroyed across an area three times the size of England.
Geologists call mountains high-energy environments and the immense physical and natural diversity in the Himalaya is reflected in a high-energy human population. Because differences in climate and environment come thick and fast as you move across terrain, the cultural habits that have arisen in response to those differences are intensely focussed. While the monsoon is the region’s major weather system, local climate, even from one side of a valley to the other, can be astonishingly different; a south-facing slope can have a growing season of a month longer. Himalayan people understand very well the French concept of terroir. This diversity is reflected in language: there are more than seventy distinct languages and dialects in Nepal alone. Yet despite this localism, people have always been on the move in the Himalaya. Across the mountains, traders have exchanged Tibetan salt for Indian grain for millennia, a trade only recently disrupted by the arrival of roads. The seasonal migration of herders taking animals to high pasture is another practice that has endured. Population growth and urbanisation is changing the region’s human face faster than ever, and politics with it. Climate change is having a greater impact here than almost anywhere else. Yet, as we shall now see, the adaptations people have made to thrive in this extreme environment not only reach the roots of their culture, they also extend to their genetic code, creating a human suture line unlike almost anything else in human history.
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The First Explorers
On the outskirts of the Chinese city of Shenzhen, just north of Hong Kong, is a former shoe factory eight storeys high that now hosts one of the defining industries of the twenty-first century: genetics. BGI, formerly known as the Beijing Genomics Institute, moved here from the capital in 2007 after it fell out with its masters at the Chinese Academy of Sciences. With one foot in academia and the other in business, it was too strange for the conservative capital: a mixture of state-owned and private enterprise. Yet the company is now the biggest player in the world of genetic sequencing, gobbling up American competitors and employing four thousand people. Those eight storeys are full of DNA sequencing machines and the data farm required to harvest the colossal amounts of information produced. Apart from human beings, including the first full sequencing of an Asian, BGI has sequenced all kinds of things: strains of rice, the cucumber, the chickpea, the giant panda, the Arabian camel, the yak and forty types of silkworm, the latter to protect China’s all-important silk industry. It has also sequenced the DNA of Tibetans.
The indigenous people of the Tibetan plateau are of immense interest to geneticists and evolutionary biologists because of their unique adaptation to the physiological challenges of living permanently at altitudes of over four thousand metres with only half the oxygen of sea level. A few other populations around the world have adapted genetically to this challenge: there are two in the uplands of Ethiopia. But no population has done it quite like Tibetans and other ethnic groups with Tibetan heritage, such as the Sherpa of Nepal. Those of us born at normal altitudes, when confronted with the task of surviving on half the air, produce a cascade of physiological responses, starting with increased breathing and a faster heart rate. Over time, the blood thickens with extra haemoglobin in red blood cells. Whether this is what we mean by acclimatisation, or is actually a problematic by-product, is a matter of debate for some researchers. Haemoglobin makes your blood sticky, causing stroke; long-term elevation can lead to chronic illness and heart attack. Tibetans manage to function perfectly well at altitude with haemoglobin levels that are sometimes lower than lowland populations. Their respiration and heart rates are also similar.
Modern genetics and the mapping of the human genome have allowed us to at least start to unpick the complex interactions and adaptations that make Tibetans so successful at altitude, from how blood vessels function in their muscles to changes in the upper respiratory tract that lets Tibetan noses breathe more easily in the thin, dry air of high altitude. Researchers from the University of Queensland in Australia and Wenzhou Medical University found nine separate genetic differences between Tibetans and lowland populations, including genes connected with haemoglobin levels and the immune system.
The most critical issue is reproduction. You can only pass on your genes if your children survive. I once met a newly born infant in the arms of his mother in a yak-hair tent at over four and a half thousand metres in the middle of the Tibetan plateau. It was a humbling experience. There was no hospital within eighty kilometres and no midwife either. The little boy’s mother had relied on her mother to help her with childbirth, but his rude health was also a product of natural selection. Tibetan women have evolved larger uterine arteries to maintain a healthy flow of oxygen to the growing foetus. Tibetan babies are born at the same weight as lowland babies but are able to extract more oxygen from the air. The birth weight of lowland babies born at altitude reduces by a hundred grams for every thousand metres of height gained. Tibetan mothers also have genetic differences that allow them to produce more folates, an essential B-vitamin, when they’re pregnant.
These discoveries deepen our understanding of history too. The ability of one group of people rather than another to thrive at high altitude is one of the organising principles in the story of the Himalaya. Trekking up valleys on the south side of the range into the high mountains, you come across an obvious ethnic switch between people from the southern lowlands of the subcontinent and South East Asia and people of the Tibetan high plateau, an invisible threshold that hovers at around 3,500 metres. North of this genetic suture line are people most often and loosely termed Bhotias, people of Bod or Bhot, the Tibetan name for Tibet. (The etymology of ‘Tibet’ is uncertain; one theory has it as a Turkic corruption of tu phod, a term from north-eastern Tibet for upper Tibet.) South of this line, and east on the lowlands of China, are populations that lack the genetic adaptations to live and thrive comfortably at altitude. Yet this differentiation doesn’t quite match any modern border between states; those run most usually along the crest of the mountains. This ethnic misalignment has been a source of political tension in recent centuries. After the Second World War, newly communist China briefly laid claim to those parts of the Himalaya with Tibetan populations that were governed by India and Nepal.
Not surprisingly, given the current political context in which Tibet is such contested territory, research into the origins of the Tibetan people is dangerous ground; it’s hard to imagine a more controversial context in which to pursue research. In 2010, the highly regarded journal Science published research that claimed to have found the fastest known example of human evolution in the shape of the Tibetan people. The research was then reported in newspapers around the world. The lead authors, Xin Yi and Jian Wang, both worked for the Beijing Genomics Institute. The paper claimed that the specific genetic differences between Tibetans and Han Chinese occurred only three thousand years ago. The previous fastest known genetic change had been tolerance for lactose among northern Europeans some 7,50
0 years ago. For those pushing a nationalist narrative making Tibet part of the Chinese motherland the implication of this research seemed obvious: the Tibetan people were an offshoot of the Han population that split within the timeframe of recorded history. To archaeologists, the claim made little or no sense. Mark Aldenderfer, an expert in the prehistory of Tibet at the University of California, Merced told the New York Times that the time frame proposed was ‘simply not tenable by anything we know from the historical, archaeological or linguistic record’. While the archaeological picture on the Tibetan plateau is far less complete than on the south side of the Himalaya, there was certainly sufficient evidence to challenge directly the notion that continuous human habitation was as young as the scientists at the Beijing Genomics Institute claimed.
Apart from altitude, a key factor in the peopling of Tibet was climate. Around fifty thousand years ago, the Tibetan plateau was dry and cold: vegetation would have been sparse and the mountains heavily glaciated. Then the climate improved with a rise in rainfall and an increase in temperature. Flourishing grasslands encouraged an expansion in the range and numbers of ungulates native to the plateau: wild yaks; chiru, the Tibetan antelope; khyang, the wild ass; and species of wild sheep. This warmer and wetter period extended north of the plateau to the Taklamakan and Gobi deserts, making migration from the north more likely. Evidence for this includes stone tools found at around 3,100 metres in the Tsaidam basin, an extensive shelf on the north-eastern corner of the plateau, dated at over thirty thousand years old. Closer to the Himalayan chain, eighty kilometres north of Lhasa, is Chusang, a late Palaeolithic site discovered in 1995. At an altitude of 4,200 metres, a series of nineteen human hand and footprints have been preserved in travertine, a sedimentary rock that forms from mineral deposits around hot springs. All the prints were made at the same time when the travertine was still a soft, calcite mud: a snapshot of a family group perhaps, since some of the prints are small enough to be made by children. All sorts of people declare themselves to be explorers; this band, you feel, really were. They faced all the problems of the early European and Asian explorers – the cold, the thin air and brutal wind – but without their technologies or scientific knowledge.