Mark S Aldenderfer. American Scientist. Volume 91, Issue 6. Nov/Dec 2003.
Standing some four kilometers above sea level, the world’s highest plateaus, the Andean altiplano and the Tibetan plateau, appear bleak, cold and uninviting. These are clearly tough places to make a living, yet large numbers of people reside there today-and not all are recent immigrants. Population estimates of indigenous highlanders are difficult to come by, but at least six or seven million reside on the altiplano, and some two million occupy the Tibetan plateau. Even larger numbers lived on these highlands in the past. Educated guesses for the pre-Columbian population in the Andes vary considerably, but it is probable that from 10 to 12 million people were living at high elevation in 1492. Estimates for the Tibetan plateau for the early 20th century suggest around six million inhabitants.
These large populations supported impressive cultural achievements. In South America from 600 to 1100 A.D., the Tiwanaku polity, with its capital in modern-day Bolivia near Lake Titicaca, controlled much of the Titicaca basin and surrounding lowlands on both flanks of the Andes. And the Tibetan Empire, with its seat of power in Lhasa, ruled much of central Asia from the mid-7th through mid-9th centuries A.D. The empire controlled access to the lucrative Silk Route (which stretched from Europe to Japan), sacked the Chinese capital at Xian in 763 and established Buddhism as the state religion. Although no central power emerged on the Tibetan plateau after the collapse of the empire, in the Andes the Tiwanaku gave way some 350 years later to the Inca. From their homeland in the Cuzco basin (at an elevation of some 3,600 meters), the Inca created the largest empire in the New World, which stretched from northern Ecuador into northwestern Argentina.
These examples make it abundantly clear that high elevations are not intrinsically inimical to the emergence and expansion of complex societies. Yet high plateaus are generally harsh and forbidding places. That is why the world’s earliest civilizations all developed in lowlands. In the Andes it took almost 3,000 years for cultures on the altiplano to reach a state of complexity comparable to what could be found on the coast, and highly advanced societies flourished at low elevations in China 4,000 years before anything similar emerged to the west on the Tibetan plateau itself.
The disparity between low- and high-elevation environments becomes even more pronounced when one examines the long archaeological record of prehistoric human migrations. Viewed from this lofty perspective, these plateaus seem to have become occupied very late in time-in a sense, almost at the last minute. Although the data are sketchy and incomplete and, as I shall discuss, controversial, the high Andes were not visited until 11,000 years ago, and the Tibetan plateau has probably had people treading on it for no more than 25,000 years. Yet humans have lived at low elevations surrounding these plateaus far longer.
Finds of Homo erectus fossils, an early hominid species, are well known from low elevation areas of China and may date to 800,000 years ago. Along the southwestern flanks of the Himalayas, stretching from Pakistan to Nepal in a geological formation known as the Siwalik Hills, is a series of archeological sites that contain 500,000-year-old hand axes. To the north of the plateau in central Mongolia, at a cave site called Tsagaan Agui, recent archaeological research has confirmed a human presence for at least 125,000 years. Indeed, farther to the north and west in Siberia, vestiges of human activity likely date to somewhere between 200,000 and 300,000 years ago. Although it is now rather clear that these early humans were not the ancestors of today’s populations [see “We Are All Africans,” page 496], there is no question that the Tibetan plateau was surrounded by people long before it was occupied.
Although human movement into the Andean highlands came very late in prehistory because the colonization of the New World itself came late-perhaps no earlier than some 15,000 years ago-the pattern of migration is similar to that of Tibet. The earliest South American sites, such as Monte Verde, which is located in southern Chile and dates to about 13,500 years ago, are all found at low elevations, along both the Atlantic and Pacific coasts. Even the interior of the Amazon basin, a notoriously difficult habitat for hunter-gatherers, gave way earlier than the Andean highlands.
Coping Mechanisms
What explains the tardiness of permanent human occupation and the slow emergence of complex societies on these high plateaus? One factor that must be considered in any explanation is that humans evolved at low elevations. The physiological processes governing the transmission of oxygen from the alveoli in our lungs into the hemoglobin in our blood and thence on to the mitochrondrial powerhouses in our cells, developed in conditions of sealevel, or near-sea level, air. Living at high altitude thus robs the human body of the abundant oxygen for which it has evolved.
For most lowlanders, the effects of this hypoxia, which is simply defined as getting less than the normal amount of oxygen in inspired air, begin at elevations around 2,500 meters. Transient hypoxic effects (sometimes known as “acute mountain sickness”) include headache, nausea, hyperventilation, dizziness and rapid fatigue. These obvious symptoms diminish for most people after 48 hours and pose no long-term threat to health. However, the lasting effects of chronic hypoxia are more subtle: It diminishes work capacity, fertility and cognition, and it generally slows growth and maturation in youngsters. Hypoxia also brings on increased exposure to altitude-induced ailments, such as chronic mountain sickness, and a host of altitude-aggravated diseases, primarily those of the pulmonary system.
Another key factor is that high-elevation environments are usually extreme ones. In addition to hypoxia, residents must manage in a habitat where little grows and where what does grow has a patchy distribution. The amount and timing of rainfall are difficult to predict, and tectonic activity often disturbs the land, exposing it to erosion and landslide. Further, high-elevation environments, even in tropical latitudes, are usually rather cold places. This Stressor exacerbates the effects of hypoxia and leads to increased energy demands as well as increased mortality rates, with many people succumbing to pulmonary infections. As altitude increases, so does the risk for frostbite, the onset of which is hastened by the typically strong winds. Snow blindness is also a significant hazard during the winter. In short, life at the top of the world can be a real struggle.
To live permanently and thrive at high elevation, a person must have two things: a set of physiological adaptations to cope with the reduced availability of oxygen and a suite of cultural adaptations to cope with the harsh environment. These include, at a minimum, fire, effective clothing and a reliable set of tools for eking out a living. Hunting and gathering peoples, who were the first inhabitants of the high plateaus, also had to work out patterns of seasonal movement that minimized exposure to environmental hazards while simultaneously providing them with sufficient calories.
Because their basic requirements are the same, one might expect that Tibetan and Andean peoples are broadly similar in their adaptations to high altitude. In fact, they are not at all alike. For example, Tibetans and Andean peoples show substantial differences in the way their bodies cope with chronic hypoxia. Cynthia M. Beall, an anthropologist at Case Western Reserve University, has identified three traits in which Tibetans and Andean Highlanders differ significantly: Tibetans can maintain very high respiratory rates while at rest (a characteristic of acute hypoxia), whereas Andean natives, as well as acclimatized lowlanders, have rates similar to those seen in sea-level natives. Tibetans also show high scores in tests of something called hypoxic ventilatory response (a reflex to increase respiratory rate under hypoxic conditions), whereas Andean natives exhibit a very low response. Finally, the concentration of hemoglobin (the protein that carries oxygen) in Tibetan blood is no different from that found in sea-level residents, whereas Andean natives show very high hemoglobin concentrations.
Beall’s observations raise some interesting questions. Does one physiological pattern work better than the other? Are these differences based on natural selection? What role, if any, has culture played in this process of biological adaptation? Although there are no definitive answers, various explanations for these differences have been offered. Some authors posit that the traits that help these people cope with chronic hypoxia are merely by-products of long-term adaptations for greater strength or endurance at any altitude. Other workers, such as Lorna G. Moore of the University of Colorado at Denver and her colleagues, have argued that these traits are indeed evolutionary adaptations to altitude and that the Tibetans have a unique set because they have lived at high elevation much longer than have Andean natives. In contrast, Beall and her co-workers argue that Tibetans and Andean natives differ because evolution just happened to take a slightly different course on two continents and that their present-day distinctness may just stem from chance differences in genetic makeup of the adventurous lowlanders who migrated upward and founded these two populations (although she accepts that the length of occupation at high elevation also may have contributed). There are plenty of ideas and opinions, but very little data with which to test them.
Digging for Answers
Archaeology can help resolve some of these issues through careful consideration of the antiquity of life at high elevations, the probable source of the founder populations that occupied the plateaus and the processes by which humans colonized them.
Unfortunately, knowledge of the prehistory of the Tibetan plateau is very sketchy. Systematic exploration did not begin there until the 1960s, when the Chinese Academy of Sciences sent in teams of geologists to conduct surveys of the local natural resources, during which they found many archaeological sites at elevations ranging from 3,600 to more than 4,800 meters. It was not until the 1970s, however, that archaeologists evaluated these sites. None were excavated, but Chinese archaeologists attempted to gauge their age by matching them up with sites in eastern and northern Asia that had been securely dated. The comparisons of artifacts led some archaeologists to conclude that the plateau had been occupied for at least 30,000 years. Until recently, Xiao Qaidam, located on the northern fringes of the plateau, was thought to be the oldest site. Although the levels that contained human artifacts-simple stone tools-were not directly dated, correlations with better-known layers nearby suggested the deposit was between 33,000 and 35,000 years old. More recent research, however, indicates that these materials are about 22,000 years old, or possibly younger.
The best current candidate for the oldest site is called Chusang. It is located in the central part of the plateau just to the west of Lhasa at an elevation of 4,200 meters. Two scientists from the University of Hong Kong, David Zhang and S. Li, discovered this site in 1995. It contains 19 human hand- and footprints, impressed into a single layer of mud. Size differences in the prints suggest that both adults and children made them. The rock with these prints is called travertine, because it began as a soft mud full of calcium carbonate. The exact environment of deposition is unclear from the observations Zhang and Li recorded, but the presence of calcium carbonate indicates that the overlying water must have come from a hot spring. The prints were formed sometime after the mud settled and before the ooze subsequently solidified, forming what is at present a hard, calcareous deposit.
Zhang and Li also found what they describe as a hearth near one concentration of prints. Unfortunately, they unearthed no artifacts of any kind, but that is not very surprising given that they did not conduct a systematic search. Zhang and Li were, however, quite successful in obtaining ages for quartz crystals extracted from the travertine and the hearth using optically stimulated luminescence dating, a method that measures the amount of time that the sediments have remained in the dark (that is, the time over which they have been buried). Their study produced dates ranging between 21,000 and 22,000 years before present.
Soon after I learned of this astonishing result, I set my sights on getting to Chusang to do more work there. I was ready to lead a team of geologists, paleoclimatologists and archaeologists to Chusang last summer to further study the site, but authorities halted all travel to the plateau after the SARS (Severe Acute Respiratory Syndrome) epidemic struck elsewhere in China. My colleagues and I are hoping for better luck this coming summer, when we plan to try again.
If Chusang indeed proves to be older than 20,000 years, this site will have a number of important implications. For most of the 20th century, many geologists believed that the Tibetan plateau was covered with a thick sheet of glacial ice during the interval between 22,000 and 18,000 years ago (a period known as the last glacial maximum). If so, Chusang could not have been occupied at this time. But more recent research suggests that, although the last glacial maximum may have been very cold and probably arid as well, no such massive ice sheet existed. Glaciers covered the high mountain peaks but did not descend into the valley floors. Life at Chusang must have been hard, but it was certainly possible.
Still, getting to the central part of the Tibetan plateau during the height of the last ice age would have been very difficult. (It’s tricky enough now.) So it makes sense to suppose that people arrived somewhat earlier. From 50,000 to 25,000 years ago, climatic conditions across the plateau were relatively benign. Geologic evidence shows that the glaciers retreated and lake levels rose, indicating a significant increase in precipitation compared with former times. It is reasonable to infer that temperatures also increased. As these processes unfolded, cold deserts transformed into steppes. This modest greening in turn would have led to the expansion in the range and numbers of various large ungulates native to the plateau. Importantly, these improved conditions may have ameliorated the extreme aridity of the large basins just to the north (the Takla Makan and Gobi deserts), thus making movement onto the plateau from those areas more feasible.
P. Jeffrey Brantingham of the University of California, Los Angeles, and his Chinese colleagues Ma Haizhou and Gao Xing propose a three-step process for the peopling of the plateau. The first step brought a source population from the low elevation zones north of the plateau (say, Inner Mongolia) into northwestern China no later than 25,000 years ago and possibly earlier. The second step moved these peoples into the eastern Qinghai lakes region (at elevations between 3,000 and 4,000 meters) after 25,000 years ago but before the extremes of the last glacial maximum. It is during this second migration that cultural and physiological adaptations to high elevation conditions commenced. The third step, movement to the higher central plateau, may have taken place just before the onset of the last glacial maximum, perhaps between 23,000 and 22,000 years ago. This view of events is consistent not only with the dates from Chusang but also with at least one hypothesis about the source of the founding population of the plateau: Antonio Torroni of the University of Pavia and his colleagues use mitochrondrial DNA evidence to argue that indigenous Tibetans have a north Asian and Siberian origin of uncertain antiquity.
There’s Llama in Them Thar Hills
The archaeological record for the colonization of highlands is more complete in the Andes than on the Tibetan plateau, but the evidence is not without controversy. Although there continue to be claims that human presence in the New World has a very deep antiquity, most archaeologists now believe that the South American continent was colonized some 15,000 years ago, or perhaps somewhat earlier, if one believes the DNA evidence. Widely accepted as the earliest known site of occupation in the New World, Monte Verde in southern Chile is dated to roughly 13,500 years ago. But Monte Verde is a considerable distance from early peoples’ point of entry into South America (through the isthmus of Panama into what is now northwestern Colombia), so most workers suppose the initial occupation began a millennium or so earlier. A series of sites ranging in date from about 13,000 to 11,000 years ago is found along the Pacific coast of the continent. Fewer sites have been as well documented along the Atlantic coast, but some ancient dwelling places in northeastern Brazil are thought to be of similar antiquity.
A number of highland sites have been said to reflect an even earlier human presence in South America. Among the most controversial of these is Pikimachay, a naturally sculpted rock shelter found in the Andean foothills at an elevation of 2,850 meters. Excavated in the 1970s by the late Richard “Scotty” MacNeish, former director of the Peabody Museum for Archaeology in Andover, Massachusetts, the earliest dates from the site range from 14,000 to 20,000 years ago. Found in its basal levels were flake tools and the bones of an extinct species of ground sloth presumed to have been butchered there. However, the purported artifacts from the earliest levels are made from the same type of stone that makes up the cave wall, and most archaeologists who have examined them carefully are not convinced they show clear signs of purposeful shaping. Further, none of the sloth bones (which produced the early dates) gave evidence of burning or modification by human hands, thus leading most archaeologists to believe the animal found its way into the rock shelter on its own.
A few other sites, such as Pachamachay, Telarmachay and Guitarrero Cave, all in the central Andean highlands, have produced radiocarbon dates in the 12,000-year range, but the investigators who reported them suggest they are not reliable and instead argue that the earliest occupation of these highland sites most likely took place between 11,500 and 11,000 years ago. If so, humans lived at the base of the altiplano for at least 2,000 years before moving up into the highlands.
As is the case for Tibet, environmental factors appear to explain in great part why humans did not move into the highlands more rapidly. After 18,000 years ago, the glaciers that covered the high mountain peaks of the Andes began to retreat. This process, however, was slow, and its progress depended on the local topography. In the central Andes, for example, significant deglaciation did not take place until about 12,000 years ago, whereas in the Lake Titicaca basin, it happened about 1,000 years earlier. At the time, temperatures throughout the Andes hovered some five to eight degrees Celsius colder than today. On the western flanks of the Andes, snowlines reached 500 meters below their current position (at 4,800 meters), and plants adapted to the cold grew well below their modern ranges. Compared with the Pacific coast, with its abundance of fish and now-extinct land mammals, these valleys would have had little to offer early hunter-gatherers.
After 12,000 years ago, temperatures continued to increase, but rainfall diminished. By 10,000 years ago, changes in wind circulation across South America created a rain shadow on the western flanks of the Andes, leading to the formation of the hyperarid Atacama Desert. Glaciers (here and elsewhere in the world) melted, raising sea level by more than 100 meters from its lowest point. Inundation of the coasts would have affected the people living along them. Fresh water, once abundant, became scarce and was found only in small springs or in permanent streams descending from the mountains. Many animals, especially large-bodied mammals once common in the Pleistocene, became extinct. But all was not bleak. The alpine valleys of South America witnessed a rebirth, as various lowland plants and animals moved into them. This biological wealth, combined with the water the valleys contained, offered lowland hunters and gatherers a new niche into which they could expand, one of considerable potential.
Although the events I have outlined here are admittedly somewhat speculative, work I did more than a decade ago at Asana, an archaeological site in southern Peru, provides a good basis for this reconstruction. The site is located at an elevation of 3,350 meters and lies approximately 40 kilometers to the west of the altiplano. Although the mouth of the river that drains this area was occupied as early as 10,500 years ago, people moved into the highlands only about 9,800 years ago.
The archaeological evidence from the earliest levels at Asana reflects very limited activity, possibly of bands of hunters or small families residing at the site for very short periods of time. These groups probably maintained more permanent camps farther down the valley. They would have experienced acute hypoxia as well as reduced work capacity, thus making it difficult for them to get very much out of the time spent at altitude.
This strategy of brief visits soon gave way, however, probably within a span of about 500 years, to a pattern of longer-term occupancy and exploitation of highland resources. By 9,300 years ago, not only were larger groups living at the site, they stayed there longer and performed a wider range of activities compared with their predecessors. One of the most common tasks was the making of high-quality leather clothing, which would have been a necessity as people began living there permanently. There is also clear evidence that at this time these foragers began exploring the altiplano to the east, which stood at even greater elevations.
Some of these pioneers would have discovered that living permanently at altitude was beneficial, in that it allowed them to acclimatize and thus increased their work capacity. But there was a hidden cost too: the higher caloric demand, which would have been especially problematic for pregnant or lactating women. Infant mortality probably rose significantly, at least until women of child-bearing age made the requisite biological adaptation to chronic hypoxia. It seems that this impediment, combined with the low productivity of the region compared with lower elevations, slowed population growth in the highlands-until people eventually learned to domesticate plants and animals. Some archaeologists have speculated that the domestication of maize brought a second wave of colonists to parts of the South American highlands sometime around 3,500 years ago.
Waves of Grain
Anthropologists working on the Tibetan plateau have hypothesized that there, too, the initial entry was followed by at least one additional migration, this time of village agriculturalists rather than hunting and gathering peoples. Bing Su of the University of Cincinnati and his colleagues, using analyses of DNA obtained from the Y chromosome, argue that the modern inhabitants of the Tibetan plateau arrived just after 6,000 years ago from a source in the upper Yellow River basin. They propose that the plateau became increasingly populated as new communities budded off from established ones-a process stoked by the exploitation of domesticated plants and animals.
Their conclusion has important implications, because it would substantially shorten the time available for natural selection to have allowed these people to adapt to the hypoxic conditions encountered at elevation. Moreover, if Su’s model is correct, the length of time Tibetans have been at altitude is similar to that of Andean natives, which lends support to the notion that the two distinct adaptations to chronic hypoxia have more to do with the nature of the founding populations than the amount of time that evolution has had to operate.
Although no cultural adaptation can overcome the effects of chronic hypoxia, it is clear that the appearance of domesticated plants and animals on the high plateaus made it possible for these populations to grow substantially and for complex societies to emerge during the first millennium A.D. And what of the present era? In Peru, many of the young Highlanders have been migrating to the lowlands, particularly to coastal cites, in huge numbers since the 1970s. The highlands thus contain an increasing proportion of elderly people, most living in desperate poverty. In Tibet, a third wave of migration, this time of Han Chinese immigrants, has seriously disrupted traditional ways of life on the plateau.
There is no question that modern clothing, improved insulation and building materials, and access to effective medicines have aided the modern immigrants (along with many of the natives), making their lives at high elevation more tolerable. My guess is that of these, the one that has made the greatest difference is modern medicine. Antibiotics are accessible in both Peru and Bolivia, and, to a lesser extent, in Tibet, and these have reduced infant mortality substantially. Improved prenatal care for expectant mothers has also helped, as has better nutrition.
It is no surprise, then, that many people today are able to thrive at high elevations. But it is a surprise that these challenging regions prove to be especially attractive places to live. Joel E. Cohen of Rockefeller University and Christopher Small of Columbia University have shown that although the most densely populated stratum in the world lies within 100 meters of sea level, second place goes to the zone between 2,200 and 2,300 meters in elevation. They speculate that this pattern might have something to do with the desire to avoid mosquito-borne diseases so prevalent in many tropical lowlands or with the great fertility of volcanic soils found in many highland locales. It would thus seem that dwelling near the top of the world remains, for one reason or another, an appealing way of life.