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date: 24 February 2020

Earth, Water, Air, and Fire: Toward an Ecological History of Premodern Inner Eurasia

Summary and Keywords

The histories of humanity and nature are deeply entangled across Inner Eurasia. Great expanses of steppe and mountain connected peoples at the far ends of the landmass and sustained unique civilizational zones of nomadic and settled societies. These are regions profoundly shaped by some of the most complex climatic regimes and by one of the most devastating disease vectors in the world. Viewed in the longue durée of the Holocene, the premodern prehistory and history of Inner Eurasia takes on new dimensions when reviewed in the context of the latest work being done in environmental, climate, and genetic science.

Keywords: Inner Eurasia, Central Asia, Holocene, ecological history, environmental history, climate history, steppe, Inner Asian Mountain Corridor, Paleolithic, Neolithic, Bronze Age, Iron Age, Transoxiana, Taklimakan, Silk Road, bubonic plague, Scythians, Mongols, nomadism, agriculture

Earth, water, air, and fire: these four central tropes in Zoroastrian cosmology, rooted deep in the Bronze Age, distantly influenced religious systems throughout the continent. They will serve as organizing principles for this commentary on the ecological history of Inner Eurasia. The deep history of humanity in this vast region has been profoundly shaped by natural circumstances. Landforms, soils, biomes, the availability of water, the shifting circulation of climatic systems, and periodic eruptions of volcanism: these formative earth system structures that would have been recognizable to ancient Eurasians as earth, water, air, and fire. These peoples might well have also accepted our metaphorical extension of fire to the forces of technology and culture, the means of human mediation of these forces of nature.

We call this discussion an ecological history, rather than an environmental history. Our concerns are primarily with the material circumstances of humanity and nature, rather than with issues of nature, culture, and politics that are so central to environmental history as typically practiced, and because we will stretch the bounds of “history” typically conceived. In keeping with recent injunctions, and given the shape of our subject, we will extend “history” into prehistory, looking briefly at the pre-agricultural past, but in considerable depth at dynamics of premodern societies and nature in the era of the Holocene.1 Modernity is a new departure across this region, unfolding in less than two centuries. If our discussion of this recent past is perhaps somewhat truncated, it is strikingly shaped by the terms established in millennia of prehistory and history. We hope that this synthesis will provide a roadmap both for developing research and ongoing teaching on the role of material and ecological circumstances in the history of Inner Eurasia.

Inner Eurasia is a domain of somewhat fluid boundaries, shaped by both culture and ecology. Perhaps it is a place where other places end: it is not Europe, China, or India; it is not the Middle East. But it is also a place with its own defining qualities. Here, we will define Inner Eurasia in ecological terms primarily, as the mountains and steppe lands bounded in the east and south by the outermost reach of the monsoons, on the north by the boundary between the forest (taiga) and the steppe, and on the west—somewhat arbitrarily—by most of the arid Middle East and the gradient of the steppe into Eastern Europe.

The space within these bounds now encompasses Ukraine, southern Russia, the former Soviet republics of Central Asia, Mongolia, northwest China, northern Afghanistan, and northeastern Iran (historical Khurasan). But, critically, it involves two broad regions, and with them, two competing interpretations of the ecological history of Inner Eurasia. On the one hand, the grassland steppe unifies the region east to west, running from Ukraine through Kazakhstan to Mongolia. Jared Diamond, in his influential Guns, Germs and Steel, describes the east-to-west orientation of Eurasia as a unique “continental endowment,” accelerating the exchanges of ideas, inventions, and trade across the Old World.2 The ancient and medieval history of this east–west axis, focusing on the spread of Indo-Europeans, the fabled Silk Road(s), the spread of the Black Death, and the rise and conquests of the Scythians, Turks, Mongols, and other nomadic powers, is critically tied to understandings of the centrality of the steppe. On the other hand, a newer understanding has coalesced around a second geographic structure that clearly should have equal importance. The chain of mountain systems running northeast from the Zagros through the Pamirs, the Hindu Kush to the Dzungar Gate and the Altai stands at a diagonal to the steppe. Now labeled “the Inner Asian Mountain Corridor” (IAMC), the valleys and edges of these mountain chains are being examined as a zone of movement and exchange at least of equal consequence, and certainly of more ancient origins, than that of the steppe.3

Behind these competing commitments to the steppe and the mountains lie important interpretive differences rooted in very different understandings of human ecology in space and in time. The steppe approach has long argued that the uniformity of the grasslands across Eurasia has encouraged movement, exchange, migrations, and conquests. In addition, very broadly, scholars who emphasize the steppe also have tended to assume that climate change had decisive impacts on the peoples of Inner Eurasia, forcing and/or facilitating large-scale migrations. Conversely, however, the emerging scholarship on the mountain corridor has focused on ecological diversity, rather than uniformity. Along its steppe edges and in its countless valleys, the mountains offered innumerable well-watered ecological niches that provide foraging, pastoral, or agricultural peoples a far wider range of subsistence opportunities than the open grasslands. These opportunities vary with elevation and with latitude and, with varying distribution of water, especially from glacial-fed streams and springs, they shape different ecological conditions and natural communities. The diversity of ecologies in and around the mountains, and also the northern forest steppe edge, might well have buffered the impact of shifting climates. Local populations might adapt rather than move: thus the new mountain-based scholarship focuses on local continuities, downplaying the stress of climate change and the drama of mass migration. This complex ecology framework is now beginning to inform interpretations of the steppes themselves, as scholarship focuses on the ways in which pastoralists, of various degrees of mobility, exploited geographically variable and annually shifting climatic conditions to find pasture for their herds.4

At the same time, genomic and climate science is expanding the range of comparable evidence about human circumstances over thousands of years of history and prehistory. Genomics is now beginning to offer an independent means of assessing the claims of either school, screening the DNA of both contemporary individuals and ancient human skeletons, and roughly mapping the complex patterns of continuity and change in Eurasian populations.5 Climate science, presenting interpretations of stratified evidence from lakes, glaciers, caves, and archaeological soil profiles, is offering an ever more detailed picture of the complexity of climate regimes and climate shifts, setting Eurasia into a wider global analysis of the shifting conditions of the earth system in geological time and in the recent millennia of the Holocene.6

What follows is a brief assessment and integration of these literatures, in light of the trajectory of the region’s prehistoric and historic societies. If these steppe and mountain interpretations have their differences, they share a set of common assumptions. First, they would agree in their rejection of core-periphery models of world systems, which place Inner Eurasia on an empty periphery of the civilizational “cores” positioned around the edges of the continent, including China, India, greater Southwest Asia, Europe, Korea, and Japan. They would argue that, until the recent past, Inner Eurasia was a co-equal player in world history.7 But they would not contest the basic reality of low population and the scarce distribution of water: the environments of Inner Eurasia, steppe, mountain, or desert, are all—in different ways and with few exceptions—water poor, and water feeds ecologies and people. There is no gainsaying the reality that Inner Eurasia was very lightly populated relative to the better-watered societies on the continental margins, fed by adjacent oceanic waters annually mobilized by atmospheric systems. From ancient times to the present, and into an uncertain future, water and its access lies at the center of the ecological history of Inner Eurasia.

Our first objective is to offer a broad overview of the natural environments involved and the sketch the outlines of their natural histories. This will require a simultaneous consideration of earth, air, water, and fire: the interplay of landforms and climate patterns that shape the meager distribution of precipitation across the different regions of Inner Eurasia. With this framework established we will turn to five ecological histories of humanity’s cultural fire. Taking a brief look at the critical post-African dispersal of Paleolithic and Mesolithic forager-hunters through Asia, we divide our analysis by regions, determined by the first emergences of Neolithic agriculture: the IAMC, the western steppe, and the eastern steppe. Finally, we will consider the near-simultaneous impacts of modern, imperial economies on fragile Inner Eurasian ecologies. In each section, as appropriate, we will be highlighting central dimensions of the new literatures in climate history, genomics, and disease, specifically that of the bubonic plague.

Earth, Air, and Water

Since the end of the Pleistocene ice ages, roughly the last 10,000 years, during what is known as the Holocene, Eurasian ecologies developed into three broad regions, defined by their seasonal patterns of precipitation:

  1. 1. A northern domain dominated by the annual flow of Atlantic westerly systems and the cold dry force of the winter Siberian High.

  2. 2. The southern and eastern coastal plains watered by the summer onset of the monsoons.

  3. 3. A southwest quadrant sharing with the Mediterranean a seasonal pattern of dry summers and wet winters.8

The westerly zone and the monsoon zone, viewed on a transect running through the Tian Shan Mountains, form a roughly horizontal layering of bioregions, or biomes, reflecting the northern hemisphere’s gradient from the Arctic to the tropics (see Figure 1).

From north to south the entire range of Asian bioregions runs from tundra permafrost to taiga forest to grassland steppe to desert, mountains, to monsoonal coastal plains. Inner Eurasia as we discuss it here is limited to steppe, desert, and mountains, though shaped and constrained by taiga forest and monsoonal plain. Beyond the mountains to the south and east lie the monsoon-watered South and East Asian lowlands; to the west the steppe grades into Eastern Europe. In the entire north–south transect of the continent, the steppe, desert, and mountains of Inner Eurasia take up a relatively narrow band, perhaps a third of the distance from the Arctic Sea to the southern tip of India. Governed by distances from oceans, and the height of the world’s youngest mountain ranges, Inner Eurasia is fundamentally a dry region. In monsoon-dominated South and East Asia, Bengal receives around 1,500 mm of rain per year, south China over 2,000 mm, and central Tibet as much as 700 mm.9 But the northern boundary of the Asian monsoon reaches only as far as northern Tibet. To the north and west of the monsoon line (red line, Figure 2a) all of Eurasia receives its precipitation from the westerly system moving to the east from the Atlantic Ocean, picking up some limited moisture from the Mediterranean, Black, Caspian, Aral (historically), and Arabian Seas. The westerlies form the dominant source of precipitation, much of it in the winter in the form of snow. But they are regularly interrupted in winter by the powerful dry, cold Siberian High, centered on Lake Baikal, which drives blasts of cold dry air out of the Arctic across the entire continent, particularly impacting the drier eastern steppes and deserts.10

Earth, Water, Air, and Fire: Toward an Ecological History of Premodern Inner Eurasia

Figures 2a and 2b. Schema of seasonal climatic patterns in Inner Eurasia. Image credits: Adapa Project, “Asia Physical Map” by, “The Global Weather Pattern Shifts” by Jim Gandy, and “Global Weather Patterns” by

Sources: See endnotes 8–20.

The results of this “continental” climate regime is a gradient from very dry to relatively dry, arid to semi-arid. Across the steppe precipitation grades off from west to east: Ukraine getting 500–600 mm, the Volga River Basin 300–400 mm, the steppe from north of the Caspian Sea and east to Mongolia between 150 and 250 mm. The deep deserts, such as the hyper-arid Kara Kum and Taklimakan receive 100 mm or less per year.

Precipitation in the form of rain or snow is only one side of the moisture equation; evaporation is the other side. For the entire Inner Eurasian region, hot summers and the cold dry northern Siberian High winter winds drive the evaporation of moisture, which reverses the effects of a minimal rainfall. “Effective moisture”—the balance of precipitation and evaporation—is the critical measure. Cooler temperatures drive less evaporation, thus much of moisture in the taiga region and high elevations is a function of lower summer temperatures as much as it is of precipitation. Cold temperatures in the high mountains are critically important in much of Central Asia: winter snowpack builds and maintains glaciers that melt back in the hot summers, providing essential run-off, watering streams, springs, and rivers throughout the mountain region.

Temperatures thus also govern moisture, leading in many places to a delicate balance of summer evaporation and glacial run-off. But evaporation in the northern mid-latitudes has another very important driver, which requires a brief excursion into the basics of atmospheric circulation. The circulation systems on either side of the equator are shaped by three climatic cells: in the northern hemisphere they are, from north to south, the Arctic Polar Cell, the Ferrel Cell, and the Hadley Cell. The northern Hadley Cell shaped a common boundary with the southern Hadley Cell, at what is called the Intertropical Convergence Zone (ITCZ) (see Figure 2a and b). Along the ITCZ warm moist air rises in both cells, hitting cold tropospheric air: this collision produces a global band of rainfall, shifting north and south with the seasons. The reverse occurs at the subtropical boundary, where the tropical Hadley Cell meets the temperate Ferrel Cell. Rising at the equator, air in the Hadley Cell falls at the subtropical boundary. So too does the air in the Ferrel Cell, which rises at the Arctic boundary. This dry air, descending in both cells at the subtropical boundary, inhibits cloud formation; without clouds, rising surface temperatures drive the evaporation that shapes the dry deserts around the earth at the northern-mid-latitudes—most importantly in the North American Southwest, the Sahara, the Middle East, and the Central Asian deserts (their counterparts at the southern hemisphere subtropical boundary are Patagonia, the Kalahari, and the Great Sandy Desert of western Australia).11

Recent research is illuminating the shifting geography of these westerly and monsoon systems. The subtropical jet is the southern anchor of the entire westerly wind system that brings moisture to all of Inner Eurasia. The subtropical jet runs typically along the dry evaporative boundary between the Hadley and Ferrel Cells, and from there the moisture-bearing westerlies spiral up toward the Arctic within the Ferrel Cell. The subtropical jet is not stable, but shifts with the seasons, and with longer periods of global climate cooling or warming. Thus the jet runs across the hyper-dry Taklimakan in the summer but through the northern Ganges Valley in the winter, shaping dry evaporative conditions in both regions in summer and winter respectively. The springtime northward shift of the subtropical jet, sometimes erratic, with rising air over a warming Tibetan Plateau, drives the advance of the monsoon in both South and East Asia.12

Here wider forces come into operation, working through time was well as space. The global cooling effects of solar minima and super-minima, such as the Maunder Minimum of the Little Ice Age, and significant volcanic eruptions, inhibit this annual cycle. A colder northern spring—measurable in ice core data—slows and disrupts the northward migration of the subtropical jet. The result is monsoon failure in South and East Asia (particularly northeast Asia), and the continuing run of moisture-bearing westerly wind systems across the steppe and Central Asia. Thus precipitation in Inner Eurasia and monsoonal Asia is reversed, with important implications that have yet to be fully worked out.13

Operating across the Pacific Ocean, the El Niño Southern Oscillation (ENSO) plays a role in regulating the monsoon systems. Generally, a strong El Niño year brings rain to the Americas but drought to South and East Asia; conversely a drier La Niña year in the America correlates with strong Asian monsoons. But the ENSO operates in rough correlation with conditions in the North Atlantic, which drive the westerlies east across Asia. Specifically, the North Atlantic Oscillation (NAO) (defined by the relationship of the Azores High and the Icelandic Low) falls into a positive mode, delivering precipitation northward along Scandinavia into Siberia, and a negative mode, in which the westerlies shift to a southern route through the Mediterranean, with influences out to the Arabian Sea, and east into Central Asia proper. These patterns are visible in both ice records in the North Atlantic and soil records in Central Asia: a colder North Atlantic meant more moisture across Inner Eurasia.14

The westerly and monsoon systems have been operating for millions of years, but during the Pleistocene they were interrupted during episodes of continental glaciation.15 With the most recent breakup of the great ice sheets over greater Scandinavia, the westerlies returned, and modern tundra-taiga-steppe-desert biome structures emerged rapidly, by c.8000 bce.16 During the Early Holocene through c. 5000 bce (see Figure 3) the orbital forces on the planet that drove the end of the most recent Pleistocene Ice Age warmed the northern hemisphere dramatically, due to the angle of the summer sun and the generally northward tilt of the westerly/monsoon system.17 Northern Asia was relatively warm and wet. Siberian forests reached 300–400 km north into what is the present tundra, and broadleaf deciduous forest displaced much of the coniferous taiga, and shifted the forest zone south into the modern steppe. It also seems clear that northern regions—forest and some of the steppe—were wetter in the warm Early Holocene, and became drier over time, while the southern desert and mountains were hyper-arid in the Early Holocene, and gradually became marginally wetter in the middle to late Holocene, after c.6000 bce.18

Earth, Water, Air, and Fire: Toward an Ecological History of Premodern Inner Eurasia

Figure 3. Inner Eurasian climate data for the Holocene, 10,000 bce to present. Image credit: The authors.

Sources: See Appendix.

The climate oscillated between warmer and cooler conditions during the Mid- to Late Holocene, and these shifts can be charted roughly across Inner Eurasian time and space. Very broadly, a dramatically warmer global climate during the immediately post-glacial Early Holocene drove a predominantly positive NAO mode of westerlies into northern Eurasia (see Figure 3). As a general cooling took effect around 6,000–4,000 years ago, the NAO shifted toward a generally negative mode, and with epochs of global cooling (and warming) the westerlies moved south (and north) across Eurasia. Between 4000 bce and the present these shifts occurred in a relatively regular pattern, broadly echoing the ~2,000-year solar Hallstatt cycle and an internal 1000-year solar half-cycle, reinforced by periods of enhanced volcanic eruption (see Figure 4).19 Measures of the Siberian High—dust particles from Asian deserts detected in the Greenland ice—map the largest framework of oscillations. Shaped by solar minima and large-scale volcanic eruptions, these large “Hallstatt” solar minimum events bracket the Bronze Age, between the fourth millennium bce and roughly 1200–600 bce, while the Little Ice Age, between roughly 1275 and 1820 ce, approximately stood between the Middle Ages and modernity. In addition, there was a significant climatic shift in the late third millennium, perhaps hinging on a dramatic episode of unknown origin at 2200 bce, as well as the volcanic-solar-driven “Dark Ages” of roughly 450–900 ce.

Earth, Water, Air, and Fire: Toward an Ecological History of Premodern Inner Eurasia

Figure 4. Inner Eurasian climate and the plague, 4000 bce to present. Image credit: The authors.

Sources: See Appendix.

These patterns can be followed in Figures 3 and 4, which present some of the data for climate change across Inner Eurasia. Generally, it appears that with cooler global conditions along the steppe the northern forests expanded south along river valleys into the modern steppe, while with warmer global conditions which impose higher evaporation in northern latitudes, the steppe and deserts expanded north into the taiga. The ecology of the taiga-steppe edge, here represented by soil and pollen syntheses from the Dnieper and Volga, roughly followed the stronger Hallstatt signals, and presumably shifted south with decreasing evaporation and the southward shift of the Atlantic westerlies. The steppe apparently was wetter in the fourth millennium bce, drier in the third, wetter at the end of the first. During the Little Ice Age the suggestion of more oak forest in the Volga region, generally a measure of moisture, may also be a result of depopulation following the Mongol conquests and the Black Death.20

To the east, studies of moisture indices for the Altai and northern Xinjiang capture the very different trajectories of the forested northeast and the desert southeast. The Altai, and Siberia in general, was particularly warm and wet during the post-glacial Early Holocene, deriving its moisture from strong westerlies flowing northeast from the Atlantic. Conversely, Xinjiang, including the Taklimakan Desert, was hyper-dry in the Early Holocene, but began to receive some moisture as the westerlies shifted south. The Indian Summer Monsoon fades after reaching well into Tibet, so provided no moisture to this region.21

The southwest region, between the Caspian Sea and the Pamirs, diverges somewhat from the southeast. The southwest, developing a Mediterranean wet winter pattern, began to shift from dry steppe to wetter—forested—conditions around 6000 bce in what is known as the Liavliakan pluvial. Here there is a debate as to whether this moisture came solely from an increased flow of westerlies or whether the monsoon had a role. The suggestion is that the monsoon had an influence, if not by direct precipitation, by sending warm air north to force increasing snowmelt in the high mountains, draining west via the Syr Darya and the Amy Darya (the ancient Oxus) to the Aral Sea. Perhaps the slow continuous retreat of the Indian Monsoon from a peak around 6000 bce brought a gradual decline in this “pluvial”; an increase in aridity that may have begun around 3000 bce and clearly intensified between 2000 and 1000 bce, as the Siberian High simultaneously also intensified, bringing dry conditions during the vital spring growing season.22

These climatic patterns—and their ecological effects—can be mapped north to south across Eurasia, from taiga forests to grassland steppe to dry deserts to the monsoon regions.

Table 1. A Schematic Overview of the Ecological Effects of the Bimodal Holocene Climate Regimes.

Higher Northern Hemisphere Temperature

Lower Northern Hemisphere Temperature

Extreme Siberian Highs on Hallstatt Minima

Westerlies North [NAO+]

Westerlies South [NAO-]


Moderate evaporation

Lower evaporation

High precipitation

Lower precipitation


less humid


High evaporation

Lower evaporation

Reduced precipitation

Increased precipitation

increasingly arid

more humid


Very high evaporation

Lower evaporation

Low precipitation

Increased precipitation

extreme aridity

more humid

South Asia

Very high evaporation

High evaporation

Strong monsoon

Weak monsoon

extreme summer humidity

more arid

Sources: See endnotes 12–20.

Three features stand out. First, the boundary between the Central Asian deserts and the South Asian Monsoon was critical: little or no monsoon moisture reaches north beyond the Tibetan Plateau. Second, precipitation in the entire region to the north is governed by the westerlies, but this westerly precipitation pivots north and south with warmer and colder global conditions. Third, evaporation varies reasonably directly with temperature, and thus has a gradient from lower in the north and higher in the south, but also shifts with change in mean temperatures. If the forest/steppe boundary was porous and shifting, the desert/monsoon region boundary was fundamental: the two climate systems stand adjacent with starkly opposite climatic systems. In the northern taiga, it seems that the evaporation is never strong enough to offset precipitation. Where it does, on the forest/steppe edge, the effect of reduced precipitation and increased evaporation with warmer conditions—droughts—would be to convert dry forest into scrubby grasslands. The steppe and the desert both seem to be affected by the north–south shift in the westerlies and the warm/cold changes in evaporation: in eras of global climatic warm “optimum” the desert would be searingly dry and the steppe would become more arid, with northward desert intrusions, such as what seems to have happened in the third millennium bce.23

Throughout these Holocene climatic shifts the basic character of bioregions across Inner Eurasia was not fundamentally altered, though the pattern of the mosaic of local ecologies along the regional boundaries may have oscillated, sometimes significantly. Changing conditions may have offered new locations of familiar micro-niches for human subsistence, especially in or adjacent to the mountains, where elevation—combined with glacial run-off—created starkly different ecologies in close proximity. On the other hand, these dynamic landscapes also forced significant changes in subsistence and adaptation by local peoples.

The two interpretive schools contending over Inner Eurasia, the steppe school and the mountain school, frame the impact of the climate as either a shattering force or an ecological opportunity. As noted, the older steppe migration school stressed change through time—the impact of climatic shifts—while the newer mountain school, arguing for continuity and resilience, stresses diverse ecological mosaics. It may well be that they are arguing on either side of the same coin, that change and diversity were indeed both critical elements of the ecological history of mountains and steppe across Inner Eurasia in premodern times.

The Paleolithic

Even the lightest survey of the longer Paleolithic in Inner Eurasia is beyond the framework of this article, but a few key points should be noted. Proto-humanity spread at least twice out of Africa, and in both cases circumstances suggest that the story in Inner Eurasia was particularly important. Lower Paleolithic peoples spread out of Africa almost 2 million years ago, though the evidence does not suggest that they penetrated Central Asia before 750,000 years ago, and, while they may have survived in the region during interglacial periods, this has not been established beyond doubt.24 It also clear that we know virtually nothing of these populations over enormous stretches of time. These peoples diverged from archaic Afro-Eurasian Homo sapiens populations as much as a million years ago.25 To the west, these are now known as the Neanderthals, to the east they were a close relation, recently rediscovered. The 2008 discovery of a child’s finger bone in a cave in the southern Siberian Altai, dating to approximately 180,000 years ago, upended our understanding of Paleolithic protohumans, or hominins. Labeled a “Denisovan” from the name of the cave, this child bore genetics that distinguished it from both modern humanity and our mutual cousins the Neanderthals, the product of a genetic split approximately 600,000 to 450,000 years ago. A Denisovan sample of the same age has now been confirmed from the eastern Tibetan Plateau. The emerging consensus is that the Denisovans occupied eastern Asia through much of the Pleistocene, overlapping Neanderthal occupation of western Eurasia.26

Both Denisovans and Neanderthals would be replaced by anatomically modern humans, leaving slight “archaic” genetic traces in non-African modern genomes. The story of the spread of modern humanity out of Africa is a complex and contested arena, with competing arguments for single or multiple waves of dispersal between 120,000 and 70,000 years ago. While a southern route along the Indian Ocean, probably somewhat earlier, led to India, Southeast Asia, and Australia, a northern route or branch led into Asia and from there to Europe and the Americas. Whether we should think of these as dispersals of modern people migrating, or of modern genetics moving though and overwhelming preexisting archaic populations, is under considerable debate. So too is the question of whether the transition from simpler and more complex stone tools of the Middle and Upper Paleolithic indicate population replacements or shifts from low population to higher populations, driven by higher birth rates, great intergroup competition, and denser contexts of innovation.27 Generally, however, the transition to the Upper Paleolithic, and the appearance of modern human anatomy, occurred virtually simultaneously along at least three routes north into Europe and Asia, roughly 40–45,000 years ago.28

The first route ran north through the Balkans into Europe; the second route ran through the Black Sea region and into the steppe. In the western steppe north of the Black Sea Russian archaeologists have excavated dozens of Upper Paleolithic sites which most famously show clear evidence of big-game hunting adapted to glacial tundra and steppe conditions. But an important survey of this evidence ends on a cautionary note: recognizing the “remarkable diversity” of Upper Paleolithic economies in the Russian steppe.29 And ecological diversity is the critical understanding in the third route, north into Central Asia along the line of mountains running from the Zagros all the way to the Altai, the IAMC. Modern humans were in the Altai perhaps as early as 45,000 years ago, and recent work is focusing on the line of sites along the northern piedmont foothills of the Pamirs and Tian Shan, leading back to the Iranian Zagros and the Levant. In particular, some argue that the diversity of ecological niches, the moderation of extreme temperatures, and the access to water all made this region a refuge during glacial episodes and a pathway toward the Altai and Siberia.30 It may well be that the IAMC had the same function earlier for archaic human populations, such as the Denisovans or their forebears. It is more than clear that all of these factors were at work from the Neolithic forward, as agricultural peoples developed an agroecological adaptation that has extended to the present to shape Central Asian economies and societies. In the long run, this mountain corridor may have been more important than the steppe corridor.

The Inner Asian Mountain Corridor: From the Neolithic into the Bronze Age

Asian history is broadly divided between the steppe and the sown, and typically treatments of Inner Eurasia focus primarily on the steppe. The great areas of the sown were certainly China, South and Southwest Asia, and Europe. But Central Asia was a unique region of the “sown” and new work is stressing its continental centrality. It is becoming clear that it is the mosaic of steppe edge, foothill, and mountain ecologies running along the IAMC, rather than the steppe grasslands, that provided the key background to the exchanges of animals, plants, and eventually trade goods that brought China into communication with the greater Mediterranean world.

The erratic warming of climates and landscapes with the transition into the Holocene shaped a general shift in human ecology toward what are called Mesolithic cultures, adapted to the increasing diversity of subsistence resources, in what is called the “broad spectrum” mode. The beginnings of domestication in the Levant, focusing on barley and wheat, and China, focusing on millet and eventually rice, developed out of the Mesolithic broad spectrum at the beginning of the Holocene, around 9000 bce. These founder crops would disperse across the continent from these centers of domestication over the ensuing millennia. Sheep and goats, barley and wheat spread to the Zagros Mountains and the regions south and east of the Caspian Sea by 7000–6000 bce, as part of a spread through Iran and Afghanistan that would establish early agriculture on the edge of the Indus Valley by 7000 bce. It would appear that these domesticates moved though exchange, rather than migration, since the early farmers at the southwest base of the IAMC were locals, not migrants from the Levant.31 At the eastern end of Inner Asia, strong Early Holocene monsoons provided the context for domestication in North China. Foxtail and broomcorn millet were domesticated in the grassy hill regions of the Yellow River Valley: sometime between 6900 and 8300 bce; by 2600 bce these crops had spread a thousand kilometers up the Hexi/Gansu Corridor, the eastern gate of the IAMC, in villages located along the foothills of the Qilian range.32

Cultures at both ends of the semi-arid IAMC independently established early agriculture that could rely on meltwaters flowing into alluvian deltas from mountain snowpacks and glaciers. Along the edge of the Kopet Dag foothills east of the Caspian Sea small communities of the Jeitun culture benefited from the shift in the climate system operating on the Central Asian mountains. Intensely arid during the Early Holocene, the Caspian/Aral regions turned wetter around 6000 bce, with the so-called “Liavliakan” pluvial, with stronger westerly winter precipitation to the Pamirs and Tian Shan, which melted back considerably in the warm summer season, perhaps enhanced by indirect influence from the South Asian Monsoon. The result was an epoch of high water availability in the rivers flowing west out of the mountains, occasionally flooding much of the Kara Kum and Kyzyl Kum deserts.33 The wider Jeitun Neolithic developed in place for at least four thousand years, with settlements growing in size and complexity, grading into Chalcolithic (Copper Age) and Bronze Age cultures, developing what has been termed a “metallurgical province.”34 It is possible that either the weakening of the Liavliakan pluvial, the dramatic global climate reversal of 2200 bce, or a combination of the two, began a process of migration to the northeast. In these centuries Bronze Age Kopet Dag peoples moved several hundred kilometers east to established a series of oasis towns in the Murghab River delta, at what is known as Margiana, now deep in the Kara Kum Desert. Soon thereafter they established another series of towns further east on the Amu Darya (Oxus) River at Bactria, which would be a center for hundreds of years.35

These Middle to Late Bronze Age towns along the Kopet Dag and at Margiana and Bactria, established by 2000 bce but only recently rediscovered, are now seen as critical centers in a wider arc of proto-urban societies mediating between Mesopotamia and the Harappan societies of the Indus Valley. Camel domestication provided critical power for long-distance trade that ranged across these regions, with walled oasis towns establishing the models for later Silk Road entrepôts.36 Far to the east the Chinese agricultural package, by 2000 bce, had moved up the Hexi Corridor to the eastern edge of the Taklimakan Desert. Across the thousand kilometers separating Margiana and Bactria from the eastern Taklimakan a new hybrid economy developed among local Mesolithic hunter-gatherer peoples, mixing herding with village agriculture, as they adopted and diffused the domesticates coming from each end of the IAMC.

Sheep led the way. A recent genetic analysis of sheep mitochondrial DNA argues that there were three distinct waves of sheep diffusion between Southwest Asia and Mongolia between 5000 and 3000 bce, with subsequent diffusion throughout China and into South Asia. Archaeologist Michael Frachetti argues that these migrations occurred not through the western steppe, but along the IAMC, and highlights the very high proportions (70–90 percent) of sheep bones among the faunal remains of fourth–third millennium sites (3500–2500 bce) near Bactria, on the northern Tian Shan foothills, and in the Altai.37

Herded sheep were followed by transported seed: wheat and barley spread from the west, drought-resistant broomcorn millet from the east. By 2500–2000 bce these crops were being grown all along the IAMC, from the Bactria region through the Taklimakan to the Hexi Corridor, spreading from the north to the Altai and southern Siberia. Millet spread into South Asia around 1800 bce, as part of what is being called a “Chinese Horizon” in the Swat Valley and Kashmir, south of the Khunjerab Pass in the Karakoram Mountains to the west of the Taklimakan; this complex included peaches, apricots, hemp, and possibly rice, along with artifacts related to the Majiayao cultures of the Gansu Corridor. Mind-altering drugs were part of the biotic mix: recent archaeology has confirmed the role of high-potency cannabis in funerary rituals in the Pamirs, at the center of the flow of people and commodities along the IAMC, no later than the first millennium bce. People, animals, seeds, and objects were all moving along the IAMC, in exchange and seasonal pastoralism. The cumulative effect was a trajectory toward transcontinental bio-connections in the Bronze Age. Sporadically colder and/or drier climatic conditions seem to have reinforced this trajectory by making agricultural exchange a resilience strategy, as local farmers adopted and altered crops that would produce a surplus in harsh conditions.38

This trajectory also included metals and horses, which reached central China at the turn of the second millennium. These connections came possibly through the Gansu Corridor, but more likely from the steppe cultures at the northern end of an arc of mountains, steppe, and desert mediating between the IAMC and the Chinese Central Plains.39

These dispersals of animals, seeds, and objects along the IAMC slowly integrated with the Bronze Age trade routes of a wider southern Asia. Much is made of the origins of the “Silk Road” in the wake of a Han Chinese military victory over their nomadic Xiongnu neighbors in 121 bce and subsequent annexation of the Gansu Corridor.40 But long-distance trade had connected Mesopotamia and India through the towns at Kopet Dag and Margiana–Bactria for centuries, as evidenced in the wide distribution of Central Asian jade and lapis-lazuli. To the northeast, a haze of local pastoral and local exchange routes were conveying domesticates and probably other goods along the IAMC. As we shall see, the rise of metallurgy played a role as well. If ores were not being transported, then processed and finished copper, tin, and bronze were moving along these routes, connecting the emerging southern Asian empires with “metallurgical provinces” to the north.41

The interplay of local pastoralism and longer-distance exchange northeast along the IAMC between valleys and oases along steppe and desert edges was extending these ancient routes toward China and their eventual label as the “Silk Road.” These connections would begin to be more formalized after 1000 bce as independent polities around the southern side of the Taklimakan began to emerge as mediators in an increasingly regular exchange linking China and greater Bactria. As it operated in Classical Antiquity, the “Silk Road” moved through this well-forged IAMC terrain, west from Gansu through the Taklimakan, south to Bactria and from there to the Indus or to Baghdad.42 It was only late in Antiquity—in a trade reaching to Byzantium sometime in the 5th to 6th centuries ce—that a direct route across the steppe north of the Aral and Caspian Seas to the Black Sea was established, so famous from medieval accounts.43 The backbone of this system of trade lay in the practices of irrigated oasis agriculture depending on run-off from winter snows, continuously elaborated from IAMC origins in the Neolithic Kopet Dag and Gansu Corridor. Along its routes oasis towns could grow into cities of some consequence: namely Balkh, Taxila, Kashgar, Merv, Shash (Tashkent), Samarkand, and Bukhara. Broadly speaking, and extrapolating from dubious population figures for 1700 ce, the premodern IAMC might have harbored as many as three to ten times as many people per kilometer as the steppe proper. And by the Middle Ages, and probably well before, urban and pastoral societies were sufficiently differentiated that their diets diverged significantly. In this regard, stable isotopic analysis of human bones indicates that urban peoples ate only locally grown food, while pastoralists had a much more widely sourced diet.44

But those populations had their vulnerabilities, first and foremost the availability of water. The mountain-fed deltaic irrigation systems that watered their agriculture, while generally predictable, had certain limits in an essentially arid environment. The earliest Jeitun Neolithic agriculture had been fed by piedmont groundwater aquifers, and was relatively stable, but to the northeast the settlements on the Murghab River delta deep in the Kara Kum were inherently unstable, solely dependent on precipitation in the Pamirs. This annual flooding could be erratic, and after 1500 bce it faded, with a general regional desiccation at the end of the Liavliakan pluvial, as did the desert delta oases.45 To the northwest, in the deltas of the Amu Darya leading to the southern shore of the Aral Sea, irrigation spread significantly during the Iron Age, and by Antiquity was diverting enough water to contribute to a drop in sea level. When the Mongols destroyed these irrigation systems in 1221, the river was diverted to the Caspian Sea.46 To the east in the hyper-dry Tarim Basin, it has been argued that mountain-fed irrigation was resilient enough to handle advancing aridity, but growing populations could be destabilizing. For example, it appears that the city of Loulan declined and collapsed as a result of population pressure on water resources due to rising populations in Han-period settlements rather than from climatic change: Lop Nur dried out while other lakes in the surrounding region remained viable. The militarization of a contested border region seems to have played an important role. In an effort to control their access to the trade routes and to contain the incursion of northern nomads, Han China established agricultural colonies of soldier households in the eastern end of the Tarim; supported by elaborate irrigation these colonies may well have contributed to the Lop Nur water crisis.47

But water may not have been the entire story. In the middle of the second millennium the desert oasis settlements in Margiana declined, but so too did the Bactrian towns directly on the Amu Darya, as did the Kopet Dag towns, fed by apparently dependable groundwater. In roughly the same early second millennium chronology one of the other legs of the Bronze Age triangle, the Harappa cities in the Indus, fell into terminal decline. What is striking is that—while waves of Indo-Europeans warriors from the steppe have been conjured up to explain the Harappan collapse—archaeological investigations into both of these civilizational declines finds no real evidence suggesting dramatic violence or conquest. Climate change is certainly a candidate. But we might also consider whether the plague—Yersinia pestis (YP)—played a role. To explore this argument we need to examine the western steppe and its relationship with the greater IAMC.

The Western Steppe

The “western steppe” may be more properly divided into the “Pontic Steppe” north of the Black Sea and the “Central Steppe” stretching through Kazakhstan to the Altai. Where they form a west–east gradient from wetter to drier, governed by the declining moisture in the Atlantic westerlies, they comprise a broadly common domain. Where the mountain corridor is understood as a mosaic of ecosystems, the steppe is typically visualized as a uniform ocean of grass. Given this uniformity, the steppe has inspired interpretations revolving around transitions and migrations shaped by climatic change as a monolithic force. In the grand scheme of things, while climate certainly played a role, steppe cultures forged the pastoral systems that would exploit variations in the ecological uniformity of the grasslands. In so doing, they set in motion forces of anthropogenic change, including population growth. The great steppe adaptations were the elaboration of wheeled carts and wagons, first drawn by cattle, and then by the horse, newly domesticated. The horse, in particular, is identified with the rise of nomadic steppe armies of popular imagination, but so are the metals, bronze, iron, and steel, that armed these warrior hordes. But farming was as important, as was the ecological impact of metallurgy, a problem that has been barely examined. In addition, new evidence for the circulation of an early form of the plague during the Bronze Age adds another dimension to the ecological history of the western steppe.

Populations of Mesolithic foraging hunter-gatherers, the descendants of Late Paleolithic peoples, were probably spread thinner on the steppe proper than along the steppe edges—the wooded valleys and marshlands that stretched north from the Black Sea and south from the northern forests. At the same time that agriculture emerged in the hill country south of the Caspian Sea, it arrived on the edge of the steppe from Southwest Asia via Anatolia and the Balkans, spreading onto the Pontic steppe around 5800 bce. To the west, long-enduring cultures of dense settlements emerged around 5200 bce, around the same time as foragers just beyond the Dnieper River in the Ukraine began to maintain herds of cattle and sheep, and about a millennium later to cultivate wheat and millet.48 David Anthony has written the most detailed account of the prehistoric western steppe, and transitions toward a colder climate stand at the center of his analysis of the fourth millennium, the first of three global “Hallstatt” solar grand minima running down to the Little Ice Age. In centuries of cold and flooding between 4200 and 3800 bce, the opening of the fourth millennium Siberian High extreme, some of the Danube Valley village societies disappeared, replaced with steppe peoples, while others—the Cucuteni–Tripolye cultures—spread east toward the Dnieper, adopting some steppe cultural features.49

Where the domestication of the horse fits into this picture is a highly contested issue. Anthony advocates an early date: as early as 4500 bce, perhaps with origins in the western steppe, but clearly manifested in the Botai culture of northern Kazakhstan between 3700 and 3000 bce. Anthony suggests a process of evolving horse management, from hunting to milking and harnessing to riding, in which small groups of mounted raiders might have dramatically accelerated movement in the western steppe in the fourth millennium.50 Others are much more conservative, stressing that the evidence for large-scale dependence on horse mobility dates to the Middle to Late Bronze Age, around 2000 bce.51

The case for the centrality of the horse in the development of these regional steppe societies is murky at best. Two broad temporal horizons dominated the western steppe into the second millennium bce: the Late Neolithic–Early Bronze Age and the Middle to Late Bronze Age. The Early Bronze Age steppe horizon was dominated by the Yamnaya/Pit Grave culture, 3300–2500 bce, who primarily herded cattle, moving in massive solid-wheel wagons with their herds. But these societies were largely semi-sedentary, movement was slow, and they focused in or near the wooded valleys cutting north and south through the steppe. Movement involved segmentary expansion rather than purposeful migration. The Corded Ware culture in Eastern Europe similarly used wagons to follow herds. The related Afanasievo culture—before the wagon horizon—did move east all the way to the Altai in an epic migration sometime after 3700 bce. Importantly, they moved through or near the territory of the Botai in north Kazakhstan, and they apparently utilized the same forest/steppe-edge environments. The horse-using Botai were genetically differentiated from the Yamnaya and the Afanasievo, part of a wider forest hunter-gatherer population, but with no significant contacts and exchanges.52

The steppe became significantly drier between 2800 and 1700 bce, conditions in which wagon-based herding would allow the exploitation of greater ranges of pasturage, essentially creating mobile villages. Nonetheless, the Yamnaya did not take to the open steppe, but stayed close to the more productive river valleys and marshlands.53 It also appears that innovations and market opportunities emerged in these centuries, shaped by the increasing reach of the Uruk cities in Mesopotamia. The wheeled cart was invented somewhere in the region in the late fourth millennium, both symptom and, literally, vehicle of a wider trade linking the steppe to Mesopotamia. Metals were an enormous driver of this system; a Copper Age Balkan-centered “Carpatho-Pontic” region faded early in the fourth millennium, and was replaced with a “Circumpontic Metallurgical Province,” running from the Caucasus to the northern Black Sea steppe.54 A trade in metals and cattle for textiles from Mesopotamia, apparently mediated by the militaristic Maikop trader-metalworkers in the north Caucasus, was the general context for the emergence of these more mobile herding cultures; the Yamnaya probably had a minor role.55

The Middle to Late Bronze Age was dominated by the expansive Andronovo culture complex, which had its origins in Urals, on the edge of the central steppe. Around 2200 bce a climate event of global impact brought severe drought to the steppe. Among various groups, peoples associated with the related Sintashta culture adapted by settling around marshland that would provide predictable winter fodder for their herds. For four hundred years, the Sintashta and related groups constructed dense, walled villages—producing prodigious volumes of bronze in a new metallurgical province. While the horse might have been domesticated much earlier, it only began to have widespread use at this time, and the Sintashta—working to defend their towns—made a critical innovation in the invention of the two-wheeled light chariot. The chariot would spread east and west over the second millennium, as Andronovo cultures spread across the central steppe through the Dzungar Gate into the east, with similar cultures—the Srubnaya (Timber-Grave)—in the Black Sea region, through roughly 1500 bce.56

The Sintashta, Andronovo, and Srubnaya cultures established a common cultural horizon across the steppe for the first time, and while they clearly remained semi-sedentary, they developed more of the features of nomadic mobility, combining horse-riding, herding snow-adapted herds dominated by sheep, and annual transhumance north in summer and south in winter, seeking out seasonal pastures in well-watered grasslands.57 As importantly, scattered among them were essentially settled groups of miners and smelters, exploiting the ores of the Urals and Altai on what has been called “a greatly expanded, nearly industrial scale.”58 Where the Early Bronze metallurgists around the Black Sea were supplying Mesopotamia, the producers in the Urals and that Altai were trading with the entire Late Bronze Age world through the towns of the Bactria–Margiana Archaeological Complex (BMAC). There were clearly environmental impacts from this “near-industrial scale” production. Ancient mining and smelting sites are pockmarked with mine shafts and littered with slag piles. In many cases, metallurgical work was not conducted at centralized locations, but in virtually every household in a settlement. This scenario suggests horrific levels of domestic indoor pollution; we can envision plumes of dense smoke rising from charcoaling and smelting pits, and indeed from the mines themselves.59 Working the metals required a massive volume of timber for charcoal to smelt the ores and refine the alloys, but also to maintain fires on the mine face to create cracks in the raw rock. In the western steppe, Srubnaya smelting apparently eliminated Scots pine in the dry Don Basin; in the Gansu Corridor the earliest bronze manufacturing site in China had to be abandoned when local timber was exhausted. However, mining and smelting seem to have had minimal impacts in Kazakhstan and the Urals, where the metal production was focused in deep forest regions.60

Recent studies of genetic variation based largely on ancient DNA (aDNA) derived from human remains recovered from archaeological contexts have been redefining our understanding of the movement of peoples in greater Eurasia since the late 2000s.61 Although the aDNA literature is too vast to review adequately here, some highlights are worth stressing, largely because they provide a critical background for addressing an important ecological problem, namely the spread of the plague among these Bronze Age peoples. The most recent work is showing a very consistent pattern, with reasonably distinct genetic signatures highlighting the differences and the connections between the steppe peoples of the Early and Middle–Late Bronze Horizons, and the peoples inhabiting Europe, East Asia, the BMAC domain, and greater South Asia. During the Neolithic, genetic signatures associated with these regions were quite distinct, “deeply structured,” except for a clear movement of Southwest Asia farmers into Europe.62 The next major linkage occurs in the Early Bronze Age, when steppe genetics, apparently carried by westward-moving Yamnaya cultures, had a huge impact on Eastern and Northern Europe. Yamnaya genetic signatures were deeply apparent in the northern European Corded Ware cultures and, at its extreme, steppe genetic patterns replaced as much as 90 percent of the Neolithic genome in the British Isles.63 In the Early Bronze Age south, Iranian farming genetic signatures dominated the BMAC region, and fed east through to the Indus Valley, combining with an ancient South Asian genetic variation. The Late Bronze Age steppe horizon was indeed transcontinental, with an Early Bronze steppe background mixed with European Neolithic genetic patterns to the west and East Asian genetic patterns to the east. Most important, however, was the movement of the Late Bronze Age steppe genetic variation to the south, into the BMAC region (which itself sent genetic variation north along the IAMC), and onto the edges of the Indus Valley, where it eventually became a central element of the north Indian genetic structure. Across large swaths of Eurasia from the steppe to Shang China, Vedic India, Southwest Asia, and Greece, these Late Bronze Age steppe societies shared a broadly common cultural signature of warrior aristocracies and chariot warfare.64

Three key points stand out in regard to major population and cultural developments: the Early Bronze Age steppe impact on Europe, the Late Bronze entry of East Asian peoples onto the central steppe, and the Late Bronze steppe diffusion down the IAMC, eventually reaching India. None of these are necessarily all that surprising, though the scale and timing of their impact is important. Early Bronze steppe genetic signatures swept into Eastern and Northern Europe with considerable force. As importantly, the movement of steppe genetic signatures north and south along the IAMC supports a sometimes fragmentary archaeology of the arrival of Andronovo outliers and the horse in southern Central Asia during the second millennium, the former either as nomadic herders settling near the walled BMAC oasis towns or involved in mining and smelting in mountain valleys.65 Late Bronze western Eurasians, probably moving east through the Pamirs and the Fergana Valley, settled in the Tarim Valley after 2000 bce, leaving a host of Caucasian mummies drying in the desert sands.66 It may be that these Late Bronze Age steppe peoples carried with them, as both considerable evidence and considerable legend suggest, both an Indo-European language and the roots of Zoroastrian and Vedic-Hindu cosmologies.67

It also might be that they carried with them the plague. Since the late 2000s, analysis of aDNA from ancient skeletons has definitively identified the plague as the cause of the 14th-century Black Death and the 6th–8th-century Justinian Plague.68 Very recent work in plague genomics establishes an even longer history of climate and the plague, reaching back to the transition from the Neolithic into the Bronze Age. This is a story that is developing rapidly and any overview interpretation is perilous. But nonetheless the outlines of our new understanding of the emergence and spread of the ancient plague is worth sketching.

The launch point of this new understanding was a review of the genomes of 20th-century plague samples published by Yujun Cui in 2013.69 The origins of a plague infectious to humans occurred when the YP bacillus diverged from a version of soil-dwelling y. pseudotuberculosis, developing an adaptation to the conditions in the guts of certain fleas. Cui’s study suggested that this event occurred somewhere in the Qinghai Plateau, east of the Tarim Basin and south of the Gansu Corridor. Their dating of this event—a coalescence to a most recent common ancestor—was 1300 bce; subsequent refinement has pushed the genetic origins of the plague to roughly 3700–4000 bce.70

Importantly, this dating put the origin event of the plague in the midst of the cold fourth millennium bce, a period when marginally higher precipitation and lower evaporation brought increased moisture to Central Asia after thousands of years of aridity. Paleoclimatologist Fahu Chen has identified these centuries as the “transition from moving sand to fixed paleosol”—the period in which desert gave way to moister, more organically rich soils, capable of supporting critical populations of rodents and their fleas.71 So the origins of the pathogen might be tied directly to the origins of Holocene wetness in Central Asia.

Cui’s study was based on the genetic histories of modern plague samples, but starting in 2015 a series of studies have identified ancient plague DNA directly in the teeth and bones from burials located from the Altai Mountains to Europe, dating to the Late Neolithic and the Bronze Age. In 2015 two YP-positive Afanasievo burials in the South Altai were dated to 2750–2800 bce, and in 2017 a YP-positive Yamnaya burial was identified near the Caspian Sea, dating approximately to 2750 bce.72 These analyses broadly supported Cui’s argument for a mid-Holocene Central Asian origin for the plague. However, a 2018 study led by Nicolas Rascovan has complicated this synthesis considerably, dating a YP-positive Late Neolithic (Funnel Beaker) burial in southern Sweden to 2900 bce, the oldest known plague dating from human remains.73

So where did the plague come from? The Swedish and Altai samples have virtually the same dates, and must have received the plague simultaneously from somewhere else. There are, as of this writing, five plague samples tightly clustered between 2700 and 2900 bce, from Sweden, the Altai, the west Caspian, and Croatia, suggesting a wave of plague at or before this time. The Rascovan study proposes that the plague might have had its origins in the fourth millennium bce in the Tripolye culture towns scattered across the Danube Valley, towns that were both densely populated and typically destroyed by burning. Plague might have evolved rapidly in these dense and probably dirty towns, towns that epidemic survivors might well have burned as cursed places. The cooler fourth millennium brought precipitation to the western steppe, as well as to arid Central Asia, so climate might have played a role. However, neither ancient YP burials nor modern plague samples show ancient origins in the Danube region or even the western steppe, and so it is still a strong possibility the plague had an eastern origin. The Yamnaya were on the move in the centuries following and had a major impact on the Corded Ware culture: the next four plague burials are dated to 2500 bce in Corded Ware and related contexts. Rascovan and others reject recent theorizing that Yamnaya migrations carried the early version of plague into Europe.74 But it remains possible that early horse utilization, if not domestication, can explain the initial spread of the plague across the steppe to the Danube Valley, from where it seems to have spread into Europe along systems of exchange and trade. It may be that such a plague dispersion comprised something of a “Black Death event.” If so, then it was probably slow-moving and incremental, compared to the more dramatic medieval Black Death, but perhaps just as decisive. Archaeologists have been debating the rise and dramatic fall of European Neolithic populations in a distinct 500-year collapse starting around 3300 bce. If such an event happened, the dates do not quite line up—yet—since the earliest European plague samples date to 2900 bce. Nonetheless, the debate over the apparent collapse of the European Neolithic has begun to shift from unsustainable population growth or the impact of climatic reversal to the early plague, suggesting an event that may have opened the way for Yamnaya cattle-herders, spreading into Europe from the steppe.75

These Late Neolithic/Early Bronze Age plague samples are followed by eleven pre-Justinian-era plague-bearing burials, located in the steppe, Eastern Europe, the German plain, the Baltics, the northern Alps, the south Caucasus, and the Tian Shan Mountains, dating between 2500 bce to 180 ce. The paleogenomic teams find that the early plague bacillus lacked the capability to block the flea’s gut, the capacity that made the Justinian and medieval plagues so lethal for human hosts. This capacity—and thus a flea vector—appears to have emerged around 1800 bce, as evidenced by samples from the proto-Srubnaya on the Volga steppe, perhaps a disease manifestation of the noted global cooling event dated to 2200–2000 bce.76

Of the seven Late Bronze Age/Early Iron Age plague samples, three are from later Bronze Age Europe and Caucasus sites. But four others are from Central Asia proper. One, from a Hun burial dated to 180 ce, is from the Tian Shan, where a dramatic concentration of pre-Black Death YP lineages survive in modern samples.77 Two, however, are from the Mid- to Late Bronze Age horizon, one from the chariot-building Sintashta, at 2200 bce, and another from an Andronovo context at 1700 bce: these three are of the less virulent early plague variety, by contrast to the similarly dated bubonic form in the Srubnaya burials on the Volga.78 If the early plague entered Europe from the steppe in the fourth millennium, then it is worth suggesting that the Late Bronze Age steppe peoples, who were appearing around the BMAC oasis towns after 1800 bce, may have introduced the plague here and to the south, now potentially in its particular virulent bubonic form. Both Margiana–Bactria and the Indus civilizations declined in these centuries. In both, increasing aridity and drought—shaped by the end of the Liavliakan pluvial—seems to have played a role, and there is apparently little to no evidence of large-scale violence. In both regions larger towns, even cities, shrank and were abandoned, giving way to smaller-scale village and nomadic populations. In both, the evidence that steppe peoples and influences were filtering in from the north is very strong.79 It is worth considering whether these steppe influences included an early version of the plague.

The Eastern Steppe

The late second millennium was one of profound global ecological crisis, broadly analogous to the fourth millennium Mid-Holocene transition and the Little Ice Age of 1275–1850 ce. Pushed by a Hallstatt solar super-minimum, and perhaps by volcanic activity, the general Bronze Age optimum was coming to a close. When Eurasia emerged into the “Classical Optimum” of roughly 500 bce to 500 ce, the age of bronze was over and the longer Iron Age was well underway. From 1500 bce the Siberian High record in the Greenland ice suggests dramatically colder winters, while a variety of records suggest that shifts in the North Atlantic would have changed the flow of the westerlies over Eurasia. These changes would have made the northern steppe drier and southern Central Asia somewhat wetter. Simultaneously, the South Asian Monsoon weakened from 1000 through 400 bce (see Figure 4).

These were the general conditions during which people of the Late Bronze Age steppe herding cultures drifted south toward India and east into the Taklimakan region. To the northeast, increasingly nomadic peoples were emerging from a forest-Andronovo background. The Seima-Turbino “complex” of bronze-producing herders and foragers of the southern Siberian forest emerged in the 17th century bce. They were replaced no later than the 12th century by the highly mobile Karasuk peoples, who extended their reach as far as northern China, followed by the Tagars. These northern peoples critically shaped the rise of the eastern steppe cultures, as did the associated dispersal of iron technology.80

Over the course of the first millennium bce the spread of iron across Asia worked a technological revolution. Bronze, requiring the careful alloying of copper and rare tin, was dominated by those with control of distant sources and long lines of trade. Iron ore was ubiquitous and, once mastered, smelting the iron using the bloomery process was a relatively simple operation. Having its roots in both the Eastern Mediterranean and India, iron-working spread through the steppe, reaching China by the 8th century bce.81

The rise of iron technology in the steppe was an important catalyst for the emergence of fully nomadic warrior cultures, the Cimmerians, Scythians, or Sakas, in the 6th century bce in the Altai-Tian Shan regions. These groups are known for their sinuous animal symbolism and their perfection of the powerful short compound bow, designed for rapid fire on horseback.82 Genetic analyses suggest that the Scythians were descended predominantly from Late Bronze (Andronovo) steppe herders (55–80 percent) with a significant contribution from Siberian foragers. Like the Late Bronze steppe peoples they spoke Iranian languages.83 Waves of steppe peoples, among them the Medes and the Achaemenids, would emerge to dominate Iran and the old BMAC region, now Parthia. Among these warrior cultures, the Kushans would move into Parthia and from there to India, to be replaced by a branch of the Huns. Gradually, as the Scythians were subsumed by the Sarmatians, the eastern Saka were eventually displaced by the Xiongnu, who were followed by the Juan-Juan, the Turks, the Uighurs, the Kitan, and finally the Mongols; East Asian peoples became the dominant force across the entire domain of Inner Eurasia.84

In these centuries the eastern steppe and Mongolia became the drivers of Inner Eurasian history. Societies in these areas were shaped by a repetitive logic of conquest settlement, stress, incipient state formation, and militarized expansion. While steppe warrior aristocracies dominate our understanding of the history of the steppe from Iron Age to modernity, these societies were fundamentally dependent on a productive base of semi-sedentary peoples. Thus there was also room for continuity, in as much as local peoples persisted at the lower ranks of new nomad-warrior societies. Therefore, there is a paradox behind our image of steppe nomadic warrior militaries: these formations rested on metallurgic, agricultural, and indeed pastoral activities rooted in the semi-sedentary practices of the Neolithic and Bronze Age IAMC. It is increasingly clear that the peoples of these societies were graded by class and mobility, with a nomadic warrior elite and soldiery supported by wider, lesser classes of minimally mobile agricultural and pastoral communities subordinated by conquest. To be sure, agriculture become increasingly difficult through space and time. The westerlies running east from the Mediterranean gradually lost their moisture, making the eastern steppe drier than the west. And as the Holocene advanced, there was a general decline in moisture in the global system, translating into increasingly arid conditions across the entire landmass. But even in Mongolia the nomadic superstructure rested on exchange and tribute relations with local agricultural communities.85

Interpretations of the rise of nomadic power in Inner Eurasia inevitably involve around questions about aridity, both in its condition and its variability. Mapping a trajectory toward aridity is a complex process, bedeviled by the effects of mountain orthography and the dynamics of west-to-east and north–south climate patterns discussed earlier in this article. As outlined, the strong consensus suggests that precipitation in the monsoon regions of South and East Asia and the westerly domain in Central Asia varied inversely, such that a stronger monsoon regime in South and East Asia co-relates with a weaker westerly regime in Central Asia: a wet south meant a dry north, and vice versa. Measures of the Indian Monsoon, Fahu Chen’s generalized synthesis of precipitation for arid Central Asia, and pollen, leaf wax, and soil records from the Tarim region all suggest the outlines of this relationship over the last four thousand years (see Figure 4).86 If the westerlies shifted south toward arid Central Asia, the steppe did not dry out. Indeed, it appears to have been wetter during cold global regimes. The explanation for this pattern lies in total “effective moisture,” the balance between precipitation and evaporation, which varies with temperature. Both cooling conditions and a shift in the summer location of the summer Ferrel–Hadley boundary and the subtropical jet would have had an effect on the balance of precipitation and evaporation on the ground. Thus, declining precipitation in the north seems to have been offset by lower temperatures, reducing evaporation. The weakening of the summer-time location of the subtropical jet over arid Central Asia would have a similar effect.

At the same time, cooling temperatures from solar declines and enormous volcanic eruptions would have made for epochs of terribly cold winters. Across Inner Eurasia, but especially in the wider eastern steppe and mountain region, severe winters known among Turkic nomads as “dzuds,” combining extreme low temperatures and deep snows, could kill hundreds of thousands of livestock in a given year. These events are caused by the intensity of the dominant climate pattern shaping Asian winters: the Siberian High centered over the Siberian–Mongolian border. Fortunately, there is a very precise record of the Siberian High derived from the Greenland ice cores: the potassium “K+” data in the Greenland cores measures dust particles derived from Asian deserts. Fluctuations in this record measure the intensity of spring dust storms set off by the impact of the winter Siberian High, a system that would be strengthened by extreme cold conditions of a solar minimum. The solar and Siberian High estimates have been in striking alignment over the past 7,000 years (see Figures 3 and 5).87 Deep spikes in the Siberian High—indicating severe winters—align with solar minima, broadly in the fourth millennium in the Iron Age transition and in the Little Ice Age, but also at unique points, specifically around 2800, 1540, 1400, 1200–1000, 815 bce, and 115, 665, 1300, and, again broadly, after 1400 ce.88

Earth, Water, Air, and Fire: Toward an Ecological History of Premodern Inner Eurasia

Figure 5. Eastern steppe: Climate and empire, 800 ce to present. Image credit: The authors.

Sources: See Appendix.

What were the impacts of these variable conditions on Iron Age societies, for whom we have increasingly detailed archaeological and documentary evidence? If general hemispheric cooling brought moister conditions to arid Central Asia, this may explain the arrival of steppe peoples on the edge of the BMAC towns from the opening phases of the Bronze Age/Iron Age Hallstatt, around 1500 bce. To the north, in the forest/steppe zone of the northern edge of the Altai, in the Minisink depression, colder, moister conditions laid the groundwork for the earliest emergence of the Scythians. A continuous series of cultures, the Andronovo, the Karasuk, and the Tagar, occupied the Minisink depression from the 18th to the 9th centuries bce. During the so-called Homeric Solar Minimum of 850–735 bce moister conditions expanded south into the Tuva region, turning scrub desert into steppe, and providing the material basis for the dramatic growth of the proto-Scythian peoples, whose culture over the next several centuries would expand south and west to build the powerful Scythian and Saka confederacies which would dominate the Iron Age steppe.89

Such were the possible implication of general tendencies, but recent historical analysis has focused on the role that sharp spikes of climate crisis played in the eruption of new social and state formations in the greater eastern steppe. The Scythian/Saka emergence was the first of a series of similar events on the eastern steppe, which developed over time into increasingly obvious imperial forms, culminating with the rise of the Mongols. The eastern steppe was drier than the western steppe, and its peoples more dependent on nomadic pastoralism. But the eastern steppe has the capacity to generate sufficient biomass in years of good rainfall to rapidly increase the numbers of livestock to levels that would prove unsustainable in leaner years. The result of the ecological pulses, G. N. Kurochkin proposed in 1994, was that the eastern steppe and especially Mongolia was a “generator of peoples” for century after century.90

Nicola Di Cosmo has developed a fuller model of historical state formation in the eastern steppe involving cycles of societal crisis and expansion, a model that may well pertain to prehistoric emergences. Di Cosmo’s cycle has the virtue of incorporating both climatic reversals and subsequent recoveries, capturing both the essential dynamics of climatic systems in arid lands and the logic of transformative events.91 His model begins with crisis, framed in ecological terms as severe winters, droughts, livestock epidemics, or overgrazing, that suddenly reduces the fortunes of nomadic pastoralists. The response to crisis was militarization—raiding, banditry, and the collapse of the legitimacy of established authority—solved by the rise of a charismatic leader. If crisis produced an enduring new social formation, grounded in ecological recovery and population expansion, a state structure might emerge, providing the authority for a cycle of conquest expansion, in which growing populations of people and animals could be projected into new regions, relieving demographic pressure on the old territory, the “generator of peoples.”92 Filling in the empirical details behind this model, Di Cosmo has been involved in a series of projects that probe the role of sudden climate change on the fortunes of eastern steppe societies. Thus the Eastern Turk Empire fell in 630 ce, in the midst of six years of dzud winter events, while the Second Uighur Empire was perhaps somewhat more resilient, surviving decades of advancing drought to collapse under the impact of intensified drought and severe dzud winter drought between 829 and 840.93 Di Cosmo’s entire model of crisis, state formation, and imperial expansion has been worked out in its fullest form for the Mongols, with a reconstruction of a drought sequence for Mongolia, establishing the pre-state crisis in a significant drought in 1180s, followed by erratic recovery in the 1190s, a second drought between 1200 and 1210, and a notable “Mongol pluvial” between 1211 and 1225. Chinggis Khan emerged as a young outcaste warrior leader in the 1180s, had consolidated a base of power by 1197, and was established as the preeminent Mongol leader in 1206. The early 13th-century droughts and ensuing pluvial may have impelled and then perpetuated the projection of new Mongol power into northern China and then to the west, devastating the Khwarazmian Empire, the descendent of the ancient oasis civilizations running back the BMAC and the Kopet Dag Neolithic, and reaching the Black Sea in the 1220s. In ecological terms, the Mongol blitzkrieg may have been propelled by the earliest turn toward the “cold north/wet Central Asia/dry South Asia” pattern that would prevail during the Little Ice Age.94

The alignment of evidence for climate crisis and steppe societal crisis is less than perfect, but nonetheless compelling. While we might not want to be “environmental determinists,” climate shifts in exceptionally dry climates leave little margin for error. There is a possibility that an episode of very cold dzud winters—perhaps during the spike of the Siberian High at 816 bce—played a role in the subsequent emergence of the Scythian/Saka cultures. The collapse of the Second Uyghur Empire does not align with a convincing marker on the Siberian High record or the solar record. But the First Uyghur Empire of 646–690 rose and fell during the Vandal Solar Minimum, and the ensuing Second Turk Empire fell, and the second Uyghurs emerged, during an extended cold Siberian High period in the 720s to the 740s. It might be significant that Islamic invasions of Central Asia also took place during the Vandal Minimum. Similarly, if we look at tree-ring data for Mongolia and East Asia, four episodes of invasion of China stand out. In each case the emergence of a northern power—the Kitan Liao in 907, the Jurchen in 1115, the Mongols between 1182 and 1210, the Manchus in the early 1640s—and their invasions of China occurred during the immediate recovery from stressful conditions in Mongolia, as expressed in tree-ring widths measuring precipitation, which coincided with a continuing downturn in China. If the Ming were ultimately destroyed by the Manchus during the stresses of the Little Ice Age, Ming resilience—and resistance to nomadic incursion—during climatic stressful centuries after their defeat of the Yuan Mongols in 1368, is particularly striking.95

These also were the centuries, broadly speaking, of what historians have tended to characterize as the classic history of the Silk Road. Very broadly speaking that history pivoted around 500 ce, as the ancient southern routes through India and Iran faded in importance, and the western route north of Caspian to the Black Sea and to Mediterranean markets emerged. The history of the Silk Road is enmeshed in the history of expanding nomad empires, a topic beyond the frame of this article, but it is also fundamental to the history of the bubonic plague, which twice in historical times surged along its routes of commerce and communication, fundamentally assaulting the human ecology of greater Eurasia.

These epidemics followed the transition from the early plague to bubonic plague that has been dated to the Mid- to Late Bronze Age and the leading opinion is that they moved along the Silk Roads. We really know very little about the origins of the Justinian Plague, which hit the Egyptian port of Pelusium on the Mediterranean, north of the Red Sea, in 541, and persisted for centuries. Available evidence suggests, however, that the Justinian Plague had its origins in Central Asia. The abundance of ancient pre-Black Death YP lineages that persist in the Tian Shan, combined with the recent confirmation of a plague burial in the Tian Shan dating to 180 ce, suggest its origins lay somewhere along these mountains. It may be that the conquest of this region by the Hephthalite White Huns in the 490s played a role. It is also possible that global cooling was a factor. Launched by the eruption of two massive volcanoes in 536 and 540, reinforcing the onset of the Vandal Minimum, this “Late Antique Little Ice Age” would have brought more humidity to arid Central Asia, thus improving the conditions for plague-bearing marmots.96

Increasing effective humidity in Central Asia at the onset of the late medieval Little Ice Age, from either more rainfall or decreased evaporation, also lies at the center of some new approaches to the origins and spread of the Black Death. Just as increased moisture encouraged the grasses that nourished Mongol cavalry horses, it might have sustained a growing population of plague pathogen-bearing marmots.97 The Tian Shan and the Silk Roads also have central roles in a long-standing interpretation of the origins of the Black Death. Standard accounts of the Black Death note the devastating epidemic that struck a Nestorian Christian community on Lake Issyk Kul in the Tian Shan in the 1330s, spreading to towns west along the northern Silk Road. The concentration of pre-Black Death plague lineages surviving in the Tian Shan gives added weight to these mountains being the epicenter of the epidemic.98 Conversely, however, Ole Benedictow has argued for a much larger origin, with the plague spreading through and erupting from rodent host populations spread across the entire steppe. Hypothetically, his model would account for the starburst “big bang” of YP genetics into three new branches at the time of the Black Death. Recently, attention has also been shifting to the fur trade running south from the Siberian “Land of Darkness” as a potential source of the plague.99 At the same time, there has been renewed focus on the spread of the plague into China proper. William McNeill, in Plagues and Peoples, suggested decades ago that the Black Death had its origins in Yunnan province in south China, spread further by the campaigns of Mongol armies. This argument has been definitively overturned by the new genetic evidence that there was no ancient plague focus in either Yunnan or Mongolia. Robert Hymes suggests that the twenty-one-year Mongol campaign against the Tangut Xi Xia state in the Gansu Corridor, beginning in 1205, was the critical point of origin, with the Mongols spreading the plague further with subsequent campaigns against the Jurchen Jin and Song domains in the 1230s and the 1270s.100 While interesting new lines of evidence and argument are being explored, and the boundaries around possible interpretations are tightening, the definitive story of the origins and spread of the Black Death in and across Inner Eurasia has not yet been told.

Modernity: Into the Anthropocene

The modern ecological history of Inner Eurasia is an enormous subject, which can only be given cursory treatment here. But what is striking is both how recently it has begun to receive scholarly attention and the degree to which this attention has been driven by the basic structures of climate and landscape—earth, water, and air—that had shaped premodern history—with the addition of the fire of external empires.101

Modernity struck Inner Eurasia with full force at the end of the 19th century, though in some ways it had deeper roots in the twin impacts of the Black Death and the Little Ice Age. The Black Death may have had as devastating an impact on cities and towns, if not nomadic peoples, in Inner Eurasia as it did across Europe, though we can only guess at many of the particulars. Uli Schamiloglu makes a cogent argument for the consequences of the Black Death across Inner Eurasia, from the splintering of the political unity of the Golden Horde, the apparent abandonment of towns and cities, the disappearance of languages, and a heightened religiosity.102 The impact of the Little Ice Age similarly is still slightly opaque. We have scattered anecdotal evidence of desperately cold winters, but the evidence on the ground is complicated. The Little Ice Age brought wetter conditions to arid Central Asia, roughly the Tarim Basin and the mountains to its east, and perhaps to the steppe as well. In any event, wet or dry, the Siberian High record may speak for itself: from 1400 through to roughly 1725 Eurasia was blasted with the most extreme cold winters since the end of the Pleistocene.103 One can imagine a seemingly endless series of dzud winters devastating sheep, cattle, and horse, interspersed with too few “normal” years for Central Asian peoples to experience any sort of lasting recovery.

Well before the Little Ice Age was over, powerful external, rimland empires were grinding away at Inner Asian autonomy in a relentless assault to appropriate land and resources. Inner Asian states, originating in the eastern steppe, perhaps with some considerable Chinese borrowing, had developed the form of empire that would ultimately overwhelm them. As Nicola Di Cosmo has suggested, starting with Kublai Khan’s advance on the Southern Song in the 1260s, Inner Asian states shifted from more indirect methods of governance and tribute extraction to direct administration and taxation. Tamerlane, the Mughals, and the Ottomans would carry this imperial form forward into the early modern period, making the great landed Eurasian empires the most feared concentrations of power in their time.104 But in the end Russia and China would swallow up and divide Inner Eurasia between them, with the British flitting around its southern periphery in hopes of sustaining their Indian domains in the “Great Game.” These powers brought modernities to the steppe and mountains at the point of a gun, with a program of extractive development that is only accelerating in the 21st century.105 Russians and their Cossack proxies started their march east into the fur-laden Siberian forests in the late 16th century, and established themselves in the 17th century on the forest/steppe edge, facing the Kazakhs, who in turn were struggling with the Dzungars, the last of the eastern nomad-warrior empires on the Mongol model.

The Oirat Dzungar Empire had emerged by the 1620s, in a gap between two episodes of extreme Siberian Highs. By the 1640s the empire had concluded a treaty with the Russians and, in the context of a wave of Siberian High winters and under the global cooling influence of massive volcanic eruptions, had launched an attack into the mountains, besieging Tashkent in 1643. The next year, the worst moment of the Little Ice Age crisis, the Manchus conquered China, and spent the next forty years consolidating their power. Over these decades the Dzungars defied the Manchu Qing Empire from their territories in modern Xinjiang, the Tian Shan, and the Tarim Basin. In the 1690s—during one of the worst periods of the Maunder Solar Minimum—they were defeated and sued for peace; in 1756, after Qing campaigns had extinguished Dzungar authority and scattered any remaining rebels into Kazakh territory, the imperial order went out to exterminate them as a people. Tens of thousands of men were massacred and women and children enslaved; the steppe nomadic power was broken.106

In Mongolia the Qing used winter and drought disasters to break pastoral traditions, gradually introducing agriculture as a prelude to Han agrarian colonization in the 18th century.107 If the exterminating war against the Dzungar Mongols was one of the last great steppe wars, carried to a new extreme, the transformation of Mongolia was the other side of the coin: the expropriation and development of land.108 Inner Eurasia under imperial rule—Qing, tsarist, Soviet, or post-communist—was scavenged for resources to fuel the economic advance of the rimland societies; if ancient peoples diverted water and mined for metals, these were pinpricks compared to the onslaught of modernity.

As the Dzungars were being destroyed, the Russians were fighting their own frontier wars to the west, starting in the 16th century and accelerating under Peter the Great in the early 18th century, controlling the steppe between the Caspian Sea and the Urals by 1734. In the mid-19th century they advanced again, first into the territory of modern Kazakhstan and then farther south into the agricultural oases of Turkestan.109 Behind the soldiers came the settlers, who attempted to transform the steppe into a neo-Europe.110 In lands of the ancient BMAC/Oxus civilization, in present-day Turkmenistan and Uzbekistan, the Russians, both tsarist and Soviet, pursued massive projects in irrigated cotton, supplying cotton factories across Europe, and virtually emptying the Aral Sea by the mid-20th century. Such projects had their indigenous analogues in the expansion of irrigated agricultural in the fertile Fergana Valley under the Khanate of Khoqand during the 18th and 19th centuries.111 Settling in Kazakhstan near the turn of the 20th century, Russian and German migrants struggled with unfamiliar soils, locusts, and drought in their efforts to turn the ancient steppe into grainlands. As populations of people and livestock grew, Kazakh pastoralists gradually lost their seasonal migration routes and, confined to collective farms by the Soviets, people and livestock suffered massive famines in the early 1930s.112

Metals and then energy would follow agriculture in Russian imperial development. Again, Peter the Great was instrumental in expanding mining for silver, copper, and iron, mostly in the Urals, shaping both the Russian economy and a massive deforestation. Well before the Aral Sea dried up in the 20th century from excess irrigation drawdown, the Caspian was being exploited for another critical recourse: Caspian oil was the bait that drew Hitler’s armies to defeat in southern Russia in 1942. As Central Asian oil flows into the world market now, iron from the Urals fed Peter the Great’s military and then British industrial demand.113 And beyond the environmental scarring left by mining and metal production, two devastating nuclear accidents poisoned vast areas of the Soviet Union, across the eastern Urals in 1957 and at Chernobyl in the Ukraine in 1986. These disasters stand as potent symbols of the massive ongoing environmental degradation unleashed by the Soviet drive toward industrial development from the 1930s forward.114

The relations of Inner Eurasia to global markets from the 19th century was a distant continuation of ancient systems. Its mines had produced precious stones, tin, copper, and iron, and the steppe had supplied horses to the coastal empires by the tens and even hundreds of thousands from the Late Bronze Age to the end of the 19th century. From the Bronze Age to the present Inner Eurasia has been both a distant source of valuable commodities, and also a daunting geographic expanse to be traversed and overcome. China’s new “One Belt, One Road” initiative is a self-conscious effort to reproduce the ancient and medieval Silk Roads with highways, high-speed rail, and maritime transport, thus consolidating Eurasian transportation networks with China at the helm.115 Like the scientific inquiry that both precipitated and followed the exploitation of other frontiers, much of the research on Inner Asian ancient climates and environments discussed here has been funded by the Chinese government as it seeks to develop, resettle, and, above all, control its own west, Xinjiang and Tibet, and extend its influence into the former Soviet Central Asian republics further to the west.

Looming over this enterprise as one considers the future of Inner Eurasia as a whole, it is the accelerating impact of anthropogenic climate change that is emerging as the most formidable factor to consider. As global climates change under the pressure of rising greenhouse gases, Inner Eurasia is warming rapidly. Rising temperatures—as they always have—suppress moisture across the region, driven by a northward shift of the Atlantic westerlies, rising rates of evaporation, and rapid melting of essential mountain glaciers. Compounding these geophysical impacts, growing populations have for some time been putting increasing burdens on very limited groundwater and the entire region is projected to suffer significant to severe water scarcity within a few decades.116 Inner Eurasia today is an ancient landscape, fragile and battered, in which resilience has its limits. In this fraught ecological zone, humanity’s fire may well be too much for nature’s earth, water, and air.


The authors thank Kyle Harper, Clark Larsen, Scott Levi, and Jessica Rawson for their careful readings. We are indebted to climate team members for sharing their climate data, and to Gregory Ginet for making the low-pass calculations.

Appendix: Full Citations for Climate Data used in Figures 3–5

Aichner, B. et al. “High-Resolution Leaf Wax Carbon and Hydrogen Isotopic Record of the Late Holocene Paleoclimate in Arid Central Asia.” Climates of the Past 11 (2015): 619–633.Find this resource:

Berger, A., and M. F. Loutre. “Insolation Values for the Climate of the Last 100 Million Years.” Quaternary Science Reviews 10 (1991): 297–317.Find this resource:

Blagoveshchenskaya, N. V., and I. E. Isaev. “The Holocene Evolution of Vegetation in the Southeast of the Volga Upland.” Russian Journal of Ecology 39 (2018): 93–101.Find this resource:

Bond, Gerard et al. “Persistent Solar Influence on North Atlantic Climate during the Holocene.” Science 294 (2001): 2130–2136.Find this resource:

Boroffka, Nikolaus G. O. “Archaeology and its Relevance to Climate and Water Level Changes: A Review.” In The Aral Sea Environment. Edited by Andrey G. Kostianoy and Aleksey N. Kosarev, 283–303. New York: Springer, 2010.Find this resource:

Boroffka, Nikolaus et al. “Archaeology and Climate: Settlement and Lake-Level Changes at the Aral Sea.” Geoarchaeology: An International Journal 21 (2006): 721–734.Find this resource:

Chen, F. H. et al. “Moisture Changes over the Last Millennium in Arid Central Asia: A Review, Synthesis and Comparison with Monsoon Region.” Quaternary Science Reviews 29 (2010): 1055–1068.Find this resource:

Fleitman, Dominik et al. “Holocene ITCZ and Indian Monsoon Dynamics Recorded in Stalagmites from Oman and Yemen (Socotra).” Quaternary Science Reviews 26 (2007): 170–188.Find this resource:

Hessel, Amy E. et al. “Past and Future Drought in Mongolia.” Science Advances 4 (2018): e1701832.Find this resource:

IPCC Box 6.4: Regional temperature proxies [c.800–present] developed by Timothy Osborn and Keith Briffa for Box 6.4, Ch. 6, Paleoclimate. In Climate Change 2007: The Physical Science Basis: Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, ed. Susan Solomon et al. Cambridge and New York, 2007.Find this resource:

Kathayat, Gayatri et al. “The Indian Monsoon Variability and Civilization Changes in the Indian Subcontinent.” Science Advances 3 (2017): e1701296.Find this resource:

Kremenetski, Konstantin V. “Steppe and Forest–Steppe Belt of Eurasia: Holocene Environmental History.” In Prehistoric Steppe Adaptation and the Horse. Edited by Marsha Levine, Colin Renfrew, and Katie Boyle, 11–27. Cambridge, U.K.: McDonald Institute, 2003.Find this resource:

Lauterbach, Stefan et al. “Climatic Imprint of the Mid-Latitude Westerlies in the Central Tian Shan of Kyrgyzstan and Teleconnections to North Atlantic Climate Variability during the Last 6000 Years.” Holocene 24 (2014): 970–984.Find this resource:

Mayewski, P. A. et al. “Major Features and Forcing of High-Latitude Northern Hemisphere Atmospheric Circulation Using a 110,000-Year-Long Glaciochemical Series.” Journal of Geophysical Research 102 (1997): 26345–26366.Find this resource:

Osborn, Timothy A., and Keith R. Briffa. “The Special Extent of 20th-Century Warmth in the Context of the Past 1200 Years.” Science 311 (2006): 841–844.Find this resource:

Pederson, Neil et al. “Pluvials, Droughts, the Mongol Empire, and Modern Mongolia.” Proceedings of the National Academy of Sciences 111 (2014): 4375–4379.Find this resource:

Seppä, H. et al. “Last Nine-Thousand Years of Temperature Variability in Northern Europe.” Climates of the Past 5 (2009): 523–535.Find this resource:

Shapiro, A. I. et al. “A New Approach to Long-Term Reconstruction of the Solar Irradiance Leads to Large Historical Forcing.” Astronomy and Astrophysics 529 (2011): A67.Find this resource:

Sigl, M. et al. “Timing and Climate Forcing of Volcanic Eruptions for the Past 2,500 Years.” Nature 523 (2015): 543–549.Find this resource:

Wang, Wei, and Zhaodong Feng. “Holocene Moisture Evolution across the Mongolian Plateau and its Surrounding Areas: A Synthesis of Climatic Records.” Earth-Science Reviews 122 (2013): 38–57.Find this resource:

Zhao, Keliang et al. “Climatic Variations over the Last 4000 Cal Yr BP in the Western Margin of the Tarim Basin, Xinjiang, Reconstructed from Pollen Data.” Palaeogeography, Palaeoclimatology, Palaeoecology 321–322 (2012): 16–23.Find this resource:

Full Citations for 66 Identified Plague Burials: Section D, Figure 4 (Sample Dates Averaged and Distributed Within Calibrated Range)

Damgaard, Peter de Barros et al. “137 Ancient Human Genomes from across the Eurasian Steppes.” Nature 557 (2018): 369–374.Find this resource:

Feldman, Michal et al. “A High-Coverage Yersinia Pestis Genome from a Sixth-Century Justinianic Plague Victim.” Molecular Biological Evolution 33 (2016): 2911–2923.Find this resource:

Keller, Marcel et al. “Ancient Yersinia Pestis Genomes from across Western Europe Reveal Early Diversification during the First Pandemic (541–750).” Proceedings of the National Academy of Sciences 116 (2019): 12363–12372.Find this resource:

Namouchi, Amin, et al. “Integrative Approach Using Yersinia Pestis Genomes to Revisit the Historical Landscape of Plague during the Medieval Period.” Proceedings of the National Academy of Sciences 115 (2018): E11790–E11797.Find this resource:

Rasmussen, Simon et al. “Early Divergent Strains of Yersinia Pestis in Eurasia 5,000 Years Ago.” Cell 163 (2015): 571–582.Find this resource:

Spyrou, Maria A. et al. “Analysis of 3800-Year-Old Yersinia Pestis Genomes Suggests Bronze Age Origin for Bubonic Plague”. Nature Communications 9 (2018): 2234.Find this resource:

Valtueña, Aida Andrades et al. “The Stone Age Plague and its Persistence in Eurasia.” Current Biology 27 (2017): 3683–3691.Find this resource:

Wagner, David M. et al. “Yersinia Pestis and the Plague of Justinian 541–543 AD: A Genomic Analysis.” Lancet Infectious Diseases 14 (2014): 319–326.Find this resource:

Further Reading

Anthony, David W. The Horse, the Wheel, and Language: How Bronze-Age Riders from the Eurasian Steppes Shaped the Modern World. Princeton, NJ: Princeton University Press, 2007.Find this resource:

Breyfogle, Nicholas B., ed. Eurasian Environments: Nature and Ecology in Imperial Russian and Soviet History. Pittsburgh, PA: University of Pittsburgh Press, 2018.Find this resource:

Brooke, John L. Climate Change and the Course of Global History: A Rough Journey. New York: Cambridge University Press, 2014.Find this resource:

Chernykh, Evgeniĭ Nikolaevich. Ancient Metallurgy in the USSR: The Early Metal Age. New York: Cambridge University Press, 1992.Find this resource:

Christian, David. “Inner Eurasia as a Unit of World History.” Journal of World History 5 (1994): 173–211.Find this resource:

Cunliffe, Barry. By Steppe, Desert, and Ocean: The Birth of Eurasia. Oxford: Oxford University Press, 2015.Find this resource:

Di Cosmo, Nicola. Ancient China and its Enemies: The Rise of Nomadic Power in East Asian History. New York: Cambridge University Press, 2002.Find this resource:

Di Cosmo, Nicola. “Ancient Inner Asian Nomads: Their Economic Basis and its Significance in Chinese History.” Journal of Asian Studies 53 (1994): 1092–1126.Find this resource:

Di Cosmo, Nicola. “New Directions in Inner Asian History: A Review Article.” Journal of the Economic and Social History of the Orient 42 (1999): 247–263.Find this resource:

Di Cosmo, Nicola. “State Formation and Periodization in Inner Asian History.” Journal of World History 10 (1999): 1–40.Find this resource:

Frachetti, Michael D. “The Multi-Regional Emergence of Mobile Pastoralism and the Growth of Non-Uniform Institutional Complexity across Eurasia.” Current Anthropology 53 (2012): 2–38.Find this resource:

Frachetti, Michael D. Pastoralist Landscapes and Social Interaction in Bronze Age Eurasia. Berkeley: University of California Press, 2008.Find this resource:

Kohl, Philip L. The Making of Bronze Age Eurasia. New York: Cambridge University Press, 2007.Find this resource:

Kohl, Philip L., ed. The Bronze Age Civilization of Central Asia: Recent Soviet Discoveries. Armonk, NY: Sharpe, 1981.Find this resource:

Kuzmina, Elena E. The Prehistory of the Silk Road. Philadelphia: University of Pennsylvania Press, 2008.Find this resource:

Levi, Scott C. The Bukharan Crisis: A Connected History of 18th-Century Central Asia. Pittsburgh: University of Pittsburgh Press, 2020.Find this resource:

Levine, Marsha et al., eds. Late Prehistoric Exploitation of the Eurasian Steppe. Cambridge, U.K.: Oxbow, 1999.Find this resource:

Lieberman, Victor. Strange Parallels: Southeast Asia in Global Context, c. 800–1300. Vol. 1, Integration on the Mainland. New York: Cambridge University Press, 2003.Find this resource:

Lieberman, Victor. Strange Parallels: Southeast Asia in Global Context, c. 800–1300. Vol. 2, Mainland Mirrors: Europe, Japan, China, South Asia and the Islands. New York: Cambridge University Press, 2009.Find this resource:

Mayewski, Paul A. et al. “Holocene Climate Variability.” Quaternary Research 62 (2004): 243–255.Find this resource:

Spengler, Robert N. “Agriculture in the Central Asian Bronze Age.” Journal of World Prehistory 28 (2015): 215–253.Find this resource:

Wanner, Heinz et al. “Mid- to Late Holocene Climate Change: A Review.” Quaternary Science Reviews 27 (2008): 1791–1828.Find this resource:

Wanner, Heinz et al. “Structure and Origin of Holocene Cold Events.” Quaternary Science Reviews 30 (2011): 3109–3123.Find this resource:


(1.) Daniel Lord Smail and Andrew Shryock, “History and the ‘Pre’,” American Historical Review 118 (2013): 709–737; Dipesh Chakrabarty, “The Climate of History: Four Theses,” Critical Inquiry 35 (2009): 197–222; and Donald Worster, “Transformations of the Earth: Toward an Agroecological Perspective in History,” Journal of American History 76 (1990): 1087–1106.

(2.) Jared Diamond, Guns, Germs, and Steel: The Fate of Human Societies (New York: Norton, 2005), 183–186; see also Christopher I. Beckwith, Empires of the Silk Road: A History of Central Eurasia from the Bronze Age to the Present (Princeton, NJ: Princeton University Press, 2009); and David W. Anthony, The Horse, the Wheel, and Language: How Bronze-Age Riders from the Eurasian Steppes Shaped the Modern World (Princeton, NJ: Princeton University Press, 2007).

(5.) For leading examples of recent work, see Vagheesh M. Narasimhan et al., “The Formation of Human Populations in South and Central Asia,” Science 365 (2019): eaat7487; Peter de Barros Damgaard et al., “137 Ancient Human Genomes from across the Eurasian Steppes,” Nature 557 (2018): 369–374; Peter de Barros Damgaard, “The First Horse Herders and the Impact of Early Bronze Age Steppe Expansions into Asia,” Science 360 (2018): eaar7711; and Morten E. Allentoft et al., “Population Genomics of Bronze Age Eurasia,” Nature 522 (2015): 167–172.

(8.) Xin-Ganga Dai and Ping Wang, “A New Classification of Large-Scale Climate Regimes around the Tibetan Plateau based on Seasonal Circulation Patterns,” Advances in Climate Change Research 8 (2017): 26–36.

(10.) Fahu Chen et al., “A Persistent Holocene Wetting Trend in Arid Central Asia, with Wettest Conditions in the Late Holocene, Revealed by Multi-Proxy Analyses of Loess-Paleosol Sequences in Xinjiang, China,” Quaternary Science Reviews 146 (2016): 134–146; Fahu Chen et al., “Moisture Changes over the Last Millennium in Arid Central Asia: A Review, Synthesis and Comparison with Monsoon Region,” Quaternary Science Reviews 29 (2010): 1055–1068; Fahu Chen et al., “Holocene Moisture Evolution in Arid Central Asia and its Out-of-Phase Relationship with Asian Monsoon History,” Quaternary Science Reviews 27 (2008): 351–364; and Judah Cohen et al., “The Role of the Siberian High in Northern Hemisphere Climate Variability,” Geophysical Research Letters 28 (2001): 299–302.

(11.) Edward Aguado and James E. Burt, Understanding Weather and Climate, 4th ed. (Upper Saddle River, NJ: Pearson, 2007), 215–216; Alta S. Walker, Deserts: Geology and Resources (Washington, DC, USGS: 1996), 7–11; Andrew S. Gouldie, Great Warm Deserts of the World: Landscapes and Evolution (Oxford: Oxford University Press, 2002), 9–10.

(12.) Wei Wei et al., “Relationship between the Asian Westerly Jet Stream and Summer Rainfall over Central Asia and North China: Roles of the Indian Monsoon and the South Asian High,” Journal of Climate 30 (2017): 537–552; Yong Zhao et al., “Relationships between the West Asian Subtropical Westerly Jet and Summer Precipitation in Northern Xinjiang,” Theoretical Applied Climatology 116 (2014): 403–411; Yong Zhao et al., “Impact of the Middle and Upper Tropospheric Cooling over Central Asia on the Summer Rainfall in the Tarim Basin, China,” Journal of Climate 27 (2014): 4721–4732; Liangcheng Tan et al., “Centennial-to Decadal-Scale Monsoon Precipitation Variations in the Upper Hanjiang River Region, China over the Past 6650 Years,” Earth and Planetary Science Letters 482 (2018): 580–590; T. Mölg et al., “Prominent Midlatitude Circulation Signature in High Asia’s Surface Climate during Monsoon,” Journal of Geophysical Research: Atmospheres 122 (2017): 127021–127712; and Jing Ge et al., “The Influence of the Asian Summer Monsoon Onset on the Northward Movement of the South Asian High towards the Tibetan Plateau and its Thermodynamic Mechanism,” International Journal of Climatology 38 (2018): 543–553.

(13.) Chen et al., “Holocene Moisture Evolution”; John C. H. Chiang et al., “Role of Seasonal Transitions and Westerly Jets in East Asian Paleoclimate,” Quaternary Science Reviews 108 (2015): 111–129; Tan et al., “Centennial-to Decadal-Scale Monsoon Precipitation Variations.”

(14.) For full discussion of the global pattern, see Brooke, Climate Change, 166–183, 276–279; for Central Asia, see: Chen et al., “Moisture Changes” ; Luca Filippi et al., “Multidecadal Variations in the Relationship between the NAO and Winter Precipitation in the Hindu Kush–Karakoram,” Journal of Climate 27 (2014): 7890–7902; Keliang Zhao et al., “Climatic Variations over the Last 4000 Cal Yr BP in the Western Margin of the Tarim Basin, Xinjiang, Reconstructed from Pollen Data,” Palaeogeography, Palaeoclimatology, Palaeoecology 321–322 (2012): 16–23; Weiguo Liu, “Wet Climate during the ‘Little Ice Age’ in the Arid Tarim Basin, Northwestern China,” Holocene 21 (2010): 409–416.

(15.) Xiaodong Liu et al., “Continental Drift and Plateau Uplift Control Origination and Evolution of Asian and Australian Monsoons,” Nature: Scientific Reports 7 (2017): 40344; A. Licht et al., “Resilience of the Asian Atmospheric Circulation Shown by Paleogene Dust Provenance,” Nature Communications 7 (2016): 12390.

(17.) Solar influences, shaped both by orbital forcing (A) and by internal fluctuation in solar intensity (B), are two of the major external drivers of climate in Eurasia, and globally, in the post-glacial Holocene period. Orbital forcing on the precession cycle (A) peaked in the Early Holocene, with exceptionally warm northern hemisphere temperatures, and then declined through the Mid and Late Holocene. During the Early Holocene the ITCZ, associated Hadley Cells, and embedded summer monsoon systems were drawn to the north, influencing much of southwest Asia, and a strong positive mode of the NAO sent Atlantic moisture over Scandinavia into Siberia. The steppe and arid Central Asia, lacking moisture from either the Indian Ocean or the Atlantic, ranged from dry to hyper arid. But with the long decline in orbital force insolation, the entire ITCZ-monsoon system shifted south (H), bringing cooler temperatures to much of Eurasia.

The impact of this general shift has been measured in a number of natural climate archives. The general temperature decline has been estimated for the Baltics (A), which broadly matches Holocene northern hemisphere estimates. Cooler temperatures and an increasing precipitation as the Atlantic westerlies shifted south (with negative NAO) brought increasing moisture to the Russian Upland forest/steppe margin (E) and to Central Asia, here summarized by an index for Northern Xinjiang (F). Conversely, the Altai (F) and much of Siberia seems to have become drier, as the westerly influence shifted south. The Aral Sea region seems to have responded these forces in a somewhat different pattern. Initially—with the rest of Central Asia—the region was hyper arid in the postglacial millennia to 6000 bce. Then it suddenly began to receive considerable moisture, apparently in the form of both local precipitation and meltwater from the Pamirs. While the causes of this “Liavliakan pluvial” are under debate, it seems likely that it was a function of both south-running westerlies interacting with indirect influences of the South Asia Monsoon, as it retreated from the Persian Gulf. The further retreat of the monsoon after roughly 3000 bce seems to have set off advancing aridity during the Bronze Age.

At the same time, the continuous but patterned oscillation of the intensity of solar output (B) had direct and indirect effects on two measures of northern hemisphere climate, the Siberian High (C) and ice-rafting in the North Atlantic (D). While the extreme Siberian High in the seventh millennium bce was probably caused by one of several collapses of the Pleistocene ice sheets, after 6000 bce these systems came into cyclical alignment, effectively running underneath the more general decline of orbital forcing. Grand solar minima in the fourth millennium, between 1500 and 800 bce, and from about 1300 to 1850 ce were echoed in the record of sustained extreme cold dry outbursts of the Siberian High. The Atlantic ice-rafting records, probably a rough measure positive and negative NAO, oscillated on this quasi-2,000-year pattern, and a notable 1,000-year cycle. These 1,000 and 2,000 shifts in colder periods brought increased moisture to acid central Asia and perhaps the steppe, via increased westerly precipitation and less evaporation. These effects are particularly evident on more recent time frames (see Figures 4 and 5), but over the Middle Holocene it is possible that a record for the Dnieper Valley steppe captures this warm/dry vs colder/wetter oscillation, with colder, wetter climate in the fourth millennium and after 1500 bce interrupted by a drier third millennium bce.

(18.) Chen et al., “Holocene Moisture Evolution”; Wei Wang and Zhaodong Feng, “Holocene Moisture Evolution across the Mongolian Plateau and its Surrounding Areas: A Synthesis of Climatic Records,” Earth-Science Reviews 122 (2013): 38–57; Liya Jin et al., “Causes of Early Holocene Desertification in Arid Central Asia,” Climate Dynamics 38 (2012): 1577–1591. There has recently been some dissent from this negative NAO/wetter Central Asia model, but see the critical commentary in Christian Wolfe et al., “Precipitation Evolution of Central Asia during the Last 5000 Years,” Holocene 27 (2017), esp. 148–152. An influential new analysis suggests that the warm polar north during the early Holocene weakened the tropical Hadley Cells, making the mid-latitudes drier, but by the Late Holocene, roughly 4,000 years ago, the conditions had reversed, with a colder north, stronger Hadley circulation, and more mid-latitude (including central Asia) precipitation. Cody C. Routson et al., “Mid-Latitude Net Precipitation Decreased with Arctic Warming during the Holocene,” Nature 568 (2019): 83–87.

(19.) Five measures of moisture suggest the patterned impact of the Atlantic westerlies (C) and the Siberian High (B) on Central Asia. The Northern Xinjiang and Lake Karkuli measure both gradually move toward moisture, apparently from the Atlantic westerlies, as conversely, the South Asian Monsoon (H) continued its southward retreat. Individual studies of lake chemistry at Sol Kol and Karakuli, and pollen in a soil profile from Kashgar, all suggest that Atlantic moisture increased during the first half of ice-rafting cycles (C), indicators of a cold North Atlantic and probably negative phase of the NAO. It is possible that these moisture increases were reversed by the impact of the dry cold winter Siberian High. However Fahu Chen’s Central Asia moisture index authoritative synthesis, charting the transition out of a dry medieval climate regime, suggests that increased moisture prevailed throughout the Little Ice Age. Strikingly, the new evidence for both the origins of the ancient plague, and for ancient and medieval plague-infected burials across Eurasia (D) seems to align with colder periods in the North Atlantic data (and perhaps wetter periods in Central Asia). (Note: the number of positive identification of YP genetics in these burials during the Justinian plague has more than doubled with the publication of Marcel Keller et al., “Ancient Yersinia Pestis Genomes from across Western Europe Reveal Early Diversification during the First Pandemic (541–750)”, Proceedings of the National Academy of Sciences 116 (2019): 12363–12372.

(20.) In a large literature on the steppe, see N. A. Khotinsky, “Holocene Vegetation History,” in Late Quaternary Environments of the Soviet Union, ed. A. A. Velichko (London: Longman, 1984), 179–200; Kremenetski, “Steppe and Forest–Steppe Belt of Eurasia,” 12–22; Constantin V. Kremenetski, Olga A. Chichagova, and Nathalia I. Shishlina, “Palaeoecological Evidence for Holocene Vegetation, Climate and Landuse Change in the Low Don Basin and Kalmuk Area, Southern Russia,” Vegetation History and Archaeobotany 8 (1999): 233–246; N. V. Blagoveshchenskaya and I. E. Isaev, “The Holocene Evolution of Vegetation in the Southeast of the Volga Upland,” Russian Journal of Ecology 39 (2018): 93–101; Elena Y. Novenko et al., “Mid- and Late-Holocene Vegetation History, Climate and Human Impact in the Forest–Steppe Ecotone of European Russia: New Data and a Regional Synthesis,” Biodiversity and Conservation 25 (2016): 2453–2472; Elena Y. Novenko et al., “The Holocene Paleoenvironmental History of Central European Russia Reconstructed from Pollen, Plant Macrofossil, and Testate Amoeba Analyses of the Klukva Peatland, Tula Region,” Quaternary Research 83 (2015): 459–468; N. N. Kashirskayaa et al., “Dynamics of Chemical and Microbiological Soil Properties in the Desert–Steppe Zone of the Southeast Russian Plain during the Second Part of the Holocene (4000 BC–XIII century AC),” Arid Ecosystems 8 (2018): 38–46; T. Alekseeva et al., “Late Holocene Climate Reconstructions for the Russian Steppe, based on Mineralogical and Magnetic Properties of Buried Palaeosols,” Palaeogeography, Palaeoclimatology, Palaeoecology 249 (2007): 103–127.

(22.) Here we draw on the chronology in Nikolaus Boroffka et al., “Archaeology and Climate: Settlement and Lake-Level Changes at the Aral Sea,” Geoarchaeology: An International Journal 21 (2006): 721–734; and Nikolaus G. O. Boroffka, “Archaeology and its Relevance to Climate and Water Level Changes: A Review,” in The Aral Sea Environment, ed. Andrey G. Kostianoy and Aleksey N. Kosarev (New York: Springer, 2010), 283–303. For important reviews of the contentious debates regarding the hydrology Aral Sea, see Ian Boomer et al., “Advances in Understanding the Late Holocene History of the Aral Sea Region,” Quaternary International 194 (2009): 79–90, and Jean-François Cretaux et al., “History of Aral Sea Level Variability and Current Scientific Debates,” Global and Planetary Change 110 (2013): 99–113. On the atmospheric dynamics in the southwest region, we are indebted to Hedi Oberhänsli et al., “Variability in Precipitation, Temperature and River Runoff in West Central Asia during the Past ~2000 Yrs,” Global and Planetary Change 76 (2011): 95–104; Suzanne A. G. Leroy et al., “Late Pleistocene and Holocene Palaeoenvironments in and around the Middle Caspian Basin as Reconstructed from a Deep-Sea Core,” Quaternary Science Reviews 101 (2014): 91–110, discussion on 107–108; S. Markofsky et al., “An Investigation of Local Scale Human/Landscape Dynamics in the Endorheic Alluvial Fan of the Murghab River, Turkmenistan,” Quaternary International 437 (2017): 1–19, discussion on 15–16; Sébastien Joannin et al., “Vegetation, Fire and Climate History of the Lesser Caucasus: A New Holocene Record from Zarishat Fen (Armenia),” Journal of Quaternary Science 29, no. 1 (2014): 70–82; Mohammad A. Hamzeh et al., “Paleolimnology of Lake Hamoun (E Iran): Implication for Past Climate Changes and Possible Impacts on Human Settlements,” Palaios 31 (2016): 616–629. The role of the monsoon in increased southwest precipitation, however indirect, is also suggested by the fact that the southwest was out of phase with the steppe to the north. The steppe had a distinct arid phase in the third millennium, and increased precipitation in the second millennium, the opposite of the Caspian region. There is also a discussion underway regarding the relationship of El Nino (warm ENSO) events (coinciding with weak South Asian monsoons) and increased precipitation in the southwest regions. See F. S. Syed et al., “Effect of Remote Forcings on the Winter Precipitation of Central Southwest Asia, Part 1: Observations,” Theoretical and Applied Climatology 86 (2006): 147–160; Andrew Hoell et al., “Cold Season Southwest Asia Precipitation Sensitivity to El Niño–Southern Oscillation Events,” Journal of Climate 31 (2018): 4463–4482.

(23.) Wang and Feng, “Holocene Moisture Evolution”; Tatiana Blyakharchuk et al., “High Resolution Palaeoecological Records for Climatic and Environmental Changes during the Last 1350 Years from Manzherok Lake, Western Foothills of the Altai Mountains, Russia,” Quaternary International 447 (2017): 59–74; Chen et al., “Moisture Changes”; Chen et al., “Holocene Moisture Evolution”; Jianhui Chen et al., “Hydroclimatic Changes in China and Surroundings during the Medieval Climate Anomaly and Little Ice Age: Spatial Patterns and Possible Mechanisms,” Quaternary Science Reviews 107 (2015): 98–111.

(24.) Robin Dennell, The Paleolithic Settlement of Asia (Cambridge, U.K.: Cambridge University Press, 2009), 325–333.

(25.) Aida Gómez-Robles et al., “Dental Evolutionary Rates and its Implications for the Neanderthal–Modern Human Divergence,” Science Advances 5 (2019): eaaw1268.

(26.) David Reich, Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past (New York: Pantheon Books, 2018), 53–74; Belen Lorente-Galdos et al., “Whole-Genome Sequence Analysis of a Pan African Set of Samples Reveals Archaic Gene Flow from an Extinct Basal Population of Modern Humans into Sub-Saharan Populations,” Genome Biology 20, no. 17 (2019): 1–15; Fahu Chen et al., “A Late Middle Pleistocene Denisovan Mandible from the Tibetan Plateau,” Nature 569 (2019): 409–412.

(27.) Huw S. Groucutt et al., “Rethinking the Dispersal of Homo Sapiens Out of Africa,” Evolutionary Anthropology 24 (2015): 149–164; Christopher J. Bae et al., “On the Origin of Modern Humans: Asian Perspectives,” Science 358 (2017): eaai9067; S. L. Kuhn, P. J. Brantingham, and K. W. Kerry, “The Early Upper Paleolithic and the Origins of Modern Human Behavior,” in The Early Upper Paleolithic beyond Western Europe, ed. S. L. Kuhn, P. J. Brantingham, and K. W. Kerry (Berkeley: University of California Press, 2004), 242–248, and chapters throughout this volume; Stephen Shennan, “Demography and Cultural Innovation: A Model and its Implications for the Emergence of Modern Culture,” Cambridge Archaeological Journal 11 (2001): 5–16.

(28.) Stefano Benazzi et al., “Early Dispersal of Modern Humans in Europe and Implications for Neanderthal Behaviour,” Nature 479 (2011): 525–529; John F. Hoffecker et al., “Kostenki 1 and the Early Upper Paleolithic of Eastern Europe,” Journal of Archaeological Science: Reports 5 (2016): 307–326; Ted Goebel, “The Overland Dispersal of Modern Humans to Eastern Asia: An Alternative Northern Route,” in Emergence and Diversity of Modern Human Behavior in Paleolithic Asia, ed. Yousuke Kaifu et al. (College Station: Texas A & M University Press, 2015), 437–452.

(29.) Natalia B. Leonova, “The Upper Paleolithic of the Russian Steppe Zone,” Journal of World Prehistory 8 (1994): 169–210, see: 206.

(30.) Goebel, “The Overland Dispersal”; Michelle Glantz, “How to Survive the Glacial Apocalypse: Hominin Mobility Strategies in Late Pleistocene Central Asia,” Quaternary International 466 (2018): 82–92.

(31.) David R. Harris, Origins of Agriculture in Western Central Asia: An Environmental-Archaeological Study (Philadelphia: University of Pennsylvania Press, 2010), 73–91; Kourosh Roustaei, “An Emerging Picture of the Neolithic of Northeast Iran,” Iranica Antiqua 51 (2016): 21–55. Harris, Origins of Agriculture, 233–236, suggests a migration following the 6200 bce climate crisis in the Levant, but this appears to be contradicted by genetic analysis: Farnaz Broushaki et al., “Early Neolithic Genomes from the Eastern Fertile Crescent,” Science 353 (2016): 499–503; Narasimhan et al., “The Formation of Human Populations.”

(32.) Liu Li and Xingcan Chen, The Archaeology of China: From the Late Paleolithic to the Early Bronze Age (New York: Cambridge University Press, 2012), 82–85; Naomi F. Miller et al., “Millet Cultivation across Eurasia: Origins, Spread, and the Influence of Seasonal Climate,” Holocene 26 (2016): 1566–1575; Guanghui Dong et al., “Prehistoric Trans-Continental Cultural Exchange in the Hexi Corridor, Northwest China,” Holocene 28 (2018): 621–628.

(33.) See citations in Note 22.

(34.) Evgenij N. Chernukh, “Metallurgical Provinces of Eurasia in the Early Metal Age: Problems of Interrelation,” ISIJ International 54 (2014): 1002–1009.

(35.) Kohl, The Making of Bronze Age Eurasia, 182–241; Anthony, The Horse, the Wheel, and Language, 421–427; V. M. Masson, “The Bronze Age in Khorasan and Transoxania,” in History of Civilizations of Central Asia. Vol. 1, The Dawn of Civilization, Earliest Time to 700 B.C. [hereafter HCCA I], ed. A. H. Dani and V. M. Masson (Paris: UNESCO, 1992), 243, 340. For key site descriptions, and a critical overview, see Fredrik T. Hiemert, Origins of Bronze Age Oasis Civilization in Central Asia (Cambridge, MA: Peabody Museum, 1994), esp. 139–178.

(36.) Masson, “The Bronze Age in Khorasan and Transoxania,” 229, 237, 241–242; Harris, Origins of Agriculture, 81–83.

(37.) Feng-Hua Lv et al., “Mitogenomic Meta-Analysis Identifies Two Phases of Migration in the History of Eastern Eurasian Sheep,” Molecular Biology 32 (2015): 2515–2533; Frachetti, “Migration Concepts,” 11–17.

(38.) Xinyi Liu et al., “Journey to the East: Diverse Routes and Variable Flowering Times for Wheat and Barley en Route to Prehistoric China,” PloS ONE 12 (2017): e0187405; Huw Jones et al., “The Trans-Eurasian Crop Exchange in Prehistory: Discerning Pathways from Barley Phylogeography,” Quaternary International 426 (2016): 26–32; John Dodson et al., “Oldest Directly Dated Remains of Sheep in China,” Scientific Reports 4 (2014): 1–4; Miller et al., “Millet Cultivation”; Meng Ren, “The Origins of Cannabis Smoking: Chemical Residue Evidence from the First Millennium BCE in the Pamirs,” Science Advances 5 (2019): eaaw1391; Dong et al., “Prehistoric Trans-Continental Cultural Exchange”; Spengler, “Agriculture in the Central Asian Bronze Age”; Robert N. Spengler et al., “The Spread of Agriculture into Northern Central Asia: Timing, Pathways, and Environmental Feedbacks,” Holocene 26 (2016): 1527–1540; Chris J. Stevens et al., “Between China and South Asia: A Middle Asian Corridor of Crop Dispersal and Agricultural Innovation in the Bronze Age,” Holocene 26 (2016): 1541–1555; Michael D. Frachetti et al., “Nomadic Ecology Shaped the Highland Geography of Asia’s Silk Roads,” Nature 543 (2017): 193–198; Jade d’Alpolm Guedes and R. Kyle Bocinsky, “Climate Change Stimulated Agricultural Innovation and Exchange across Asia,” Science Advances 4 (2018): eear4491.

(39.) R. L. Thorp, China in the Early Bronze Age: Shang Civilization (Philadelphia: University of Pennsylvania Press, 2005); John Dodson, “Early Bronze in Two Holocene Archaeological Sites in Gansu, NW China,” Quaternary Research 72 (2009): 309–314; Jessica Rawson, “China and the Steppe: Reception and Resistance,” Antiquity 91 (2017): 375–388; Jessica Rawson, “Shimao and Erlitou: New Perspectives on the Origins of the Bronze Industry in Central China,” Antiquity 91, no. 355 (2017): e5; C. Zhang et al., “China’s Major Late Neolithic Centres and the Rise of Erlitou,” Antiquity 93 (2019): 588–603.

(40.) See critiques in Craig Benjamin, Empires of Ancient Eurasia: The First Silk Roads Era, 100BCE–250CE (New York: Cambridge University Press, 2018), 91–117, and Armin Selbitschka, “The Early Silk Road(s),” in Oxford Research Encyclopedias. Asian History, ed. David Ludden, 1–25.

(41.) David A. Warburton, “What Might the Bronze Age World-System Look Like?” in Interweaving Worlds: Systemic Interactions in Eurasia, 7th to 1st Millennia BC, ed. Toby C. Wilkinson, Susan Sherratt, and John Bennet (Oakville, CT: Oxbow Books, 2011), 120–134; Elena E. Kuzmina, The Prehistory of the Silk Road (Philadelphia: University of Pennsylvania Press, 2008), 71–87; Y. Majidzadeh, “Lapis Lazuli and the Great Khorasan Road,” Paléorient 8 (1982): 59–69; Evgeniĭ Nikolaevich Chernykh, Ancient Metallurgy in the USSR: The Early Metal Age (New York: Cambridge University Press, 1992), 172–189.

(42.) Kwang-tzuu Chen and Fredrik T. Hiebert, “The Late Prehistory of Xinjiang in Relation to its Neighbors,” Journal of World Prehistory 9 (1995): 243–300; Tomas Larsen Høisæter, “Polities and Nomads: The Emergence of the Silk Road Exchange in the Tarim Basin Region during Late Prehistory (2000–400 BCE),” Bulletin of SOAS 80, no. 2 (2017): 339–363.

(43.) Valerie Hansen, The Silk Road: A New History with Documents (New York: Oxford University Press, 2017), 158–159; Benjamin, Empires of Ancient Eurasia, 251–253; Étienne de la Vaissière, “Trans-Asian Trade, or the Silk Road Deconstructed (Antiquity, Middle Ages),” in The Cambridge History of Capitalism, ed. Larry Neal and Jeffrey G. Williamson (Cambridge: Cambridge University Press, 2014), 101–124.

(44.) Colin McEvedy and Richard Jones, Atlas of World Population History (New York: Penguin, 1978), 152–174; Taylor R. Hermes et al., “Urban and Nomadic Isotopic Niches Reveal Dietary Connectivities along Central Asia’s Silk Roads,” Scientific Reports 8 (2018): 5177.

(45.) Boomer et al., “Advances in Understanding,” 1264; P. M. Dolukhanov, “Ecological Prerequisites for Early Farming in Southern Turkmenistan,” in Philip L. Kohl, ed., The Bronze Age Civilization of Central Asia: Recent Soviet Discoveries (Armonk, NY: Sharpe, 1981), esp. 275–382.

(46.) Ian Boomer et al., “The Palaeolimnology of the Aral Sea: A Review,” Quaternary Science Reviews 19 (2000): 1259–1278; Hedi Oberhänsli et al., “Climate Variability during the Past 2,000 Years and Past Economic and Irrigation Activities in the Aral Sea Basin,” Irrigation and Drainage Systems 21 (2007): 167–183.

(47.) Høisæter, “Polities and Nomads,” 353; James A. Millward, “Toward a Xinjiang Environmental History: Evidence from Space, the Ground, and in Between,” in Studies on Xinjiang Historical Sources in 17–20th Centuries, ed. James A Millward; Yasushi Shinmen; Jun Sugawara (Tokyo: Toyo Bunko, 2010), 279–303; Steffen Mischke et al., “The World’s Earliest Aral-Sea Type Disaster: The Decline of the Loulan Kingdom in the Tarim Basin,” Nature Scientific Reports 7 (2017): 43102; Ma Yong and Sun Yutang, “The Western Regions under the Hsiung-nu and the Han,” in History of Civilizations of Central Asia, Vol. 2, The Development of Sedentary and Nomadic Civilizations, 700 B.C. to A.D. 250, ed. János Harmatta, B. N. Puri, and G. F. Etemadi (Paris: UNESCO, 1994), 238–246. For a contrasting view, see Zhi Li et al., “Drought Promoted the Disappearance of Civilizations along the Ancient Silk Road,” Environmental Earth Science 75 (2016): 1116.

(48.) Anthony, The Horse, the Wheel, and Language, 134–192.

(49.) Anthony, The Horse, the Wheel, and Language, 225–236, 254.

(50.) Anthony, The Horse, the Wheel, and Language, 220–221, 237–249, 258–262, 273–275; Alan K. Outram et al., “The Earliest Horse Harnessing and Milking,” Science 323 (2009): 1332–1335.

(51.) See Marsha Levine, “The Origins of Horse Husbandry on the Eurasian Steppe,” in Late Prehistoric Exploitation of the Eurasian Steppe, ed. Marsha Levine et al. (Cambridge, U.K.: Oxbow, 1999), 5–58. See overviews in Kohl, The Making of Bronze Age Eurasia, 137–144; Pita Kelekna, The Horse in Human History (New York: Cambridge University Press, 2009), 21–66; and the debate in Levine, Renfrew, and Boyle, eds., Prehistoric Steppe Adaptation.

(52.) Anthony, The Horse, the Wheel, and Language, 307–311; compare with Frachetti, “The Multi-Regional Emergence,” esp. 339; Damgaard, “The First Horse Herders.”

(53.) Kohl, The Making of Bronze Age Eurasia, 145; Anthony, The Horse, the Wheel, and Language, 300–339; Elena E. Kuzmina, “Origins of Pastoralism in the Eurasian Steppes,” in Prehistoric Steppe Adaptation, ed. Levine, Renfrew, and Boyle, 203–232; Kateryna P. Bunyatyan, “Correlations between Agriculture and Pastoralism in the North Pontic Steppe Area,” in Prehistoric Steppe Adaptation, ed. Levine, Renfrew, and Boyle, 275–276.

(54.) Kuzmina, “Origins of Pastoralism”; Andrew Sherrat, “The Horse and the Wheel: The Dialectics of Change in the Circum-Pontic Region,” in Prehistoric Steppe Adaptation, ed. Levine, Renfrew, and Boyle, 233–252; Anthony, The Horse, the Wheel, and Language, 282–285; Chernykh, Ancient Metallurgy in the USSR, 48–171; Chernukh, “Metallurgical Provinces of Eurasia.”

(55.) Kohl, The Making of Bronze Age Eurasia, 72–86, 164–166; Anthony, The Horse, the Wheel, and Language, 282–295; Chernykh, Ancient Metallurgy in the USSR, 67–83.

(56.) Hanks and Doonan, “From Scale to Practice”; Anthony, The Horse, the Wheel, and Language, 371–405, 435–457; Kohl, The Making of Bronze Age Eurasia, 126–158; Chernykh, Ancient Metallurgy in the USSR, 190–215.

(57.) Kuzmina, Prehistory of the Silk Road, 34–70; Frachetti, Pastoralist Landscapes, 48–49.

(58.) Anthony, The Horse, the Wheel, and Language, 456–457; quote from Kohl, The Making of Bronze Age Eurasia, 169.

(59.) Kohl, The Making of Bronze Age Eurasia, 150, 152, 177–178.

(60.) Kremenetski, “Steppe and Forest–Steppe Belt of Eurasia,” 23–25; Kremenetski, Chichagova, and Shishlina, “Palaeoecological Evidence,” 244; Dodson, “Early Bronze,” at 314. Victor H. Mair, “The Horse in Late Prehistoric China: Wrestling Culture and Control from the ‘Barbarians’,” in Prehistoric Steppe Adaptation, ed. Levine, Renfrew, and Boyle, 163–187.

(61.) For reviews of methods and approach, see Ashot Margaryan et al., “Ancient Pathogen DNA in Human Teeth and Petrous Bones,” Ecology and Evolution 8 (2018): 3534–3542; and Cheryl P. Andam et al., “Microbial Genomics of Ancient Plagues and Outbreaks,” Trends in Microbiology 24 (2016): 978–990.

(62.) I. Lazaridis et al., “Genomic Insights into the Origin of Farming in the Ancient Near East,” Nature 536 (2016): 419–424; Z. Hofmanová et al., “Early Farmers from across Europe Directly Descended from Neolithic Aegeans,” Proceedings of the National Academy of Sciences of the United States of America 113 (2016): 6886–6891.

(63.) Allentoft et al., “Population Genomics of Bronze Age Eurasia”; Wolfgang Haak et al., “Massive Migration from the Steppe was a Source for Indo-European Languages in Europe,” Nature 522 (2015): 207–211; Inigo Olalde et al., “The Beaker Phenomenon and the Genomic Transformation of Northwest Europe,” Nature 555 (2018): 190–196; Damgaard et al., “137 Ancient Human Genomes”; Damgaard, “First Horse Herders”; but see Martin Furholt, “Massive Migrations? The Impact of Recent aDNA Studies on our View of Third Millennium Europe,” European Journal of Archaeology 21 (2018): 159–191.

(64.) Narasimhan et al., “The Formation of Human Populations”; Damgaard, “First Horse Herders”; Kristen Khristiansen, “Bridging India and Scandinavia: Institutional Transmission and Elite Conquest during the Bronze Age,” in Interweaving Worlds, ed. Wilkinson, Sherratt, and Bennet, 243–265.

(65.) Anthony, The Horse, the Wheel, and Language, 427, 450–451; Masson, “The Bronze Age in Khorasan and Transoxania”; V. M. Masson, “The Decline of Bronze Age Civilization and Movements of the Tribes,” J. Harmatta, “The Emergence of the Indo-Iranians: The Indo-Iranian Languages,” and B. A. Litvinsky and L. T. P’yankova, “Pastoral Tribes of the Bronze Age in the Oxus Valley (Bactria),” in HCCA I, 347–350; 352, 370, 388; Masson: 247–350, 352; Harmatta, 370; Litvinsky, 388; C. C. Lamberg-Karlovsky, “Archaeology and Language: The IndoIranians,” Current Anthropology 43 (2002): 63–88; Julie Di Cristofaro et al. “Afghan Hindu Kush: Where Eurasian Sub-Continent Gene Flows Converge,” PLoS ONE 8 (2013): e76748.

(66.) J. P. Mallory and V. H. Mair, The Tarim Mummies (London: Thames & Hudson, 2000); Qidi Feng et al., “Genetic History of Xinjiang’s Uyghurs Suggests Bronze Age Multiple-Way Contacts in Eurasia,” Molecular Biology and Evolution 34 (2017): 2572–2582.

(67.) Compare Anthony, The Horse, the Wheel, and Language, with Lamberg-Karlovsky, “Archaeology and Language.” Here it should be noted that we are avoiding discussion of Indo-European language dispersal and its vast literature. For a short recent overview, see Paul Heggarty, “Europe and Western Asia: Indo-European Linguistic History,” in The Encyclopedia of Global Human Migration, ed. Immanuel Ness (Oxford and Malden, MA: Blackwell, 2013).

(68.) M. Drancourt and D. Raoult, “Molecular History of Plague,” Clinical Microbiology and Infection 22 (2016): 911–915.

(69.) Yujun Cui et al., “Historical Variations in Mutation Rate in an Epidemic Pathogen, Yersinia Pestis,” Proceedings of the National Academy of Sciences 110 (2013): 577–582.

(71.) Chen et al., “A Persistent Holocene Wetting Trend,” 139.

(72.) Rasmussen et al., “Early Divergent Strains”; Valtueña et al., “The Stone Age Plague.”

(73.) Nicolás Rascovan et al., “Emergence and Spread of Basal Lineages of Yersinia Pestis during the Neolithic Decline,” Cell 176 (2019): 295–305.

(74.) Anthony, The Horse, the Wheel, and Language, 225–370; Allentoft et al., “Population Genomics of Bronze Age Eurasia”; Haak et al., “Massive Migration”; L. Cassidy et al., “Neolithic and Bronze Age Migration to Ireland and Establishment of the Insular Atlantic Genome,” Proceedings of the National Academy of Sciences 113 (2016): 368–373.

(75.) Kristian Kristiansen et al., “Re-theorising Mobility and the Formation of Culture and Language among the Corded Ware Culture in Europe,” Antiquity 91 (2017): 334–347, at 335; Volker Heyd, “Kossina’s Smile,” Antiquity 91 (2017): 348–359, at 249; Furholt, “Massive Migrations?” 168.

(76.) Maria A. Spyrou et al. “Analysis of 3800-Year-Old Yersinia Pestis Genomes Suggests Bronze Age Origin for Bubonic Plague,” Nature Communications 9 (2018): 2234.

(77.) Damgaard et al., “137 Ancient Human Genomes,” 372–373; Galina A. Eroshenko et al., “Yersinia Pestis Strains of Ancient Phylogenetic Branch 0.ANT are Widely Spread in the High-Mountain Plague Foci of Kyrgyzstan,” PLoS ONE 12 (2017): e0187230.

(78.) Rasmussen et al., “Early Divergent Strains.”

(79.) Masson, “The Bronze Age in Khorasan and Transoxania”; Masson, “The Decline”; Harmatta, “The Emergence”; Litvinsky and P’yankova, “Pastoral Tribes,” in HCCA I, esp. 243–244, 344–345, 347, 348, 349, 350, 356, 374–376; Anthony, The Horse, the Wheel, and Language, 427, 450–451; Lamberg-Karlovsky, “Language and Archeology,” 71–73; Leonid T. Yablosky, “Some Ethnogenetical Hypotheses,” in Nomads of the Eurasian Steppes in the Early Iron Age, ed. Jeannine Kimball-Davis et al. (Berkeley, CA: Zinat Press, 1995), 242; Aska Parpola, The Roots of Hinduism: The Early Aryans and the Indus Civilization (New York: Oxford University Press, 2015), 35–51; Di Cristofaro et al., “Afghan Hindu Kush”; Narasimhan et al., “The Formation of Human Populations.”

(81.) Rawson, “China and the Steppe”; Donald B. Wagner, Iron and Steel in Ancient China (Leiden, The Netherlands: Brill, 1993), 97–146; Bennet Bronson, “The Transition to Iron in Ancient China,” in The Archaeometallurgy of the Asian Old World, ed. Vincent C. Pigott (Philadelphia: University Museum, University of Pennsylvania, 1999), 177–198; generally, see Theodore A. Wertime and James D. Muhly, eds., The Coming of the Age of Iron (New Haven, CT: Yale University Press, 1980). Bloomeries were the first iron-producing system, universally used from c1200 bce until the development of the blast furnace in the Middle Ages. Built of clay and stone, bloomeries were small structures that after firing would broken apart to extract the “bloom” of iron and slag, which in turn would be purified into wrought-iron by hammering.

(82.) The earliest compound bow is dated to 1900–1750 bce, in the Volga steppe, but the form was not perfected and adapted to mass warfare until the Early Iron Age. Anthony, The Horse, the Wheel, and Language, 222–224, n39.

(83.) Di Cosmo, Ancient China and its Enemies, 35–37; Damgaard, “First Horse Herders.” Ludmila Koryakova and Andrej V. Epimakhov, The Urals and Western Siberia in the Bronze and Iron Ages (New York: Cambridge University Press, 2007), 220–250.

(84.) Here see the essays in Denis Sinor, ed., The Cambridge History of Early Inner Asia (New York: Cambridge University Press, 1990); Nicola Di Cosmo, Allen J. Frank, and Peter B. Golden, eds., The Cambridge History of Inner Asia: The Chinggisid Age (New York: Cambridge University Press, 2009); and David Christian, History of Russia, Central Asia and Mongolia (Oxford: Blackwell, 1998).

(85.) Nicola Di Cosmo, “Ancient Inner Asian Nomads: Their Economic Basis and its Significance in Chinese History,” Journal of Asian Studies 53 (1994): 1092–1126; Frachetti, Pastoral Landscapes, 141–142; Arlene Miller Rosen et al., “Palaeoenvironments and Economy of Iron Age Saka-Wusun Agro-Pastoralists in Southeastern Kazakhstan,” Antiquity 74 (2000): 611–623; Claudia Chang and Perry A. Tourtellotte, “The Role of Agro-Pastoralism in the Evolution of the Steppe Culture of the Semirechye Area of Southern Kazakhstan during the Saka/Wusan Period (600BCE–400CE),” in The Bronze Age and Early Iron Age Peoples of Eastern Central Asia, ed. Victor H. Mair (Washington, DC: Institute for the Study of Man, 1998), 264–279; Mark G. Macklin et al., “The Influence of Late Pleistocene Geomorphological Inheritance and Holocene Hydromorphic Regimes on Floodwater Farming in the Talgar Catchment, Southeast Kazakhstan, Central Asia,” Quaternary Science Reviews 129 (2015): 85–95.

(86.) Gayatri Kathayat et al., “The Indian Monsoon Variability and Civilization Changes in the Indian Subcontinent,” Science Advances 3 (2017): e1701296; Ashish Sinha et al., “The Leading Mode of Indian Summer Monsoon Precipitation Variability during the Last Millennium,” Geophysical Research Letters 38 (2011): L15703; Chen et al., “Moisture Changes”; Zhao et al., “Climatic Variations over the Last 4000 Cal Yr BP”; Aichner et al., “High-Resolution Leaf Wax Carbon”; Wei Zhong et al., “Climatic Change during the Last 4000 Years in the Southern Tarim Basin, Xinjiang, Northwest China,” Journal of Quaternary Science 22 (2007): 659–665 (see also Millward, “Toward a Xinjiang Environmental History,” 284); Lonnie G. Thompson et al., “Ice Core Records of Climate Variability on the Third Pole with Emphasis on the Guliya Ice Cap, Western Kunlun Mountains,” Quaternary Science Reviews 188 (2018): 1–14; Liu, “Wet Climate”; Nicola Di Cosmo et al., “Environmental Stress and Steppe Nomads: Rethinking the History of the Uyghur Empire (744–840) with Paleoclimate Data,” Journal of Interdisciplinary History 48 (2018): 439–463, see Fig. 6a.

(87.) Sudden climate disruptions, perhaps intensifying preexisting stresses shaped by ongoing solar minima, seem to have played a role in both steppe state emergence and collapse. Volcanic eruptions, perhaps working with outbursts of the Siberian High, played a critical role at various junctures. In particular, the emergence of steppe states seems to have come during the upswing of Mongolia precipitation and invasions of China during the continued downswing of conditions across China.

(88.) S. Begzsuren et al., “Livestock Responses to Droughts and Severe Winter Weather in the Gobi Three Beauty National Park, Mongolia,” Journal of Arid Environments 59 (2004): 785–796; Cohen, “The Role of the Siberian High”; P. A. Mayewski et al., “Major Features and Forcing of High-Latitude Northern Hemisphere Atmospheric Circulation using a 110,000-Year-Long Glaciochemical Series,” Journal of Geophysical Research 102 (1997): 26345–26366. The Siberian High measure does not, however, align obviously with the second Greenland Ice Sheet Project (GISP2) volcanic data, as re-dated by Sigl (private communication). Compare with the tree-ring volcanic alignment in Nicola Di Cosmo et al., “Interplay of Environmental and Socio-Political Factors in the Downfall of the Eastern Türk Empire in 630 CE”, Climatic Change (2017): 383–395.

(89.) B. van Geel et al., “Climate Change and the Expansion of the Scythian Culture after 850 BC: A Hypothesis,” Journal of Archaeological Science 31 (2004): 1735–1742; Blyakharchuk et al., “High Resolution Palaeoecological Records.”

(90.) See Koryakova and Epimakhov, The Urals and Western Siberia, 211–212, discussing the work of G. N. Kurochkin and N. A. Gavriluk.

(91.) Nicola Di Cosmo, “State Formation and Periodization in Inner Asian History,” Journal of World History 10 (1999): 1–40; for a wider understanding of stalemates and ruptures in historical time, see William H. Sewell, Logics of History: Social Theory and Social Transformation (Chicago: University of Chicago Press, 2005).

(92.) Di Cosmo, “State Formation,” 15–26. Looking at the three great climatic downturns of the past 6,000 years, Brooke has suggested a similar interpretation of innovation as a response to stress: for quick summaries, see Brooke, Climate Change, 278–279, 285–287. On cycles of climate and resource stress and violence more widely, see David G. Anderson et al., “Paleoclimate and the Potential Food Reserves of Mississippian Societies: A Case Study from the Savannah River Valley,” American Antiquity 60 (1995): 258–286; David D. Zhang et al., “Global Climate Change, War, and Population Decline in Recent Human History,” Proceedings of the National Academy of Sciences 104 (2007): 19214–19219; Zhibin Zhang et al., “Periodic Climate Cooling Enhanced Natural Disasters and Wars in China during AD 10–1900,” Proceedings of the Royal Society B (2010): 3745–3753; Qing Pei et al., “Long-Term Association between Climate Change and Agriculturalists’ Migration in Historical China,” Holocene 28 (2018): 208–216; and the essays in Richard J. Chaconand and Rubén G. Mendoza, eds., Feast, Famine or Fighting? Multiple Pathways to Social Complexity (Cham, Switzerland: Springer, 2017).

(93.) Di Cosmo et al., “Interplay of Environmental and Socio-Political Factors”; Di Cosmo et al., “Environmental Stress and Steppe Nomads.” See also Rustam Ganiev and Vladimir Kukarskih, “Climate Extremes and the Eastern Turkic Empire in Central Asia,” Climatic Change 149 (2018): 385–397; and Chao Zhang and Qi-Bin Zhang, “Is there a Link between the Rise and Fall of the Tuyuhun Tribe (Northwestern China) and Climatic Variations in the Fourth–7th Centuries AD?” Journal of Arid Environments 125 (2016): 145–150.

(94.) Neil Pederson et al., “Pluvials, Droughts, the Mongol Empire, and Modern Mongolia,” Proceedings of the National Academy of Sciences 111 (2014): 4375–4379; Amy E. Hessl et al., “Past and Future Drought in Mongolia,” Science Advances 4 (2018): e1701832; see also Aaron E. Putnam et al., “Little Ice Age Wetting of Interior Asian Deserts and the Rise of the Mongol Empire,” Quaternary Science Reviews 131 (2016): 33–50; Chen et al., “Moisture Changes.”

(95.) Timothy Brook, The Troubled Empire: China in the Yuan and Ming Dynasties (Cambridge, MA: Belknap Press of Harvard University Press, 2010); Geoffrey Parker, The Global Crisis: War, Climate and Catastrophe in the Seventeenth-Century World (New Haven, CT: Yale University Press, 2013); Brooke, Climate Change, 444–446.

(96.) Cui et al., “Historical Variations”; Rasmussen et al., “Early Divergent Strains”; Damgaard et al., “137 Ancient Human Genomes,” 372–373; Uli Schamiloglu, “The Plague in the Time of Justinian and Central Asian History: An Agenda for Research,” in Central Eurasia in the Middle Ages: Studies in Honour of Peter B. Golden, ed. István Zimonyi and Osman Karatay (Wiesbaden, Germany: Harrassowitz Verlag, 2016), 294–311; Kyle Harper, The Fate of Rome: Climate, Disease, and the End of an Empire (Princeton, NJ: Princeton University Press, 2017), 215–235, 249–259; Keller et al., “Ancient Yersinia Pestis Genomes”; Ulf Büntgen et al., “Cooling and Societal Change during the Late Antique Little Ice Age from 536 to around 660 AD,” Nature Geoscience 9 (2016): 231–236.

(97.) The conclusions of Nils C. Stenseth et al., “Plague Dynamics are Driven by Climate Variation,” Proceedings of the National Academy of Sciences 103 (2006): 13110–13115 might be somewhat complicated if evidence for drier conditions north of the Tian Shan in the Little Ice Age develops further. See Jens Fohlmeister et al., “Winter Precipitation Changes during the Medieval Climate Anomaly and the Little Ice Age in Arid Central Asia,” Quaternary Science Reviews 178 (2017): 24–36.

(98.) Uli Schamiloglu, “The Impact of the Black Death on the Golden Horde: Politics, Economy, Society, Civilization,” in The Golden Horde in World History, ed. Marie Favereau-Doumenjou (Kazan, Russia: Sh. Marjani Institute, 2017), 674–685; Michael Dols,The Black Death in the Middle East (Princeton, NJ: Princeton University Press, 1977), 36. Philip Slavin, “Death by the Lake: Mortality Crisis in Early Fourteenth-Century Central Asia,” Journal of Interdisciplinary History 50 (2019): 59–90, provides a careful assessment of the historical and paleogenetic context of the Issyk-Kul epidemic.

(99.) Ole J. Benedoctow, “Yersinia Pestis, the Bacterium of Plague, Arose in East Asia: Did it Spread Westwards via the Silk Roads, the Chinese Maritime Expeditions of Zheng He or over the Vast Eurasian Populations of Sylvatic (Wild) Rodents?” Journal of Asian History 47 (2013): 1–31; Cui et al., “Historical Variations,” 578; on the “Land of Darkness” see Amine Namouchi et al., “Integrative Approach Using Yersinia Pestis Genomes to Revisit the Historical Landscape of Plague during the Medieval Period,” Proceedings of the National Academy of Sciences 115 (2019): E11795; Slavin, “Death by the Lake,” 86–87.

(100.) Robert Hymes, “Epilogue: A Hypothesis on the East Asian Beginning of the Yersinia Pestis Polytomy,” Medieval Globe 1 (2014): 285–308.

(101.) Here and below we are indebted to the editor and authors in Nicholas B. Breyfogle, ed., Eurasian Environments: Nature and Ecology in Imperial Russian and Soviet History (Pittsburgh, PA: University of Pittsburgh Press, 2018), and for John Brooke’s opportunity to comment at the conference leading this volume.

(102.) Schamiloglu, “The Impact of the Black Death on the Golden Horde,” 674–685.

(103.) Parker, Global Crisis; Levi, The Bukharan Crisis; Chen et al., “Moisture Changes”; Mayewski et al., “Major Features and Forcing.”

(104.) Douglas E. Streusand, Islamic Gunpowder Empires: Ottomans, Safavids, and Mughals (Boulder, CO: Westview Press, 2011); Beatrice Forbes Manz, The Rise and Rule of Tamerlane (New York: Cambridge University Press, 1989); Di Cosmo, “State Formation.”

(105.) Scott C. Levi, “Asia in the Gunpowder Revolution,” in Oxford Research Encyclopedias. Asian History. ed. David Ludden, 1–31. (April 2018).

(106.) Peter C. Purdue, China Marches West: The Qing conquest of Central Eurasia (Cambridge, MA: Belknap Press of Harvard University Press, 2005), 94–107, 138–173, 174–292.

(107.) David A. Bello, Across Forest, Steppe, and Mountain: Environment, Identity, and Empire in Qing China’s Borderlands (New York: Cambridge University Press, 2016); David A. Bello, “Relieving Mongols of their Pastoral Identity: Disaster Management on the Eighteenth-Century Qing China Steppe,” Environmental History 19 (2014): 480–504.

(108.) D. R. Weiner, “The Predatory Tribute-Taking State: A Framework for Understanding Russian Environmental History,” in The Environment and World History, ed. Edmund Burke III and Kenneth Pomeranz (Berkeley: University of California Press, 2009), 276–315.

(109.) Alexander Morrison, Russian Rule in Samarkand 1868–1910: A Comparison with British India (Oxford: Oxford University Press, 2008), 201–243.

(110.) On neo-Europe: Alfred Crosby, Ecological Imperialism: The Biological Expansion of Europe, 900–1900 (New York: Cambridge University Press, 1986); Alfred J. Rieber, The Struggle for the Eurasian Borderlands: From the Rise of Early Modern Empires to the End of the First World War (New York: Cambridge University Press, 2014); Michael Khodarkovsky, Russia’s Steppe Frontier: The Making of a Colonial Empire, 1500–1800 (Bloomington: Indiana University Press, 2002); Willard Sunderland, Taming the Wild Field: Colonization and Empire on the Russian Steppe (Ithaca, NY: Cornell University Press, 2004).

(111.) Scott Levi, The Rise and Fall of Khoqand, 1709–1876: Central Asia in the Global Age (Pittsburgh, PA: Pittsburgh University Press, 2017); Julia Obertreis, Imperial Desert Dreams: Cotton Growing and Irrigation in Central Asia, 1860–1991 (Göttingen, Germany: V&R unipress, 2017); Daniel Brower, Turkestan and the Fate of the Russian Empire (New York: Routledge Curzon, 2003).

(112.) Sarah Cameron, The Hungry Steppe: Famine, Violence and the Making of Soviet Kazakhstan (Ithaca, NY: Cornell University Press, 2018); Gulnar Kendirbai, Land and People: The Russian Colonization of the Kazak Steppe (Berlin: Klaus Schwarz Verlag, 2002); David Moon, The Plough that Broke the Steppes: Agriculture and Environment on Russia’s Grasslands, 1700–1914 (Oxford: Oxford University Press, 2013); Nicholas B. Breyfogle, Abby Schrader, and Willard Sunderland, eds., Peopling the Russian Periphery: Borderland Colonization in Eurasian History (London and New York: Routledge, 2007).

(113.) Ian Blanchard, Russia’s “Age of Silver”: Precious-Metal Production and Economic Growth in the Eighteenth Century (London: Routledge, 1989); Malcolm R. Hill, “Russian Iron Production in the Eighteenth Century,” Icon 12 (2006): 118–167; Malcolm R. Hill, “Russian Iron and Steel Production from 1800–1860,” Icon 20 (2014): 125–150; Malcolm R. Hill, “Russian Iron Production from the Repeal of Serfdom to the First World War,” Icon 22 (2016): 115–138; Michael Williams, Deforesting the Earth: From Prehistory to Global Crisis (Chicago: University of Chicago Press, 2003), 186–193, 285–291; Daniel Yergin, The Prize: The Epic Quest for Oil, Money, and Power (New York: Simon & Schuster, 1991), 57–63, 131–133, 237–243.

(114.) Douglas R. Weiner, Models of Nature: Ecology, Conservation, and Cultural Revolution in Soviet Russia (Bloomington: Indiana University Press, 1988); Paul Josephson, “Industrial Deserts: Industry, Science and the Destruction of Nature in the Soviet Union,” Slavonic and East European Review 85 (2007): 294–321.

(115.) Scott C. Levi, The Indian Diaspora in Central Asia and its Trade, 1550–1900 (Leiden, The Netherlands: E. J. Brill, 2002).

(116.) Andrew Maddocks, Robert Samuel Yung, and Paul Reig, Ranking the World’s Most Water-Stressed Countries in 2040, World Resources Institute Report, August 26, 2015.