Afroeurasia in Geological Time¹

David Christian
San Diego State University

"Live in London for a year and you will not learn much about England, but you will learn a lot about France: you see because you have distanced yourself."²

Geography and the longue durée

     As any photographer knows, photographs show what your lens allows them to show. The great French historian, Fernand Braudel, famously argued that different scales reveal different aspects of the past. Though close-ups are revealing, you can also learn a lot by standing way back. This means that the choice of scale is a matter of strategic importance for the historian. In this essay, I'd like to suggest what we can learn about Eurasia and Afroeurasia by using the widest possible lenses.³

     In two immensely influential studies of the early modern world, Braudel distinguished between three distinct scales.⁴ The smallest focused on particular events and individual actors. He called it the scale of "events" or "individual" time. This has been the main for research in political history, and it has dominated modern historical scholarship and teaching. It is the scale best illuminated by official archives, and the scale at which historians usually feel most comfortable . (It is also the scale at which they get to meet the famous and powerful. Working as a doctoral student in the state archives in St. Petersburg/Leningrad in 1973, I read a project of political reform dating from the early 1790s. Its handwritten pages were held together by a beautiful pearl-headed pin. Who else but Catherine the Great could have put it there? I've always imagined her reaching into her hair for a pin to hold the pages together, and forgetting to retrieve it when she'd finished.) However, as Braudel argued, an exaggerated focus on the scale of events can deprive history of much of its meaning, for individual events can easily obscure their own context. If geography, the environment, lifeways, demography, large social and economic cycles, eating habits, and basic technologies are treated as a sort of static backdrop to history, it is all too easy to suppose that Chinggis Khan personally created the Mongol Empire, or that Catherine the Great personally conquered the last remnants of the Golden Horde in the Crimea in 1783, or that Lenin personally set world history off on a new course in Petrograd (as it was then called) in 1917. Seen through a wider lens, things look very different. Suddenly, all those parts of the past that had seemed fixed, begin to move too, and eventually we may realize that the frame itself is moving.

     Braudel's second scale (which he sometimes described as "social" time) highlights large social and economic changes over many decades. At this scale, economies expand and contract, populations grow and shrink, and wealth and power shift from region to region, transforming the international balance of power as they do so. At this scale it is easier to see that individuals are acting within contexts that constrain or empower them. In 1905, Lenin and the Bolsheviks were minor actors in a huge revolutionary crisis. In 1917, the Bolsheviks surfed international waves of change whipped up by world war, industrialization and several decades of European imperialism. Without understanding these huge waves of change, the October Revolution makes little sense. Chinggis Khan could not have conquered such vast territories if he had not been born in a period of competing Chinese dynasties and civil war in the Mongolian steppes. And Catherine the Great could not have conquered the Crimea if she had not faced a weakened Ottoman Empire.

     Then Braudel takes a third step, by moving to what he calls the "longue durée", or "geographical time." This is a scale of many centuries. It is the scale of "material life," and those "structures of everyday life" that changed very slowly between the fifteenth and eighteenth centuries, and set limits to human action: the diets, lifeways and productive methods of the peasants who made up most of society, the methods of transportation, and patterns of demographic growth. "Ever-present, all-pervasive, repetitive, material life is run according to routine: people go on sowing wheat as they always have done, planting maize as they always have done, terracing the paddy-fields as they always have done, sailing in the Red Sea as they always have done. . . .And this layer of stagnant history is enormous: all rural life, that is 80 to 90% of the world's population, belongs to it for the most part."⁵ Over the four centuries covered in Braudel's two major books, little changed at this level. But widen the lens just slightly and change re-appears even at this scale, for today, the material world Braudel described so well has largely vanished.

     It is quite easy to imagine being transported to, say, Voltaire's house at Ferney, and talking to him for a long time without being too surprised. . . .But if the patriarch of Ferney invited us to stay with him for a few days, the details of his everyday life, even the way he looked after himself, would greatly shock us. Between his world and ours, a great gulf would open up: lighting at night, heating, transport, food, illness, medicine.⁶

     At the outer limits of Braudel's longue durée, even the environment begins to shift. Weather patterns and sea currents alter, driving Viking colonists from Greenland, creating droughts and famines in Russia, or building and tearing down entire civilizations in Peru as El Niño currents alternate with La Niñas. Widen the lens even more and glaciers expand and contract, the Sahara is transformed from a fertile savanna into an arid desert, and estuaries silt up leaving the ancient port cities of Sumer high and dry. Geography and the biosphere cease to be merely the backdrop to human history; they, too, become historical actors.

     This is why world history and environmental history are such natural allies. While history at the scale of events is all about people and societies, what comes into view at the larger scales of world history is the complex and endlessly fascinating relationship between human beings, geography, and the biosphere.

Creating Afroeurasia

     By Afroeurasia, we mean the huge, connected landmass that includes both Africa and Eurasia. To understand the significance of Afroeurasia in world history, we need the widest possible lens. Indeed, we need to go further than Braudel and shift to a fourth scale, that of geological time. At this scale, Afroeurasia appears not just as a stage on which human history has been played out, but as a large structure with a history of its own, and a history which, like human history, might have turned out differently. Afroeurasia's history is dominated by the slow movement of tectonic plates across the earth's surface over hundreds of millions of years. Geologists can now track these movements back almost a billion years, though the accuracy of their reconstructions fades as they move back in time. But for the last 500 million years or so they can tell a fascinating story.⁷

     Let's begin about 540 million years ago in the Cambrian era, as the first multi-celled organisms appeared in the oceans and left their shells behind to form the first naked-eye fossils.⁸ The Cambrian world was formed as a result of the slow disintegration of an ancient super-continent now known as "Rodinia." [PowerPoint Slide #2] Most of the world's continental plates were gathered in the southern hemisphere. The plates that will interest us here are those still preserved, more or less intact, within modern Africa, India, China, Europe and Siberia. In the Cambrian period, the African, Indian and Chinese plates were still locked within the fragmenting body of Rodinia. But the European and Siberian plates formed separate small continents at middle latitudes. The Chinese and Siberian plates were both split into two separate plates. To modern eyes, this is an utterly unfamiliar world.

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     300 million years ago, in the Silurian period, trees are spreading on land and vertebrates are thriving at sea. [PowerPoint Slide #3] The European and Siberian plates are traveling north in convoy, and the two Chinese plates have budded off from the remains of Rodinia, which now lie over the southern polar region. 120 million years later, in the Carboniferous period, reptiles are spreading on land, huge forests are beginning to lay down the fossil fuels we burn today, and the remnants of Rodinia have swung north to form a large north-south aligned continent. [PowerPoint Slide #4] To its north, the Siberian plates have joined to form a smaller continent, while the two Chinese plates are almost touching each other off to the east.

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     180 million years ago, in the Jurassic period, the dinosaurs are strutting their stuff on land, and most of the continental plates have fused together again to form a new super-continent known today as Pangaea. [PowerPoint Slide #5] Africa is roughly in its present latitudes, with Antarctica adjoining it to the south, India tacked on to its southeastern corner, and Australia tacked on to India. The American plates are fused to Africa's western coast. The European plate adjoins Africa to the north. To the northeast, the European plate has fused with the Siberian plates, which in turn are fused to the Chinese plates. The Chinese plates form a large, thin peninsula that loops off to the east and south again. In some reconstructions, this peninsula stretches so far south that it nearly touches plates that would later form Southeast Asia and Indonesia, in an arc pointing south towards the Australian plate. This thin strip of land nearly closes off the inner sea, known to geologists as Tethys, after the sea-goddess that Greek mythology portrayed as the mother of the world's great rivers.

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     By 120 million years ago, in the Cretaceous period, Pangaea is beginning to crack and split along several tectonic seams. [PowerPoint Slide #6] A large east-west fissure has developed, dividing the southern continents (joined in a supercontinent that geologists will call "Gondwanaland") from the northern continents of N. America, Europe, Siberia and China (joined in a supercontinent known as "Laurasia"). To the South, Gondwanaland, with its distinctive cargo of marsupials, is splitting as Antarctica, India and Australia tear away. By 60 million years ago, S. America has broken from Africa. To modern eyes, this world is beginning to look familiar. This is when an asteroid landed off the northern coast of Mexico's Yucatan peninsula at a place known today as Chicxulub (cheech-uh-loob). [PowerPoint Slide #7] The explosion and the nuclear winter that followed killed off most large species of animals, including all species of dinosaurs (though many smaller species survived—we know them as birds).⁹ It was in the many niches vacated by the great dinosaurs that our mammal and primate ancestors would flourish in a post-Chicxulub world. One of the geological oddities of this world is that the Indian plate began to move northwards very fast, as if it had had a violent quarrel with its partners in Gondwanaland. Eventually, the Indian plate would collide with the Siberian and Chinese plates in a tectonic crash that would drive up a huge chain of mountains reaching from eastern China to the Caucasus, with the Himalayas at its center. In a similar but less violent crash, Africa will eventually rear-end Europe, creating the Alps. The mountainous east-west aligned skeleton of Eurasia, perhaps its most dominant single geographical structure, is a product of tectonic events within the last 60 million years. [PowerPoint Slide #8]

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A Pangaean World

     What would world history have been like if humans had evolved in a Pangaean world? [Back to PowerPoint Slide #5] This counter-factual exercise can help us see more clearly some of the distinctive features of world geography as we know it today.

     First, we notice that the Pangaean world consists of a single, huge landmass. Almost all of it can be reached by land. At the very worst, it might have required some modest sea-voyages to reach the thin strip of land at the eastern edge of the Tethys sea, or the islands that cross the center of that sea. Second, the Pangaean world was almost certainly warmer than ours, because the distribution of landmasses did nothing to prevent the free flow of sea currents between tropical and polar regions. Presumably, this would have made it easier for humans to migrate from their evolutionary homeland, wherever that was, to most other parts of the world. If recent arguments about the critical impact of warmer and more stable climates on the origins of agriculture are correct, this might also have been a world in which agriculture would have appeared quite early in human history. Human history might have been speeded up, with dense, agrarian communities, cities and states appearing after a relatively short Paleolithic era.¹⁰[PowerPoint Slides #9 and #10]

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     The most striking feature of this Pangean world is its tectonic unity. To clarify the significance of this, it might help to imagine three possibilities. The first is a Pangaean world, in which all landmasses are connected. The second, is a world of extreme dispersion, a vast archipelago, in which humans evolve in one of many quite separate island worlds between which travel is impossible until the evolution of sophisticated sea-going techniques. The third is a world like ours, in which landmasses are separated but contact between them is just possible at a few points, and when conditions are right.

     In a world of absolute dispersion, there would have been hardly any biological links between different regions, and each land mass would have had its own distinct flora and fauna, as Australia does today. In such a world, human history would surely have been confined for long periods to the one island or semi-continent on which humans happened to have evolved. Presumably, this would have limited possibilities for technological and social change, just as it would have limited the biodiversity of each of the world's islands. (However, the world as a whole would surely have contained greater biodiversity than our world, as the flora and fauna of each region would have evolved separately for many millions of years.) Eventually, we can imagine a revolutionary moment of breakthrough when humans developed sea-going technologies that allowed them to leave their homeland and enter many new worlds of whose existence they had no idea before, lands with entirely different plants and animals from those they were used to. In this imaginary history, our species would have exploded on the rest of the world with extraordinary suddenness and violence, creating a sharp divide between two major phases of human history.

     In the opposite scenario of a Pangean world, global expansion would probably have occurred much earlier, so the histories of all human communities would have been unified very early in human history. In some form, "globalization" would have been a reality for most of human history. Because our species evolved in Africa, it is not unreasonable to imagine a Pangaean version of Homo Sapiens appearing right in the middle of Pangaea. From here, they could have spread to most parts of the world by relatively easy steps. Like our own ancestors, they would have encountered a great diversity of landscapes, but if our own ancestors managed to adapt to ice-age Siberia then we can easily imagine Pangaean humans adapting to the cold, but still quite benign environments of the far north and south. We can also imagine them doing some island hopping in the far east, just as Paleolithic humans succeeded in reaching Australia some 60,000 years ago. In a Pangaean world, globalization would have also been a much less startling process as total biodiversity would have been less than in more dispersed worlds, such as our own. This suggests that there would have been no equivalent of the "Columbian exchange" and few great geographical discoveries to be made at a later phase of human history.¹¹

     Our world is between these two extremes. [PowerPoint Slide #11] Its great land masses touch at the extremities, coming just close enough for occasional exchanges of species across the Bering straits or between the Americas. But the links are tenuous, and can easily break. And large parts of our world, including Australia, Antarctica, and the islands of the Pacific, are almost completely out of touch with the major land masses. The result is a world of much greater biodiversity than Pangea, and probably a world of greater social and cultural diversity, too. In our world, as in Pangaea, humans could reach most of the world's great landmasses, but the links could be snapped by something as simple as a period of global warming that allowed sea-levels to rise, as they did at the end of the last ice age. Furthermore, the sea crossing from southeast Asia to Australasia was difficult enough that, most of the time, Australia remained a quite separate world after its initial settlement. Much the same is true, too, of the Americas. And eventually, it would be true, also, of the great diaspora of the Pacific. So, in the history we know, human communities in different parts of the world existed and evolved for long periods in isolation from each other. In effect, human history played itself out in a number of almost entirely separate worlds, almost as if humans had settled a number of different planets.

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     The relative dispersion of landmasses in our world set up a series of interesting natural historical experiments, because, while it allowed humans to spread throughout the world, it limited contacts between different regions, creating almost entirely separate "world zones." The differences between these zones can help us understand better the range of different histories possible for a species such as ours, and that ought to allow us to gauge the relative importance of the different world zones.¹² In particular, these comparisons may help us gauge the relative importance of environmental factors, of contingency, and of the distinctive features of our species. If contingency rules we should expect to find the histories of each region diverging very significantly. If environmental factors dominate we should expect to find that the distinctive features of large regions such as Eurasia or Afroeurasia play a dominant role in explaining the larger patterns of human history. Finally, if we find significant similarities in the histories of separate regions, despite the workings of contingency and despite their different environments, we may have to conclude that there are long term drivers of human history that have something to do with the nature of our species. So quite a lot is at stake in these thought experiments, including our evaluation of the role of Afroeurasia in world history.

     We can identify three major stages or "world zones." These are large regions within which there were connections between human communities, but between which connections were so sporadic that they don't really count. The three zones are Afroeurasia, Australasia, and the Americas. Within the last three or four thousand years, a fourth zone has appeared in the Pacific.

Afroeurasia in World History

     The idea of a history that took place within separate "world zones" (each its own petri dish!) should help us identify some of the more important distinctive features of the Afroeurasian world zone. [PowerPoint Slide #12]

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     The Afroeurasian zone is the oldest of all the world zones because that is where our species evolved. To be precise, our species probably evolved within the African part of the Afroeurasian zone. Some paleo-anthropologists still argue that our species evolved simultaneously in many parts of Afroeurasia, and we do know that predecessor species, such as Homo erectus and Homo neanderthalis flourished outside of Africa for long periods. But the most recent evidence, both genetic and archaeological, suggests that our species evolved in Africa from about 200,000 years ago.¹³ And Africa is where most humans lived for most of the 200,000 years or so of our existence. This is why it is in Africa that you find the greatest genetic diversity within our species. Human colonization of the Eurasian part of Afroeurasia began between 100,000 and 60,000 years ago, and colonization of Eurasia's cold northern zones probably began after 30-40,000 years ago. The other world zones are much younger. Humans have been in the Australasian zone for perhaps 60,000 years and in the American world zone for probably no more than 13-15,000 years.¹⁴ The Pacific zone was colonized only in the last three or four thousand years in a process that was completed just 1,000 years ago with the settlement of New Zealand.¹⁵[PowerPoint Slide #13]

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     Afroeurasia is by far the largest of the world zones. It covers about 83.6 m square kilometers, while the Americas cover about 40 m square kilometers; Australia and Papua New Guinea together cover c. 8.5 m square kilometers; and the Pacific Islands and New Zealand cover only c. 0.4 m square kilometers.¹⁶

     Afroeurasia has also been the most populous of the world zones in all eras of human history. This is where most humans have lived throughout human history. Until 100,000 years ago, and perhaps as late as 60,000 years ago (in other words, for more than half of our history), humans lived only in Africa. At some point after about 100,000 years ago, small numbers of humans migrated beyond Africa, but the limited genetic variety of all modern populations outside of Africa suggests that this was, indeed, a small group. It is a reasonable bet that most humans continued to inhabit Africa until some time within the last ten thousand years, well after the first appearance of agriculture, at the end of the last ice age. When agriculture first appeared, it appeared in the Nile valley and perhaps in other parts of N. Africa (in parts of the Sahara that were warmer and lusher than they are today), but it also appeared in the fertile crescent of southwest Asia, in the north of the Indian sub-continent, and in parts of east Asia. Eventually, from perhaps 5,000 years ago, agriculture also evolved in Mesoamerica and in parts of S. America, but in the Americas agriculture appeared in fewer regions than in Afroeurasia and never diffused as far.¹⁷ Agriculture also appeared in the Australasian zone, but only in Papua New Guinea. Here, perhaps because it was based on root crops that resisted long-term storage, it never generated surpluses large enough to support large, urbanized and state-organized communities of the kind that appeared in several regions of Afroeurasia and the Americas. Agriculture also appeared in the Pacific zone, but here it appeared within the last 4,000 years, through diffusion from southeast Asia. In the Pacific, it was probably the small size of most islands that explains the absence of large imperial states, though powerful, agrarian-based polities did emerge on several of the larger island groups including Tahiti, Hawaii and New Zealand.

     Agricultural technologies could support much larger populations than foraging technologies; even the earliest farming techniques may have been able to support populations densities up to 50 times as large as those of foraging societies.¹⁸ So the fact that there were more agrarian regions in Afroeurasia than in the other world zones guaranteed that its demographic preponderance would be preserved. The spread of agriculture in Eurasia eventually tipped the demographic balance within Afroeurasia itself. By 5,000 years ago, and possibly several millennia earlier, more humans lived outside of Africa than within it. So the spread of agriculture finally ended the demographic dominance of Africa in world history. But it also sustained the demographic importance of Afroeurasia as a whole. The well-known estimates of world populations by J.N. Biraben suggest that 2,500 years ago, almost 94% of all humans lived in Afroeurasia, while most of the rest lived in the Americas.¹⁹ According to the same estimates, Afroeurasia's share of world population had fallen to c. 90% by 1500 CE. Even the massive migrations out of Afroeurasia in recent centuries appear not to have lowered this ratio greatly, dropping it merely to c. 86% of global population by the late 20^th century.

     The populations of Afroeurasia were also better networked than those of other world zones. Though there were barriers to movement, such as the Himalayas, or the great deserts such as the Gobi desert and the Sahara, none were insurmountable, and often they could be circumvented by sea voyages. In Australia, too, there were no insurmountable barriers to extensive exchanges, and indeed we know that goods could travel across the entire continent as a result of relay exchanges, for ceremonial objects such as shells from N. Queensland have been found more than 3,000 kilometers away in Western Australia.²⁰ Yet we also know that what contacts there were in the Australasian zone were slow, indirect and intermittent. The volume of information and of goods that flowed in this way seems to have been extremely limited. In the Americas, too, there were certainly links of some kind throughout the world zone. After entering the Americas about 13,000 years ago, it took humans only two or three millennia to reach the far south of S. America. Five hundred years ago, there were certainly connections between the major agrarian regions of Mesoamerica and the Andes, for European diseases appear to have reached the Andes before Francisco Pizarro and his men. But these links were limited, and there is as yet no significant evidence of extensive cultural or commercial exchanges between the two regions.

     In Afroeurasia, on the other hand, evidence of technological exchanges or the movement of precious goods between the Mediterranean and East Asia dates back several thousand years. By two thousand years ago there was extensive relay trade along the "Silk Roads" between the Mediterranean and China, and the Mongol conquests encouraged individuals to travel the entire length of the Eurasian landmass.²¹ Regions at the edges of the system, in northern Siberia or southern Africa, were certainly less well connected, but by 1,000 years ago, and perhaps even as early as 2,000 years ago, it made sense to claim that much of Afroeurasia belonged to a single "world system." Indeed, Barry Gills and the late A.G. Frank have argued that such a claim can be made as early as 2,000 BCE.²² What this meant in practice was that there was a chain of core regions with dense populations, cities, powerful states, and extensive trade systems that were joined all the way from China, through Central Asia or around the sea routes through S.E. Asia to India, and then linked to S.W. Asia, E. Asia, and the large region of agrarian lands around the Mediterranean.

     One important reason for the efficiency of these networks was the success in the Afroeurasian zone of what the late Andrew Sherratt called the "secondary products revolution".²³ Sherratt pointed out that the first domesticators of large herbivores seem to have used them rather inefficiently, keeping them perhaps for many years, and getting value from them only when they slaughtered them and used their meat and hides. From about 6,000 years ago, he lists a series of innovations that made it possible to exploit large herbivores even while they were alive, by using the fibers, milk and blood they produced (as well as their manure), or by using their draft power either for ploughing or transportation. These changes would revolutionize agriculture in Afroeurasia, making it possible to farm regions that could not be farmed before either for lack of fertilizer or because hand-held hoes could not reach the fertile sub-soils. They also revolutionized transportation and warfare. And they made possible the emergence of an entirely new life-way, pastoralism.²⁴ While farmers lived mainly off domesticated plants, pastoralists lived mainly off domesticated animals that could turn the world's grasslands into products that humans could use. Suddenly, entire societies began to settle the grasslands. But to do so they had to move their animals over large areas so they had to be highly mobile. And it is their mobility that explains why pastoralists provided natural links between the major agricultural regions, exchanging technologies (such as the technologies of horse-warfare), and protecting caravans that exchanged goods. Certainly by 1,000 BCE, and possibly much earlier, pastoralists played a vital role in stitching together the exchange networks both of Eurasia and of Africa.

     But the secondary products revolution would be confined to Afroeurasia. Why? The most likely explanation at present is the simple fact that there were very few promising large animal domesticates in the other two large zones, the Americas and Australasia, because large numbers of large mammals became extinct within the last 50,000 years.²⁵ In North America, horses (which had evolved in the Americas), elephants, giant armadillos, sloths and several species of camelids, vanished. The result was that until 500 years ago (after which the technologies of the secondary products revolution began to be introduced into the Americas and Australasia from Afroeurasia), humans were the primary beasts of burden, except in the Andes, where surviving camelids such as alpaca, allowed a modest repetition of the secondary products revolution. Though the reasons for these "megafaunal" extinctions remain a matter of controversy, the fact that their dating broadly coincides with the arrival of humans in different regions suggests that humans, with improved hunting techniques, were the major culprits. In Africa, where humans and large herbivores had co-evolved for many millions of years, each species had developed protocols for dealing with the others. Large herbivores learned that humans could be dangerous and humans probably learned that over-hunting could lead to extinction. But in Siberia, Australia and the Americas, where humans entered regions that had little experience of this new species, the results were devastating.²⁶

     The exceptional interconnectedness of Afroeurasia shaped the history of this world zone in profound ways. Exchanges of technologies such as crop-growing, metallurgy, pottery, or the technologies associated with horse domestication, or paper making or gunpowder hastened technological, commercial and military innovation in each of the major urbanized zones within Afroeurasia. And, as William McNeill pointed out, trans-Eurasian exchanges also had major epidemiological consequences.²⁷ On the one hand, they undoubtedly led to significant demographic downturns as diseases entered regions whose populations lacked immunities; yet over time, they also created a broader range of immunities in the major urbanized zones of Eurasia. And this explains why, when the different world zones finally converged in a new, artificial, Pangaea, in the sixteenth century CE, it was overwhelmingly the inhabitants of the smaller world zones that suffered through the exchange of diseases.

     This combination of advantages explains a fact that is all too often taken for granted in world history: Afroeurasia's dominant role in world history. It may seem as if, temporarily, this is no longer true, with the world's single superpower being based in North America. But there is plenty of evidence that the dominant role of the American world zone will prove short-lived. The illustrations accompanying this article include a graph based on estimates from Angus Maddison's The World Economy of the shares of global production generated by two different regions of the world since 1700.²⁸ [PowerPoint Slides #14 and #15] The first group of figures combines the output of India and China, two of the economic superpowers of the pre-modern world; the second group combines the output of Britain and the USA, two of the economic superpowers of the modern era. As the graph shows, the old economic superpowers of China and India continued to dominate world production until the middle of the nineteenth century. But it also suggests that they are now recovering a significant share of global production. In which case, the distinctive distribution of wealth and power that has shaped the history of the last two centuries may prove to be an aberration within the longer frame of world history, as the Eurasian heartlands that had dominated world history since the Neolithic era reassert their economic, political, military and even cultural centrality in world history.

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Conclusions? Strange Parallels in Human History

     What can we learn from the natural experiments set up by our world's serendipitous arrangement into separate "world zones"?

     First, it is clear that geography was indeed a powerful shaping force in human history. A Pangaean world would surely have generated a different history from the more diverse and fragmented world of today. The significant differences in size, populations, and connectedness of the different world zones generated very different histories in each of the zones. And these differences really mattered when the zones finally converged five hundred years ago. Perhaps most spectacularly, these differences help explain why that convergence led, in the smaller of the world zones, to some of the most catastrophic demographic declines ever seen in the course of human history.

     Second, we learn that sheer scale matters in human history. That it was the Afroeurasian zone rather than the American, Australasian or Pacific zones that have dominated most of human history had much to do with the number of people who lived there, the amount of time they lived there, the region's cultural, technological and social diversity, and the extent and intensity of exchanges between different parts of Afroeurasia. When the different world zones finally converged in the sixteenth century, the technological, commercial, demographic, military and even epidemiological advantages of societies from Afroeurasia were overwhelming.

     Third, however, we also learn that geography is not everything. Despite the striking differences in the histories of the three world zones, there are also some remarkable similarities in the general historical trajectory of each region, even if the timing differed. Agriculture appeared independently in both the Afroeurasian and American zones within just a few millennia, despite the fact that there is no evidence of technological diffusion between them. Agriculture also appeared within the Australasian world zone, in Papua New Guinea. And even in Australia, though agriculture in its fully developed forms did not appear before modern times, there are plenty of signs in recent millennia of forms of intensification that might well have led, in the absence of global convergence, to flourishing agrarian societies. In the state of Victoria, for example, extensive systems were created for the herding of eels, and so successful were these systems that around them there emerged sedentary communities, similar, in some ways, to the "affluent" foraging communities of the Natufians in Mesopotamia, or the more sedentary communities of the American Pacific Northwest.²⁹

     The near-simultaneity of the appearance of agriculture in these different world zones is itself striking when seen on a scale of some 200,000 years. But even more striking are the many parallels that appear after the emergence of agrarian societies. In both Afroeurasia and the Americas, agriculture supported numerous small sedentary communities which expanded in range and size until some developed towns and cities. As societies expanded, there appeared a more complex division of labor, and social hierarchies organized by lineage, wealth, ethnicity and gender. Eventually, states emerged, power structures in which elites exacted tributes from farmers, artisans and traders, imposing their will through powerful religious and civilian bureaucracies and large armies. To manage the resources they collected, elites developed accounting systems which evolved into the earliest forms of writing. (Even the Inca quipu, a way of recording information on knotted strings, was evolving into a writing system.) Elites also built large monumental structures, usually in the same basic form: that of a pyramid. The transition from early forms of agriculture to cities and states took 5-6,000 years in Mesopotamia, Egypt and parts of China; in the Americas it may have taken slightly longer, but the broad similarities in sequence and timing are impressive nonetheless. And at present there is no good reason to think they can be explained as a result of diffusion.

     At the scales of conventional historiography, it is all too easy to take these similarities for granted because they appear as a sort of immobile frame within which human history has taken place. But at the larger scales of world history, we can no longer take them for granted because agriculture itself can no longer be taken for granted. The frame itself begins to move at the large scale. And once this happens, we are forced to notice that these parallels are peculiar, and require explanation. What these "strange parallels" suggest (to borrow from the title of a book by Victor Lieberman) is that, despite differences of geography, many large patterns in human history may have more to do with the nature of our species than with differences of geography.³⁰

     Here is the best simple explanation I know for these strange parallels. I have argued elsewhere that what makes our species distinctive is our unique ability to exchange information in great volume and with great precision.³¹ This means that humans can share what each individual has learned, and that allows the learning of individuals to be pooled within the cultural traditions of each community. The result is that human knowledge can accumulate over time, as the insights of many individuals over many generations are stored within the culture of each community. It is this ability to accumulate information within culture that explains why, over time, humans slowly develop more and more powerful and varied ways of extracting resources and energy from their environment. The result was that humans could "adapt" through cultural change, rather than through the much slower method of genetic change (a.k.a "natural selection"). This is why change has been so rapid in human history.

     The results are apparent even in the Paleolithic era. In this era, there developed an astonishing diversity of human cultures as humans acquired the ecological skills needed to settle an increasing range of different environments, from the savanna lands in which our species had evolved, to the arid steppes, the sea shores and the tropical forests of Africa, and eventually to the equally diverse environments of Eurasia, including the tundra lands of the north. From Eurasia humans who had learnt to exploit coastal environments and coastal waters migrated to the very different world of Australia, while those that had learned to live in the ice-age environments of Siberia eventually crossed into the Americas, and spread throughout the American world zone. It is all too easy to think of the Paleolithic era as an era of changelessness. The reason is that, while humans made these astonishing migrations, migrations that have no parallels in the histories of other species, the basic social structures of human communities did not change greatly because most human societies remained small and nomadic. It was agriculture that changed all that. And agriculture itself was in some sense an unavoidable result of the slow spread of human communities throughout the world during the Paleolithic. By 11,000 years ago, some regions of the world were as densely settled as they could be given the foraging technologies that had supported human societies for 200,000 years. Further exploitation of environments would have to take new forms. Extensive technologies would have to give way to intensive technologies. Humans would have to work more closely with selected species, by clearing away rival species, protecting the chosen species, and harvesting them regularly. In other words, they would have to start farming.

     With the emergence of agricultural technologies within the last 11,000 years or so, humans in different parts of the world began to develop technologies that made it possible to support larger and denser communities. Wherever these larger communities appeared, they faced similar problems; the problems of feeding large communities sustainably, of managing conflicts, and of sharing resources as humans began to play diverse roles within each community. There is, after all, a huge difference between a family group of ten to fifteen people, and a sedentary community of more than a thousand individuals. Rules of kinship are all you need to manage the small group. They no longer work within much larger communities. In other words, the very rules of association were transformed by the advent of agriculture. The parallels we find in all the different world zones arise from the fact that similar solutions emerged to similar problems.

     The appearance of a species capable of collective learning, while it did not determine in detail the trajectory of human history, did make certain long-term outcomes very likely. It was likely, first, that this species would migrate to all accessible parts of the world. Within all these regions it was likely that, eventually, human populations would grow, creating communities that were denser, more complex and more interdependent. The striking parallels that we see in human history at very large scales, despite the play of contingency, and despite the very different environmental conditions within different world zones and different regions within those world zones, suggest that there was a limited number of solutions to the problems of sustaining large, and increasingly dense, human communities. Cities, states, armies, monumental architecture, organized religion, writing systems, and an extensive division of labor—these appear over and over again, even where there is no evidence or even possibility of diffusion, for the simple reason that there were a limited number of solutions to the problems generated by our species' astonishing ecological precocity. And what that suggests is that we might have followed a similar trajectory, albeit with rather different timing, even in a Pangaean world, or a highly dispersed world of many small continents. Geography matters, but so does the astonishing ecological creativity of our species.

     I hope this somewhat breathless survey of the history of Afroeurasia has brought out some of the more distinctive features of Afroeurasia's role in world history. But I hope it has also helped bring into focus some of the important patterns in human history that are not determined by geography, but are outcomes of the peculiar nature of human beings in general, in particular our unique ability to keep finding new ways of exploiting the natural environment. Whether we should admire or fear this unique gift is something we may find out in the coming decades.