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With few exceptions, the spread of plants, animals, and diseases was limited to geographically bound regions for much of the Earth’s history. Humans facilitated biological exchange, intentionally and accidentally carrying species across natural borders. As opportunities for human travel increased, so did the opportunities for biological exchange, often with dramatic consequences.
During most of the history of life on Earth, geographic barriers such as oceans and mountain ranges divided the planet and inhibited migrations of most kinds. Only birds, bats, flying insects, and good swimmers consistently bucked the trend. A few other species did so occasionally, thanks to sea-level changes and land bridges or to chance voyages on driftwood. However, for most species most of the time, biological evolution took place in separate biogeographical provinces.
Intracontinental Biological Exchange
This long phase of separation ended when human beings began their long-distance migrations. Deep in prehistory hominids (erect bipedal primate mammals) walked throughout Africa and Eurasia, occasionally bringing a plant, seed, insect, microbe, or rodent to a place it would not have reached on its own. With plant and animal domestication ten to twelve thousand years ago, people began to transport such things on purpose and more frequently. Most of the plants and animals susceptible to domestication lived in Eurasia, and those sensitive to climate or day length (several flowering plants take their cues to bloom from day length) spread most easily along the east-west axis of that continent. The sets of domesticated plants and animals on which Eurasian and North African agriculture and herding are based spread almost instantaneously by the standards of the past, although in fact the spread took a few millennia. This process of spreading no doubt proved highly disruptive biologically as local biogeographic provinces were invaded by alien creatures spread by humanity. It also proved highly disruptive historically, obliterating peoples who did not adapt to the changing biogeography, the changing disease regimes, and the changing political situations brought on by the spread of farmers, herders, and eventually states. Out of this turmoil of Afro-Eurasian biological exchange emerged the great ancient civilizations from China to the Mediterranean. They all based their societies on intersecting but not identical sets of plants and animals.
Biological homogenization within Afro-Eurasia had its limits. The links between northern Africa, say, and eastern Asia before 500 BCE were slender. Varying topography and climate also checked the spread of species. The process presumably accelerated when interregional contacts flourished, for example, when empires created favorable conditions for the movement of goods and people. The era of the Han dynasty (206 BCE–220 CE) in China and the Roman Empire, for example, when the trans-Asian trade route called the Silk Roads was a well-beaten path, unleashed a small flood of biological exchanges. The Mediterranean acquired cherries at this time, and possibly smallpox and measles, too; sorghum made its way from east Africa to India to China, and grapes, camels, and donkeys arrived in China from southwest Asia and northern Africa.
Within Eurasian history two more moments of heightened biological exchange occurred. The next moment occurred during the early Tang dynasty (618–907 CE) in China. The Tang rulers came from various ethnic and cultural traditions and for a century and a half showed keen interest in things foreign: trade, technology, culture (e.g., Buddhism), and plants and animals. The court imported exotica: curious creatures, aromatic plants, ornamental flowers. Much of this exotica was inconsequential in social and economic terms, but some of it, such as cotton (imported from India), was not. The Tang were culturally receptive to strange plants and animals, but political conditions helped too: their political power on the western frontier, and the geopolitical situation generally before 750, promoted the trade, travel, and transport that make biological exchange likely.
For roughly a century and a half (600–750 CE) the numerous political organizations of Central Asia were frequently consolidated into only a few, simplifying travel by lowering protection costs. A handful of large empires held sway throughout Central Asia, making the connections between China, India, and Persia (modern-day Iran) safer than usual. These geopolitical arrangements fell apart after 751, when Muslims defeated Tang armies, and after 755, when rebellion shook the Tang dynasty to its foundations. Thereafter, both the stability of the geopolitical situation and the receptivity of the Tang to things foreign changed, waning more often than waxing, and the opportunities for biological exchange grew scarcer.
Another moment of heightened biological exchange within Eurasia occurred with the Pax Mongolica (Mongol Peace) of the thirteenth and fourteenth centuries. By this time most of the feasible exchanges of plants and animals had already taken place. However, the heightened transport across the desert-steppe corridor of Central Asia may have brought carrots and a species of lemon to China and a form of millet to Persia. Quite possibly this transport also allowed the quick diffusion from Central Asia of the bacillus that causes bubonic plague, provoking the famous Black Death, the worst bout of epidemics in the recorded history of western Eurasia and northern Africa. Plague may also have afflicted China during these centuries, although the evidence is ambiguous.
Although this process of Eurasian (and North African) biological exchange never truly came to an end, it slowed whenever political conditions weakened interregional contacts. It also slowed in general after around 200 CE, with the erosion of two eras of peace—the Pax Romana (Roman Peace) and Pax Sinica (Chinese Peace), which had encouraged long-distance travel and trade within Eurasia. By that time sugarcane had taken root in India, spreading from its New Guinea home. Wheat had spread widely throughout most of its potential range, as had cattle, pigs, horses, sheep, and goats. Less and less was left to do even when political and economic conditions encouraged biological exchange.
Meanwhile on other continents, similar, if smaller-scale, processes of biological exchange and homogenization were in train. In the Americas maize spread both north and south from its Mesoamerican home (the region of southern North America that was occupied during pre-Columbian times by peoples with shared cultural features), slowed, it seems, by difficulties in adapting to different day lengths at different latitudes. In Africa the Bantu migrations of two thousand years ago probably diffused several crops throughout eastern and southern Africa and possibly brought infectious diseases that ravaged the indigenous, previously isolated, populations of southern Africa. These events in Africa and the Americas, too, must have been biologically and politically tumultuous, although the evidence is sparse.
In biological terms the process of human-promoted biological exchange selected for certain kinds of species, those that co-existed easily with human activity: domesticates, commensals (organisms that obtain food or other benefits from another without damaging or benefiting it), and plants that thrive on disturbed ground, most of which we usually call “weeds.” These species prospered under the new regimes of expanded human migration and interaction; for them, history had taken a favorable turn. Indeed, humanity was in a sense working for them, spreading their genetic footprints far and wide within the continents and into the future.
Biological Exchange and Biological Invasion
Intercontinental biological exchange also has a long pedigree. The first people to migrate to Australia may have accidentally brought some species with them forty thousand to sixty thousand years ago. About thirty-five hundred years ago, later migrants to Australia purposely brought the dingo (a large dog), the first domesticate in Australian history. The dingo quickly spread to all Aboriginal groups outside of isolated Tasmania and also formed feral packs. It proved an effective hunting dog and led to the extinction of some indigenous mammals. The dog (not the dingo) was also the first domesticated animal in the Americas, brought across the Siberian-Alaskan land bridge with some of the first settlers during the last Ice Age. Here dogs probably played a significant role in reducing the populations of large mammals, many of which became extinct soon after humans arrived in North and South America. Initial human settlement of unpopulated islands also wrought major ecological changes throughout the southwest Pacific and Polynesia, including numerous extinctions, from about four thousand years ago until the colonization of New Zealand roughly seven hundred or one thousand years ago.
All of these instances were invasions of “naive” lands—continents and islands that had no prior exposure to humanity and its fellow travelers or to the intensified fire regimes that human presence normally brought. This fact helps to explain the dramatic effects, particularly the rash of extinctions that followed upon human settlement of Australia, New Zealand, and the Americas.
Eventually people began to transport animals, plants, and pathogens from one human community to another across the seas. In many cases the only evidence for such transfers is the existence of the imported species. The sweet potato, a native of South America, somehow arrived in central Polynesia by 1000 CE and subsequently spread widely throughout Oceania (lands of the central and southern Pacific). It is a delicate crop and could not survive a driftwood voyage: no one doubts that people transported it, although no one knows just when, how, or even who. It eventually became a staple food in the western Pacific, highland New Guinea, and to a lesser extent the eastern Asian archipelagoes and mainland.
A second mysterious transoceanic crop transfer took place across the Indian Ocean some time before 500 CE. Somebody brought bananas, Asian yams, and taro to eastern Africa. These crops had much to recommend them because they do well in moist conditions, whereas the millets and sorghum that Bantu expansion brought into central and southeastern Africa were adapted to dry conditions. Plantains, of which bananas are one variety, had existed in the wild from India to New Guinea. Linguistic and genetic evidence suggests they arrived on the east African coast as early as three thousand years ago and reached the forest zone to the west of Africa’s Great Lakes around two thousand years ago, just about the time of the Bantu migrations. Quite possibly the success of Bantu speakers, often attributed to their use of iron, owed something to their successful adoption of these exotic crops. As relative newcomers to eastern and southern Africa, they had less invested in prevailing ecological patterns and fewer disincentives to experiment. Bananas, taro, and yams were probably introduced to eastern Africa more than once and almost surely were brought again in the settlement of Madagascar that took place not long before 500 CE. These Asian crops assisted in the epic (but unrecorded) colonization of central Africa’s moist tropical forests by farmers, as well as in the settlement of Madagascar.
Several other significant intercontinental biological transfers took place before 1400 CE, mainly between Africa and Asia, a route that posed minimal obstacles to sailors. Africa’s pearl millet, derived from a western African savanna grass, is the world’s sixth most-important cereal today. It was introduced into India three thousand years ago and today accounts for about 10 percent of India’s cereal acreage. East African sorghum entered India at about the same time and eventually became India’s second-most-important grain after rice. Sorghum stalks were also useful as fodder for India’s cattle. Finger millet, also from Africa, arrived in India only around one thousand years ago. It became the staple in Himalayan foothill communities and in far southern India. The transfer of African crops to south Asia mainly provided India with drought-resistant dryland crops, opening new areas to settlement and providing a more reliable harvest where water supplies were uncertain. These examples suggest a lively world of crop exchange—and probably weeds, diseases, and animals, too—around the Indian Ocean rim from about three thousand to fifteen hundred years ago. The regular monsoon winds of the Indian Ocean helped make this region of the world precocious in its maritime development and hence biological exchange.
Whereas south Asia received new crops from Africa, it sent new crops to the Middle East and the Mediterranean. Facilitated by the relative peace supervised by the Abbasid dynasty (749/750–1258), between the tenth and thirteenth centuries Arab trading networks brought sugar, cotton, rice, and citrus fruits from India to Egypt and the Mediterranean. These plants, and the cultivation techniques that came with them, worked a small revolution on the hot and often malarial coastlands of northern Africa, Anatolia in Turkey, and southern Europe. They caused many coastal plains to be brought under cultivation on a regular basis, often for the first time since the Roman Empire. Sugar and cotton could flourish with unskilled and unmotivated slave labor; their introduction may have quickened the slave raiding that kept Mediterranean and Black Sea populations anxious for centuries. Keeping an army of laborers at work on deadly malarial coasts—in the Levant (countries bordering on the eastern Mediterranean), Egypt, Cyprus, Crete, Sicily, Tunisia, and Andalusia in Spain, to mention a few centers of sugar production—required constant topping up from poorly defended peasantries. This quest took slave merchants and raiders to the Black Sea coasts but also across the Sahara Desert and along Africa’s Atlantic coast. Saadian Morocco, a state originally based on plantations in the Sous and Draa River valleys, brought sugar and African slaves together in a profitable mix that would soon be transplanted to Atlantic islands such as the Canaries and Madeira and then to the Americas.
A second avenue of exchange linked the Mediterranean basin to western Africa. Although this exchange was not genuinely intercontinental, the Sahara Desert for several millennia functioned somewhat like a sea, as the Arabic term for “shore” (sahel) for the western African desert edge implies. One thousand years before Christopher Columbus crossed the Atlantic, some unknown soul crossed the Sahara, reuniting the Mediterranean and the sahel, which the increasingly arid Sahara had divided since about 3000 BCE. Trans-Saharan trade developed in salt, slaves, and gold. However, this reunification no doubt included a biological dimension. Large horses seem to have made their debut in west Africa via trans-Saharan trade. Linguistic evidence suggests they came from the Maghreb region in the north. Horses eventually became a decisive element in a military revolution in the sahel, creating a mounted aristocracy who by the fourteenth century built imperial states. The Jolof, Mali, and Songhai empires of west Africa depended on horse cavalry, which underwrote their military power and, via slave raiding, their economies. When ecological conditions permitted, these empires bred their own warhorses, and when conditions did not permit, the empires had to import them, usually from Morocco. In any case, the social, economic, and political history of west Africa took a new direction with the arrival of large horses.
These events show that long before the great age of oceanic navigation, the links of trade and colonization in the Pacific Ocean, in the Indian Ocean, and across the Sahara Desert brought biological exchanges that powerfully influenced the course of history. The further exchanges attendant upon the voyages of Columbus, the Portuguese navigator Ferdinand Magellan, the British sea captain James Cook, and others extended this process, wrenchingly, to lands formerly quite separate in biological (as in other) terms.
After 1400 CE mariners linked almost every nook and cranny of the humanly inhabitable Earth into a biologically interactive unit. The world’s seas and deserts were no longer isolated biogeographical provinces. The world became one without biological borders as plants, animals and diseases migrated wherever ecological conditions permitted their spread, although how soon and how thoroughly they did so often depended on patterns of trade, production, and politics.
Columbus inaugurated regular exchanges across the Atlantic whereby the Americas acquired a large set of new plants and animals as well as devastating diseases that severely depopulated the Americas between 1500 and 1650. Simultaneously, Africa and Eurasia acquired some useful crops from the Americas, most notably potatoes, maize, and cassava (manioc). Ecosystems and societies in the Americas were remade with new biologies and new cultures. The same thing was true, however, even if less catastrophically, in Africa and Eurasia. The new food crops fed population growth in Europe and China and possibly in Africa, too (no firm evidence exists). Maize and potatoes changed agriculture in Europe, as did maize and sweet potatoes in China, allowing more intensive production and allowing lands not suited to wheat, barley, rye, or rice to come into production. In Africa maize, cassava, and peanuts became important crops. Today 200 million Africans rely on cassava as their staple food. Many of the rest, mainly in the south and east, rely on maize.
These modern biological exchanges had political meanings and contexts. European imperialism, in the Americas, Australia, and New Zealand, simultaneously promoted, and was promoted by, the spread of European (or more usually Eurasian) animals, plants, and diseases. Europeans brought a biota (the flora and fauna of a region) that worked to favor the spread of European settlers, European power, and Eurasian species and thereby to create what Alfred Crosby, the foremost historian of these processes, called “neo- Europes”—including Australia, New Zealand, most of North America, southern Brazil, Uruguay, and Argentina.
Beyond the neo-Europes, in the Americas something of a neo-Africa emerged. More than 10 million Africans arrived in the Americas in slave ships. In those same ships came yellow fever and malaria, which profoundly influenced settlement patterns in the Americas. The ships also brought African rice from the west coast, which became the foundation of the coastal economy in South Carolina and Georgia during the eighteenth century and was important in Suriname in South America as well. Other African crops came, too: okra, sesame, and (although not in slave ships) coffee. African biological impact on the Americas did not cease with the end of the slave trade. Much later African honeybees imported into Brazil crossbred to create an “Africanized” bee that since the 1950s has colonized much of the Americas.
The age of sail brought the continents together as never before. But sailing ships did not prove hospitable carriers to every form of life. They filtered out a few, those that could not for one reason or another survive a long journey or that required conditions that sailing ships could not provide. The age of steam and then the age of air travel broke down yet further barriers to biological exchange, adding new creatures to the roster of alien intruders and accelerating the dispersal of old and new migratory species alike.
The advent of iron ships toward the end of the nineteenth century, for example, opened a new era in biological exchange involving species of the world’s harbors and estuaries. After the 1880s iron ships began to carry water as ballast. Soon special water ballast tanks became standard, and thus, for example, a ship from Yokohama, Japan, bound for Vancouver, Canada, would suck up a tankful of water and, more than likely, a few marine species from Japanese shores, cross the wide Pacific, then release its Japanese water and sea creatures in Puget Sound before taking on a Canadian cargo. During the 1930s Japanese clams hitched such a ride and upon arrival began to colonize the seabeds of Puget Sound, creating a multimillion-dollar clam fishery in British Columbia and Washington State. A jellyfish that devastated Black Sea fisheries came from the East Coast of the United States in about 1980. The zebra mussel, a Black and Caspian seas native, colonized the North American Great Lakes and river system from a beachhead established near Detroit in 1985 or 1986. It has cost the United States and Canada billions of dollars by blocking water intakes on city water systems, factories, and nuclear power plants.
A more recent invader of the North American Great Lakes is the fishhook flea, a crustacean that is a native of Caspian and Black sea waters. It first appeared in Lake Ontario in 1998 and is now in the all the Great Lakes and New York’s Finger Lakes, menacing sport and commercial fisheries and disrupting the lakes’ food web. The failures of Soviet agriculture and the expanded grain trade from North America during the 1970s and 1980s created a new pattern of ship traffic that quickly brought disruptive biological exchanges. Nowadays thirty-five thousand ocean-going ships and three thousand marine species are in transit at any given time, linking the world’s harbor and estuarine ecosystems as never before. The exchanges via ballast water are but a single variety of the swirl of biological exchange going on in modern times. Transport, travel, and trade take place on such a scale now and with such rapidity that a vast homogenization of the planet’s flora and fauna is under way.
From the Olympian height that allows a view of all life on Earth over its entire history, the last ten thousand years appear as an instantaneous homogenization of ecosystems, a new era in Earth history. Humankind has connected formerly distinct spheres of life through trade and travel, reprising in the blink of an eye what previously happened through continental drift. Some 300 to 250 million years ago the world’s continents fused to form a single supercontinent, called “Pangaea.” Creatures formerly kept apart from one another now rubbed shoulders. Large numbers of them became extinct by about 220 million years ago, perhaps in part on account of this new familiarity (although other theories exist). Reptiles inherited the Earth, spreading throughout the globe. During the last few millennia the human species has once again fused the continents, and to some extent the seas, and is probably provoking (through this and other means) the sixth great extinction spasm in the history of Earth.
From a less Olympian height, other vistas present themselves. The process of biological exchange is much influenced by the technology of transportation. The invention of ships, of ocean-going ships, of ballast tanks, of railroads and airplanes all led to changes and surges in the pattern of biological exchange. Transport technology provides one rhythm. Another rhythm is political.
Some states and societies showed great eagerness to import exotic species. Monarchs of ancient Egypt and Mesopotamia buttressed their prestige by maintaining gardens and zoos filled with exotic plants and animals. The Tang dynasty, as noted, showed a similar enthusiasm. Thomas Jefferson tried his best to establish rice and silkworms in Virginia. Later, the U.S. government employed an army of plant prospectors, who scoured the globe for potentially useful species and brought tens of thousands to the United States. During the nineteenth century Australia and New Zealand featured “acclimatization societies,” which imported species that met with their approval (usually from Britain). Nowadays the United States, Australia, New Zealand, and many other countries spend vast sums trying to prevent the importation of unwanted species, hoping to forestall biological invasions rather than foment them. Altogether, biological invasions now cost the United States more than all other natural hazards combined, including floods, hurricanes, tornadoes, earthquakes, and so forth.
Beyond the disposition that any society might have toward exotic species, the changing nature of geo politics also affected biological exchange. Trade and travel—and presumably biological exchange—expanded in times of peace and contracted in times of war, brigandage, and piracy. Probably eras of imperial unification provided the best political environment for biological exchange, when a single power enforced a general peace. Anarchic systems of competing states probably checked biological exchange by slowing trade and travel, notwithstanding the effects of mobile armies and navies. Furthermore, imperialism also seems to have inspired, as well as eased, the process of collection: botanical gardens and the like. Kew Gardens outside of London proved a crucial link in transferring rubber seeds from Brazil to Malaya at the end of the nineteenth century, starting a new plantation economy in Southeast Asia. The swings between moments of consolidated imperialism and anarchic struggle established another rhythm governing the history of biological exchange. This rhythm, of course, was influenced in turn by biological exchanges, as in the case of horses on the African savanna.
One can only postulate such patterns in the history of biological exchange. Demonstrating their validity would require quantitative evidence beyond what one can reasonably hope to find. Yet, one may be sure that time and again during the past ten millennia biological exchange has altered history. The next ten millennia will be quite different: fewer exchanges of existing species will take place because so many already have. Newly engineered species will occasionally depart from their creators’ scripts, however, fashioning unpredictable biological dramas. Some of these surely will help shape the future.
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