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code How, then, does all this explain the outcome of 1492--the Europeans conquered and depopulated the New World, instead of Native Americans conquering and depopulating Europe? Part of the answer, of course, goes back to the invaders' technological advantages. European guns and steel swords were more effective weapons than Native American stone axes and wooden clubs. Only Europeans had ships capable of crossing the ocean and horses that could provide a decisive advantage in battle. But that's not the whole answer. Far more Native Americans died in bed than on the battlefield--the victims of germs, not of guns and swords. Those germs undermined Indian resistance by killing most Indians and their leaders and by demoralizing the survivors. The role of disease in the Spanish conquests of the Aztec and Inca empires is especially well documented. In 1519 Cortes landed on the coast of Mexico with 600 Spaniards to conquer the fiercely militaristic Aztec Empire, which at the time had a population of many millions. That Cortes reached the Aztec capitol of Tenochtitlan, escaped with the loss of "only" two- thirds of his force, and managed to fight his way back to the coast demonstrates both Spanish military advantages and the initial naivete of the Aztecs. But when Cortes next onslaught came, in 1521, the Aztecs were no longer naive, they fought street by street with the utmost tenacity. What gave the Spaniards a decisive advantage this time was smallpox, which reached Mexico in 1520 with the arrival of one infected slave from Spanish Cuba. The resulting epidemic proceeded to kill nearly half the Aztecs. The survivors were demoralized by the mysterious illness that killed Indians and spared Spaniards, as if advertising the Spaniards' invincibility. By 1618 Mexico's initial population of 20 million had plummeted to about 1.6 million. Pizarro had similarly grim luck when he landed on the coast of Peru in 1531 with about 200 men to conquer the Inca Empire. Fortunately for Pizarro, and unfortunately for the Incas, smallpox had arrived overland around 1524, killing much of the Incan population, including both Emperor Huayna Capac and his son and designated successor, Ninan Cuyoche. Because of the vacant throne, two other sons of Huayna Capac, Atahuallpa and Huascar, became embroiled in a civil war that Pizarro exploited to conquer the divided Incas. When we in the United States think of the most populous New World societies existing in 1492, only the Aztecs and Incas come to mind. We forget that North America also supported populous Indian societies in the Mississippi Valley. Sadly, these societies too would disappear. But in the case conquistadores contributed nothing directly to the societies' destruction; the conquistadores' germs, spreading in advance, did everything. When De Soto marched through the Southeast in 1540, he came across Indian towns abandoned two years previously because nearly all the inhabitants had died in epidemics. However, he was still able to see some of the densely populated towns lining the lower Mississippi. By a century and a half later, though, when French settlers returned to the lower Mississippi, almost all those towns had vanished. Their relics are the great mound sites of the Mississippi valley. Only recently have we come to realize that the mound-building societies were largely intact when Columbus arrived, and that they collapsed between 1492 and the systematic European exploration of the Mississippi. When I was a child in school, we were taught that North America had originally been occupied by about one million Indians. That low number helped justify the white conquest of what could then be viewed as an almost empty continent. However, archeological excavations and descriptions left by the first European explorers on our coasts now suggests an initial number of around 20 million. In the century or two following Columbus's arrival in the New World, the Indian population is estimated to have declined by about 95 percent. The main killers were European germs, to which the Indians had never been exposed and against which they therefore had neither immunologic nor genetic resistance. Smallpox, measles, influenza, and typhus competed for top rank among the killers. As if those were not enough, pertussis, plague, tuberculosis, diphtheria, mumps, malaria, and yellow fever came close behind. In countless cases Europeans were actually there to witness the decimation that occurred when the germs arrived. For example, in 1837 the mandan Indian tribe, with one of the most elaborate cultures in the Great Plains, contracted smallpox thanks to a steamboat traveling up the Missouri River form St. Louis. The population of one Mandan village crashed from 2,000 to less than 40 within a few weeks. The one-sided exchange of lethal germs between the Old and New Worlds is among the most striking and consequence-laden facts of recent history. Whereas over a dozen major infectious diseases of Old World origins became established in the New World, not a single major killer reached Europe from the Americas. The sole possible exception is syphilis, whose origin still remains controversial. That one-sidedness is more striking with the knowledge that large, dense human populations are a prerequisite for the evolution of crowd diseases. If recent reappraisals of the pre- Columbian New World population are correct, that population was not far below the contemporaneous population of Eurasia. Some New World cities, like Tenochtitlan, were among the world's most populous cities at that time. Yet Tenochtitlan didn't have awful germs waiting in store for the Spaniards. Why not? One possible factor is that the rise of dense human populations began somewhat later in the New World than in the Old. Another is that the three most populous American centers-- the Andes, Mexico, and the Mississippi Valley--were never connected by regular fast trade into one gigantic breeding ground for microbes, in the way that Europe, North Africa, India, and China became connected in late Roman times. The main reasons becomes clear, however, if we ask a simple question: From what microbes could any crowd diseases of the Americas have evolved? We've seen that Eurasian crowd diseases evolved from diseases of domesticated herd animals. Significantly, there were many such animals in Eurasia. But there were only five animals that became domesticated in the Americas: the turkey in Mexico and parts of North America, the guinea pig and llama/alpaca (probably derived form the same original wild species) in the Andes, the Muscovy duck in tropical South America, and the dog throughout the Americas. That extreme paucity of New World domestic animals reflects the paucity of wild starting material. About 80 percent of the big wild mammals of the Americas became extinct at the end of the last ice age, around 11,000 years ago, at approximately the same time that the first well-attested wave of Indian hunters spread over the Americas. Among the species that disappeared were ones that would have yielded useful domesticates, such as American horses and camels. Debate still rages as to whether those extinctions were due to climatic changes or to the impact of Indian hunters on prey that had never seen hunters. Whatever the reason, the extinctions removed most of the basis for Native American animal domestication--and for crowd diseases. The few domesticates that remained were not likely sources of such diseases. Muscovy ducks and turkeys don't live in enormous flocks, and they're not naturally endearing species (like young lambs) with which we have much physical contact. Guinea pigs may have contributed a trypanosome infection like Chagas' disease or leishmaniasis to our catalog of woes, but that's uncertain. Initially the most surprising absence is of any human disease derived from llamas (or alpacas), which are tempting to consider as Andean equivalent of Eurasian livestock. However, llamas had three strikes against them as a source of human pathogens: their wild relatives don't occur in big herds as do wild sheep, goats, and pigs; their total numbers were never remotely as large as the Eurasian populations of domestic livestock, since llamas never spread beyond the Andes; and llamas aren't as cuddly as piglets and lambs and aren't kept in such close association with people. (You may not think of piglets as cuddly, but human mothers in the New Guinea highlands often nurse them, and they frequently live right in the huts of peasant farmers.) The importance of animal-derived diseases for human history extends far beyond the Americas. Eurasian germs played a key role in decimating native peoples in many parts of the world as well, including the Pacific islands, Australia, and southern Africa. Racist Europeans used to attribute those conquests to their supposedly better brains. But no evidence for such better brains has been forthcoming. Instead, the conquests were made possible by Europeans' nastier germs, and by the technological advances and denser populations that Europeans ultimately acquired by means of their domesticated plants and animals. So on the 500th anniversary of Columbus's discovery, let's try to regain our sense of perspective about his hotly debated achievements. There's no doubt that Columbus was a great visionary, seaman, and leader. There's also no doubt that he and his successors often behaved as bestial murderers. But those facts alone don't fully explain why it took so few European immigrants to initially conquer and ultimately supplant so much of the native population of the Americas. Without the germs Europeans brought with them--germs that were derived from their animals--such conquests might not have been possible.

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