Suddenly exposed to this avalanche of unfamiliar infectious diseases, Amerindians were devastated. Some estimates suggest that the indigenous population of the Americas dropped by more than 90 percent over a few centuries, with almost all of the loss due to infectious diseases.5
This Amerindian vulnerability was a primary reason for European success in the Americas. Epidemic disease, particularly smallpox, interfered with armed resistance by Amerindians and thus played an important part in the early Spanish conquests. In Mexico, where Hernan Cortes and his troops had made the Aztec emperor their puppet, the Aztecs rose against them, killing Moctezuma II and two-thirds of the Spanish force in the famous "Noche Triste."The Aztecs probably would have utterly destroyed the invaders, were it not for the smallpox epidemic under way at the same time. The leader of the Aztec defense died in the epidemic, and Cortes and his men conquered the Aztec Empire.
It is hard to see how Cortes could have won without those microscopic allies, since he was trying to conquer an empire of millions with a few hundred men. Moreover, major Indianpolities such as the Mayan city-states were still intact after the defeat of the Aztecs, and the Spaniards might well have lost control except for the series of epidemics that followed. Francisco Pizarro's conquest of the Incan Empire was also aided by a smallpox epidemic. It killed the emperor and his heir, causing a convenient succession struggle. Considering that Pizarro was invading another empire of millions with only 168 soldiers, it's obvious that he needed all the help he could get.
Amerindian vulnerability to infectious disease shaped history again and again. The first Spanish attempts at colonization in the West Indies were actually jeopardized by it, since the Taino and Arawak peoples diminished so rapidly (they were almost gone by 1530) that the Spanish were left without a labor force. Inhabitants of those Caribbean islands had been even more isolated and shielded from disease than the Amerindians of the mainland, and so were even more vulnerable.
The Pilgrims' first settlement was on land that already had been cleared by an Indian tribe that had been ravaged by some plague (possibly smallpox) just three years earlier. Squanto, the Indian who taught the Pilgrims survival skills, seems to have been one of the few survivors of that tribe. The later Puritan settlement of New England was also furthered by devastating epidemics among the Amerindians, while Jamestown's safety was only secured when epidemic disease had weakened the local tribes.
The Amerindians survived best in the highlands, where they could avoid most of the new African diseases.6 In fact, in the Altiplano of South America, which has an altitude of over 11,000 feet, local Amerindians had a compensating advantage over the Spaniards because they were better adapted to the thin air.
Europeans had multiple advantages, of course. They had superior weapons and tactics, honed over thousands of years of organized conflict. Their form of warfare was more realistic and less ritualistic, at least in comparison to that of the Aztecs, who had "flower wars"—wars aimed at acquiring captives for sacrifice rather than achieving decisive results—with neighboring city-states such as Tlaxcala. Europeans had a varied and useful set of domesticated animals to use for food, raw materials such as wool and leather, and transportation. They had advanced metallurgy (iron and steel) and large sailing ships. They were the heirs of literate cultures going back thousands of years, and although there were exceptions, such as Pizarro, who never learned to read or write, many of the early explorers and settlers—the Puritans, for example—were highly literate and well educated.
The most complex Amerindian civilizations in 1492 were similar to civilizations found in the Middle East 3,000 to 4,000 years earlier—so the Europeans were, in a sense, invaders from the future.
The European advantage in disease resistance was particularly important because those early attempts at conquest and colonization were marginal. Shipping men and equipment across the Atlantic Ocean presented huge logistical difficulties. European military expeditions to the New World were tiny and poorly supplied. The successes of the conquistadors are reminiscent of ridiculous action movies in which one man defeats a small army—and that's a lot harder to do with an arquebus than an Uzi. Early colonization efforts often teetered on the edge of disaster, as when half the Pilgrims died in their first winter, or when most of the settlers in Jamestown starved to death in the winter of 1609.
Epidemic disease didn't just grease the skids for the initial conquests: It reduced Amerindian populations and made later revolts far weaker than they would have been otherwise. If they had not died of disease, the Amerindians would have had time to copy and use many European military innovations in the second or third round of fighting.
We know a lot about the genetic basis of resistance to malaria, but relatively little about the genetic basis of European resistance to diseases like smallpox, although there are some hints. As we have said before, there is plenty of evidence for selection acting recently on many genes involved with disease defense, but in most cases we don't know the biochemical details—for example, which particular infectious organism a particular selected allele defended against. We suspect that delta CCR5 (for chemokine receptor 5), a common mutation among northern Europeans, protects against smallpox, but since smallpox is dangerous to work with and now exists only in a couple of genetic repositories, it's hard to be sure.7 Some recessive genetic diseases that are common in Europe and the Middle East also probably have conferred resistance to some infectious diseases: That list would include cystic fibrosis, alpha-1-antitrypsin deficiency, familial Mediterranean fever, connexin-26 deafness, and hemochromatosis. All are nonexistent in Amerindians, discounting recent admixture.
There is another way we may be able to detect some of the alleles that helped protect Europeans from infectious diseases that devastated the Amerindians: admixture studies. Many of the present inhabitants of Latin America are descended from both Europeans and Amerindians, along with a smaller amount of African ancestry. In the absence of natural selection, you'd expect that admixture would be the same at each gene, allowingfor chance—if 40 percent of overall ancestry was European, then 40 percent of the copies of each gene would also be European. But as we pointed out in Chapter 2, this no longer remains the case if a particular allele of some gene causes increased reproductive fitness. For example, if the European allele of some gene conferred substantial protection against smallpox, the average inhabitant of Mexico might be considerably more European at that locus than average—even though the admixed population is no more than 500 years old. In other words, there could be adaptive introgression of the European version of that gene. Some Amerindian or African alleles might also have advantages: But considering the relatively short time available for introgression (at most twenty generations) and the sheer destructive power of smallpox, a European smallpox defense seems one of the more likely candidates for detectable introgression. There is evidence of such an unusually European chunk of the genome in at least one Mexican American population.8 It is remarkable that the same principles apply to Neanderthals and conquistadors.
We do know a bit about immunological differences between Amerindians and other peoples. We know that the Yanomamo (a much-studied group of Amerindians in the backwoods of Venezuela) tend to produce high levels of antibodies against tuberculosis antigens rather than the more effective cell-mediated responses seen in Europeans. Even though tuberculosis is common among the Yanomamo, few individuals have a positive response to a tuberculin test. This is important, because most Old Worlders exposed to tuberculosis mount an effective immune response (which causes a positive response on a tuberculin test) and avoid symptoms. Only a minority develop active disease.
The Yanomamo also have extremely high levels of Immunoglobulin E (a molecule involved in defense against parasitic worms)—much higher than levels seen in Europeans with the same level of infestation. Pre-Columbian Amerindians were subject to some parasitic worm infections but relatively few bacterial or viral diseases, and it may be that natural selection adjusted their immune s
ystem to face those threats.
We know more about the practical consequences of these genetic differences than we do about their biochemical details, in part because of historical accounts of the relative impact of infectious diseases in European and Amerindian populations, but also because of well-documented epidemics during the era of scientific medicine—the past 100 years or so.
Even during the twentieth century, first contacts between Amerindians and people of European descent killed one-third to one-half of the natives in the first five years unless there was high-quality medical care available.9 This was the case during a period in which some of the worst Eurasian diseases (smallpox, bubonic plague, and typhus) were no longer major threats. For example, of the 800 Surui contacted in 1980 in Brazil, 600 had died by 1986, most of tuberculosis.
Judging from historical accounts, the fatality rate of smallpox was much higher among Amerindians than among Europeans. Roughly 30 percent of the Europeans who were infected died, whereas for the Amerindians, the fatality rate sometimes reached 90 percent. For example, in an epidemic in 1827, smallpox spared only 125 out of 1,600 Mandan Indians in what later became North Dakota.
Some historians have argued that a virulent epidemic hitting an epidemiologically inexperienced population would beespecially damaging because it kills adults rather than children. It takes a long time and a lot of investment to produce an adult, so they are hard to replace. Since adults do most of the productive labor and produce most of the food, this matters. A population can survive a disease that kills 20 percent of the population in childhood more easily than it can survive one that kills 20 percent of the population in early adulthood. This effect may have increased the impact of the first wave of epidemics in the New World. Along the same lines, an epidemic that sickens nearly everybody may leave too few caretakers to nurse those who could survive if they were fed and kept warm. However, this effect can't explain why infectious diseases kept hitting Amerindians harder than Europeans in epidemic after epidemic over hundreds of years.
Although factors such as a paucity of domesticated animals decreased the probability that the Amerindians would develop really potent infectious diseases of their own, it must have been possible. In principle, they might have had their own equivalents of smallpox and malaria. But if such potent diseases had been brought back to the Old World by European explorers, civilization would have fallen and you wouldn't be reading this right now. Some have also said that cultural inexperience might have worsened these epidemics, as when stricken Indians ran from epidemics (thus spreading the disease further) or tried various ineffective therapies. But Europeans ran from epidemics, too (as in Boccaccio's Decameron), and European medicine in those days was generally useless. (Charles II's doctors in the 1600s treated his fits with bleeding, cupping, emetics, laxatives, enemas, blistering plasters, Spanish Fly, and more bleeding, then plastered the soles of his feet with tar and pigeon dung,gave him a bezoar [a concretion found in the stomach of a goat, thought to neutralize any poison], and followed this up with more bleeding. None of it worked.)10
But wasn't Spanish oppression of the Amerindians the main factor reducing their numbers? We think not. Of course, the Spanish did oppress the Amerindians, but they were hoping to become the new lords of those lands, rather than farming the land themselves. Lords need serfs—live serfs. Spanish demands for labor and food must have made the situation worse, but depopulation raced far ahead of Spanish administrative control. For example, when Hernando de Soto explored the American South in 1539, he found many fair-sized towns, but also ghost towns that had been recently abandoned. Old World diseases (likely smallpox) had gotten there first, just as smallpox had reached Peru before Pizarro.
Furthermore, the Spanish began to conquer the Philippines in the sixteenth century, and there's no sign that they caused a population collapse there.11 Whenever Europeans made contact with a long-isolated people, whether Amerindians or Australian Aborigines or Polynesians, there was a population crash. When Europeans conquered peoples who had already had extensive contact with other Old World populations, as the British did in India or the Dutch in Indonesia, there was no crash. As Charles Darwin said, "Wherever the European has trod, death seems to pursue the aboriginal. We may look to the wide extent of the Americas, the Cape of Good Hope, and Australia, and we find the same result."12
Those who refuse to acknowledge the crucial role of biological differences in the European conquest and settlement of the Americas are rejecting Darwinian evolution. Thousands of yearsof high disease load among Eurasian agriculturalists had to select for increased disease resistance. This isn't particularly unusual or unorthodox, yet many who claim to accept the idea of natural selection reject most of the obvious implications of the theory when it is applied to humans.
Of course, as the Duke of Wellington said, two can play at this game. When the Europeans tried to conquer and settle sub-Saharan Africa, the shoe was on the other foot.
HEART OF DARKNESS
Europeans had long been aware of the lands south of the Sahara, but in the fifteenth century they knew very little about them. The information they did have was largely an inheritance from classical civilization. Hanno the Navigator (of Carthage) had explored the west coast of Africa, and Herodotus tells us of an earlier Phoenician expedition, sent out by the Pharaoh Necho around 600 BC, that seems to have circumnavigated the continent. Somehow, the Greeks acquired some information about central Africa, including facts about the Pygmies: Aristotle said, "These birds migrate from the steppes of Scythia to the marshlands south of Egypt where the Nile has its source. And it is here, by the way, that they are said to fight with the pygmies; and the story is not fabulous, but there is in reality a race of dwarfish men."13 Another interesting hint is a 2,500-year-old frieze in Persepolis, the capital of the Persian Empire, which shows peoples from many lands bringing tribute. One panel shows a Pygmy with an okapi, a deep-forest relative of the giraffe that was only rediscovered by Europeans in 1901.
Sub-Saharan Africa may actually have been easier to reach and explore in classical times than it is today. The Sahara hadnot yet become as bone-dry as it is now: Horses could cross the desert until Roman times, and there are old, shallow wells in places where the water table is now thousands of feet below the surface. It's also possible—although this is speculation—that falciparum malaria wasn't as widespread in Africa in classical times as it is today. If true, African exploration might have been safer in those days.
When modern Europeans (Portuguese in the beginning) did start showing up along the coast of West Africa around 1500, they found wealth (mainly in gold and slaves), but they also faced incredible disease risks. The king of Portugal sent an expedition of eight up the Gambia River in about 1500: One came back alive. Joao de Barros, a Portuguese historian of the sixteenth century, said, "But it seems that for our sins, or for some inscrutable judgment of God, in all the entrances of this great Ethiopia we navigate along, He has placed a striking angel with a flaming sword of deadly fevers, who prevents us from penetrating into the interior to the springs of this garden, whence proceed these rivers of gold that flow to the sea in so many parts of our conquest."14 Europeans typically bought slaves in coastal outposts or islands: Going inland to seize them was just too dangerous to their health. Arab slavers ranged further, but many Arabs had genetic malaria defenses such as alpha- thalassemia, and many were part African.
These difficulties persisted for centuries. British soldiers stationed on the Gold Coast would lose half their numbers in a year. Early explorers did no better. Mungo Park began his second attempt at African exploration (in 1805) with a party of forty-five Europeans; only eleven were alive by the time they reached the Niger. He was eventually killed at the Bussa rapids—by Africans, not parasites—but when his son Thomaswent in search of him, he died of fever before getting far. We presume you've heard of Dr. Livingston—Dr. David Livingston, that is, the nineteenth-century British medical missionary to central Africa. His wife died of malaria
during their travels, and the doctor himself later died of malaria and dysentery. John Speke and Sir Richard Francis Burton, nineteenth-century British explorers, sought and eventually found the sources of the Nile—but both men fell ill of tropical diseases. Speke suffered greatly when a beetle crawled into his ear. He removed it with a knife, but he became temporarily deaf and later temporarily blind. Consider that these are the famous explorers, the ones who enjoyed some degree of success. What happened to the unlucky ones?
Europeans had a vast technological edge over most of the inhabitants of sub-Saharan Africa. In most ways (except for their use of iron tools), African technology and social organization were simpler than that of the Amerindians—at any rate simpler than the Andean and Mesoamerican civilizations. (Here we're speaking of the inhabitants of what has been called the "isolated zone," areas that had not been much influenced by Islamic civilization—especially west, central, and southern Africa.) Literacy, the wheel, sailing ships, and guns gave the Europeans a huge military advantage, but nothing came of it for hundreds of years, except in the far south, where a temperate climate allowed Dutch colonization.
In the 1800s, quinine became widely available, and that allowed Europeans to venture into interior Africa with moderate success, since falciparum malaria had been the deadliest of many African diseases. Later scientific advances controlled or eliminated a number of other local diseases, including yellow feverand sleeping sickness. This made possible the "scramble for Africa," in which European countries ranging from Great Britain to Italy conquered almost the entire continent. In these efforts, European military technology was a trump card. As Hilaire Belloc wrote in a poem, "Whatever happens / We have got / The Maxim gun /And they have not."
The 10,000 Year Explosion Page 15