could lead to disaster. With purebred strains (monocultures), disruption becomes that much easier. Living in villages or cities—that is, in permanent settlements—involves the risk of parasite invasion. Those infected with intestinal parasites can through their feces more easily transmit disease to others, and where the water supply becomes contaminated through the use of either night soil or fecally contaminated streams, the spread of disease can be great indeed. A single contaminated water source serving a large population can be a much greater threat than several sources supplying smaller bands of hunters and gatherers. Irrigation practices thus created a favorable environment for transmission of parasites—moisture was abundant; there was a liquid medium in which parasites and/or their cysts and eggs could persist; and the water could also be used for drinking, bathing, washing of clothes, and waste disposal.
The disruptive effects of an epidemic disease are more than simply the loss of individual lives. Often the survivors are demoralized, they lose faith in inherited customs, and if it affects the working age group, it can lead to a material as well as a spiritual decline. As a consequence, the cohesion of the community may collapse and it may become susceptible to invasion from neighbors. Once disease is widespread in an agricultural community, it can produce a listless and debilitated peasantry, handicapped for sustained work in the fields, for digging irrigation canals, and for resisting military attack or throwing off alien political domination. All this may allow for economic exploitation.
The smaller population size of hunters and gatherers makes it seem probable that person- to-person “civilized” infectious diseases, such as measles, influenza, smallpox, and polio, could not have established themselves, because these are density-dependent diseases requiring a critical number of individuals for transmission. Although there is no hard literary or archeological evidence, it does seem reasonable to suggest, as did William McNeill, that “the major civilized regions of the Old World each developed its own peculiar mix of infectious, person-to-person diseases between the time when cities first arose (about 3000 BC) and about 500 BC. Such diseases and disease-resistant populations were biologically dangerous to neighbors unaccustomed to so formidable an array of infections. This fact made territorial expansion of civilized populations much easier than would otherwise have been the case.”
The Accident That Caused Societal Differences
Imagine for a moment that you are living in the year 1492 and you have just graduated from the University of Padua in Italy with a degree in medicine. Before you are able to set up your practice, you receive a letter from an old friend, Giuseppe Diamonte, who writes from Spain that King Ferdinand and Queen Isabella are about to provide funds for the discovery of a new route to India; the expedition is to be under the command of a fellow Italian, Christopher Columbus, and he is in need of a naturalist to collect plants and animals and to act as the ship’s doctor. Giuseppe writes that he has recommended you for the position. You are enthusiastic, so you board a ship in Venice, land in Barcelona, and travel by horseback to Palos, Spain. The journey across the Atlantic Ocean begins on August 3. Upon arrival in “India” (in actual fact the present-day Dominican Republic) on October 12, you are astounded to be greeted by a band of near-naked “Indians” who have paddled their canoes to greet the ship and its crew. No iron tools or oceangoing vessels can be seen, the village consists of a scattering of huts, and there is little that could be considered a city. The natives have no writing, and what agriculture there is is on an entirely different scale from that with which you were familiar in Europe. Ten weeks earlier you left iron tools and weapons, agriculture, oceangoing ships, large cities, horses, carts and carriages, writing, money, banking, painting, sculpture, cathedrals, palaces, buildings of brick and stone, established religion and ritual, and music. You are perplexed. You ask yourself: Why has the rate of technological and political development been so much faster in Europe-Asia than in the Americas? In short, why were the Americas technologically a few thousand years behind Europe? Why were stone tools, comparable to those used by Europeans and Asians 10,000 years earlier, being used?
Fast forward to the future. In Guns, Germs, and Steel, Diamond argues persuasively that it was not biological differences but geography that was the decisive element. It was differences not in the braininess or genetics of the human populations but in the plant and animal resources available on a particular continent—an accident of geography—that made the difference. Diamond believes that the fortuitous accident began in the Fertile Crescent, which contained a suitable array of plants and animals, called “founders,” and that these formed the basis for domestication. What were the founder plants? Those locally available in Southwest Asia/Fertile Crescent that would not serve as the basis for domestication were plants with a large amount of indigestible material such as bark or that were poisonous, low in nutritional value, or tedious to prepare and gather. The desirable attributes of plants that make them suitable for domestication include having a larger proportion of edible parts (large seeds) and a lower proportion of woody, inedible parts; being easy to harvest en masse (with a sickle); a seasonal nature; being easy to grind, easy to sow, and easily stored; and being high in yield and high in calories. Plants with these characteristics were selected for domestication. They fall into four categories: grasses (wheat, barley, oats, millet, and rice), legumes (peas and beans), fruit and nut trees (olives, figs, dates, pomegranates, grapes, apples, pears, and cherries), and fiber crops (flax, hemp, and cotton).
Of the 148 big wild terrestrial plant-eating animals (herbivores)—those suitable for domestication—only 14 were able to serve as founders. What were these founder animals that could be domesticated? In Europe and Asia in about 4000 B.C. it was the “Big Five”—sheep, goats, pigs, cows, and horses. In East Asia the cow was replaced by the yak, water buffalo, and gaur. On the other hand, although in the Americas there were mountain sheep and goats, llamas, bison, peccaries, and tapirs and in Australia there were kangaroos and in Africa zebras, buffaloes, giraffes, gazelle, antelope, elephants, and rhinoceroses, all of the latter were unsuitable for domestication. Domestication involves more than taming and requires a special suite of animal characters: social species that occupy territories and animals that are herbivores. A potentially domesticated animal species must also have the right reflexes—it must be predictable and not panic easily, and it must not be ferocious or nasty in disposition. It must grow quickly, and it must be able to breed in captivity. The appropriate domesticated animals were the cow, goat, horse, sheep, donkey, yak, and camel. Once these animals were domesticated, what benefits did they provide? Food in the form of meat and milk, clothing and fiber from wool and hides, manure for use as a fertilizer, and animal power for land transport of goods and people, as well as for plowing fields. Indeed, before there were domesticated beasts of burden, the only means for moving goods and people across the land was on another person’s back! Domesticated horses, goats, camels, and cows were hitched to wagons to move humans and their possessions, and reindeer and dogs were used to pull sleds across the snow. Horse-drawn chariots revolutionized warfare, and after the invention of saddles and stirrups, it became possible for marauding Huns on horseback to strike fear into the legions of Rome.
There was a downside to animal domestication. Domesticated animals could be the source of human disease. As human populations settled down, they created heaps of waste—middens of animal bones, garbage, and feces. These served as the breeding grounds for and a source of microparasites; they also attracted insects that could act as vectors of disease, as well as wild birds and rodents carrying their own parasites and potential new sources of human disease. With each domesticated species of animal came the possible human exposure to new disease agents—parasites. For example, the numbers of diseases acquired from domestic animals (zoonotically) has been estimated to be: dogs, 65; cattle, 45; sheep and goats, 46; pigs, 42; horses, 35; rats, 32; and poultry, 26. Specifically, the human measles virus has its counterpart in the distemper virus of dogs and rinderpest in cattle. Smallpox has its closest relatives in the virus of cows and poxviruses in pigs and fowl, and human tuberculosis is a cousin of bovine tuberculosis. More recent examples of the “jump” from one animal species to another include HIV, in which a chimpanzee virus became humanized; monkey pox transmitted to humans by the bite of pet prairie dogs; SARS from civet cats; and Ebola from bats.
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