The Lethal Gifts of Agriculture

Permanent settlements developed independently in several parts of the world, including the Middle East, China, and the Americas. But those that have been best studied are found in the so-called Fertile Crescent, a region bounded by the Tigris and Euphrates Rivers and curving around the Mediterranean and the Nile Valley to include present-day Syria, Lebanon, Israel, Egypt, Turkey, Jordan, Iran, and Iraq. The oldest village known, just outside present-day Jericho in Israel, may have sprung up around a shrine used by roving bands of hunters and gatherers. By 10,500 years ago it had evolved into a small farming village. At first this settlement and others like it were simply collections of villages on the banks of natural streams, but soon they were able to spread out via networks of irrigation canals. The surplus of food and the practice of irrigation contributed to larger and larger concentrations of people, allowing some people to quit farming and to become full-time artisans, priests, or members of other professions. Meanwhile, the farmers who provided the food for these ever-enlarging villages continued to live on their outskirts. And 5,500 years ago the first undisputed city—a place where farmers do not live—Uruk in Mesopotamia (present-day Iraq), was established.

We have little precise information about the parasitic diseases that afflicted our ancestors more than 10,000 years ago. To be sure, they had their parasites, but we believe their impact may not have been quite as severe as in the period that stretches from the present back to the time of the earliest cities. The reason for this is that the pattern and the impact of disease depend on several factors: the population density, the character and quality of the water supply, food and shelter, the frequency of contact among individuals, human contact with animals, and the climate. Once human populations were concentrated into larger and larger communities and their numbers increased, however, then the enhanced potential for infectious disease organisms to be transmitted could (and did) affect the size and well-being of the population. Farming and domestication also reduce the biological variability of animals and crop plants, leading to purebred strains, making local populations of plants and animals more and more uniform. As a consequence, any upset in the balance could decimate an entire population, whether that was a crop or a flock or a herd. (A modern example of this is the 1845-1851 potato famine in Ireland.) Agriculture necessitated artificial flooding of land, which was naturally devoid of adequate amounts of water, enabling longer growing seasons; it also required tilling or plowing and replenishing the soil with fertilizer. The cheapest and most available of fertilizers is human waste or animal feces. Intensive agricultural practices, requiring crop protection from weeds and pests, demand some kind of control measures. Tilling the soil necessitates some kind of work force: animal power, human power, or machines. Disruptions in the availability of these 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.”