Plagues and Peoples (7 page)

Obviously, local conditions were capable of distorting this
general pattern. Densities of human populations, the character and quality of available water supply, food, and shelter, together with the frequency and range of contacts among individuals all could affect disease patterns significantly. Great cities were, until recently, always unhealthy, even when situated in cool or dry climates. Generally speaking, though, all such local disturbances of ecological relations have worked within a biological gradient characterized by an increase in the variety and frequency of infections as temperatures and moisture increased.
¹⁶

While it lasted, the expansion of Paleolithic hunting bands throughout the temperate and sub-Arctic zones of the earth constituted a period of unprecedented biological success for humankind. But by the time all available hunting grounds had been occupied, the most suitable game animals in older regions had been depleted and in some cases entirely destroyed by overkill.

Depletion of big-game food resources obviously created a crisis of survival for hunters at different times in different parts of the world. Such crisis coincided with radical changes of climate associated with the retreat (since about 20,000
B.C.)
of the most recent ice cap. These two factors presented human hunting communities with a series of severe environmental challenges. Wherever older ways ceased to work, the response was intensification of the search for food and experimentation with new sorts of things to eat. Exploitation of the sea marges, for example, led to the development of boats and fishing; gathering of edible seeds led other groups to develop agriculture.

Paleolithic hunters and gatherers in a rough way presumably recapitulated the experience of the earliest humanoids in their tropical cradleland. That is, once the obvious possibilities of new ecological niches had been exploited, a kind of rough equilibrium set in, whereby checks of various kinds supervened to halt the growth of human populations. What these were varied from place to place, community to community,
and time to time. Nonetheless, it seems probable that outside the tropical zones where humanity had itself evolved, disease organisms were not very important. Parasites that could spread from host to host by direct bodily contact, like lice, or the spirochete of yaws, could survive in temperate climates within small and migratory hunting communities. As long as the infection acted slowly and did not incapacitate the human host too severely or too suddenly, such parasitisms could and probably did travel with hunting communities from humanity’s tropical cradlelands throughout the earth. But the array of such infections and infestations was vastly diminished from what had thriven in the tropical luxuriance of humanity’s oldest habitat.

As a result, ancient hunters of the temperate zone were most probably healthy folk, despite what appear to be comparatively short life spans.
¹⁷
That they were healthy is also supported by what is known about the life of contemporary hunting peoples in Australia and the Americas. Except for formidable illnesses traceable to recent contacts with the outside world, these peoples, too, seem to have been quite free from infectious disease and from infestation by multicelled parasites.
¹⁸
Anything else would be very surprising, for there was not enough time for the slow work of biological evolution to devise organisms and patterns of transfer from host to host suitable for cool and dry conditions such as would be needed to maintain a tropical level of infection and infestation among the small and relatively isolated communities of hunters who penetrated the world’s temperate and sub-Arctic climates.

Before such adjustments could affect human life, new and fateful inventions again revolutionized humanity’s relationships with the environment. Food production permitted a vast and rapid increase in the number of people, and soon sustained the rise of cities and civilizations. Human populations, once concentrated into such large communities, offered potential disease organisms a rich and accessible food supply that was quite as unusual, in its way, as the big game of the African savanna had been for our remoter ancestors. Micro-organisms
in their turn could expect good hunting under the new conditions created by the development of human villages, cities and civilizations. How they took advantage of the new possibilities offered by human aggregation into large communities will be the theme of the next chapter.

II

Breakthrough to History

T

he numerous extinctions of large-bodied game animals that began in Africa about 50,000 years ago, spread to Asia and Europe about 20,000 years ago and became especially pronounced in the Americas some 11,000 years ago must have been a severe blow to human hunters whose skills had concentrated on killing big animals.
¹
Indeed, the disappearance of one species of large-bodied prey after another probably led to sharp local reductions in human numbers. It was one thing for a band to feed on a single mammoth for a week or more, and quite a different task to kill sufficient small game, day after day, to keep the same number of human beings alive. Simultaneously, climatic changes altered the balance of nature, both in northern regions along the fringes of the retreating glaciers, and in the subtropics where a northward shift of the trade winds spread desiccation across what had earlier been good hunting territory in the African Sahara and adjacent parts of western Asia.

Everywhere, therefore, ancient hunters had to readjust their habits to make fuller use of whatever they could find in changing
landscapes. When large-bodied animals disappeared, other foods had to be searched out. Under these pressures, our ancestors became omnivorous again like their distant primate forebears, feeding on an expanded number of plant and animal species. In particular, the food resources of shore and sea were for the first time systematically exploited, as numerous middens of discarded mollusk shells and far less conspicuous fishbones attest. Not only that; new ways of preparing food were developed. Certain groups learned, for instance, that by prolonged soaking, they could remove poisonous chemicals from olives and cassava, thus making them edible. Other vegetable matter could also be rendered more palatable or digestible by grinding, cooking, fermenting.
²

All these palliatives were, however, soon eclipsed by the development of food production, through domestication of animals and plants. Many communities in different parts of the earth moved in this direction, with results that varied in accordance with what was available in a wild state to start from. Generally speaking, although the New World was remarkably impoverished in domesticable animals, it did have a number of useful plants, whereas the Old World offered human ingenuity both a wide range of domesticable animals and an impressive array of potential food plants.

Details of early domestications remain unclear. One must assume a process of mutual accommodation between humanity and the various domesticable species. This involved rapid and sometimes far-reaching changes in the biological character of domesticated plants and animals as a result of both accidental and deliberate selection for particular traits. Conversely, one can assume that a radical, if rarely deliberate, selection among human beings occurred as well. Individuals who refused to submit to the laborious routines of farming, for instance, must often have failed to survive, and those who could not or would not save seed for next year’s planting, and instead ate all they had, were quickly eliminated from communities that became dependent on annual crops.

Herdsmen and farmers, together with their varying array of
domesticated animals and plants, fitted into the wild background of plant and animal life in different ways, depending on climate, soils and human skills (or lack thereof). Results varied markedly from village to village, field to field, and even, for that matter, within a single field.

Nevertheless, there are some general phenomena worth noting. First of all, as men made over natural landscapes by causing some animals and plants to multiply, others were displaced. The general effect was to reduce biological variety and to make local plant and animal populations more uniform. Simultaneously, food chains shortened as human action reduced the roles of rival predators and reserved an increased amount of food for the consumption of a single species:
Homo sapiens
.

Shortening natural food chains involved humankind in never-ending effort. Protecting herds and crops from animal predators was not a serious problem for skilled hunters, though it required perpetual vigilance. Protection from other men, however, was a different matter, and efforts to achieve safety from human marauders provided the chief stimulus to political organization—a process by no means completed yet.

More significant for human life, because it involved more continual effort by a larger proportion of the entire population, was the work of reducing weeds, i.e., trying to eliminate rival species competing with domesticated varieties of plants and animals for living space. Weeding by hand may indeed have been the first form of “agriculture,” but human powers achieved a new range when people learned how to remodel natural environments more radically, widening the ecological niche available to their preferred crops by eliminating natural climax vegetation. Two methods proved effective: artificial flooding of land naturally dry, and mechanical alteration of soil surfaces by digging and plowing.

Flooding allowed humans to drown out the competing species. When the agricultural year could be arranged so that part of the time fields lay under water while at other times the water was allowed to run off so that the land dried out, weeds
were not much of a problem. Few plant species could thrive under alternating extreme conditions of wet and dry; fewer still could survive when farmers deliberately adjusted periods of flood and drought to suit the needs of the desired crop by simply opening and closing cunningly arranged sluices. Of course, only crops that flourish under shallow water benefited by such a regime: rice above all. But other less valuable root crops can be raised in this fashion also.

The mechanical disturbance of soil by digging-stick, hoe, spade, or plow is far more familiar to Westerners, since this was the type of agriculture that established itself in the ancient Near East and spread thence to Europe. It also prevailed at the other centers of early agricultural development in the Americas and Africa. An initial phase—slash-and-burn cultivation—depended on destroying deciduous forest by girdling the trees. This allowed sunlight to flood the forest floor and sustain the growth of grains in an environment from which competing grasses were absent. This style of cultivation, however, even when supplemented by burning the dead trees and scattering ashes on the soil to renew fertility, was not stable. Air-borne seeds soon established a lush growth of thistles and similar weeds in forest clearings. Given a year or two in which to establish themselves, these intruders were fully capable of crowding out the crop. Only by moving on to start anew with a first year’s weed-free crop on virgin land could the most ancient Near Eastern, Amerindian, and African farmers keep going.

These initial limitations were transcended in the ancient Near East by the invention of plowing, not long before 3000
B.C
. Plowing allowed effective weed control, year in and year out, so that fields could be cultivated indefinitely. The secret was simple. By substituting animal for human muscles, the plow allowed ancient Near Eastern farmers to cultivate twice the area they needed for cropland, so that when the extra land was fallowed (i.e., plowed during the growing season so as to destroy weeds before seeds had formed), it created a suitably empty ecological niche into which next year’s crop might
safely move without being too severely infested by locally formidable weed species.

It is a testimony to humanity’s animistic propensities that most textbooks still explain how fallowing allows the earth to restore fertility by having a rest. A moment’s thought will convince anyone that whatever processes a geological weathering and consequent chemical change occur in a single season would make no noticeable difference for the following year’s plant growth. To be sure, in the case of “dry farming,” soil kept in a bare fallow can store moisture that would otherwise be dispersed into the air by passage of water from the soil through the roots and leafy parts of plants. In regions where deficient moisture limits crop yields, a year’s fallowing can, therefore, increase fertility by letting subsoil moisture accumulate. Elsewhere, however, where moisture is not the critical limit to plant growth, the great advantage of fallowing is that it allows farmers to keep weeds at bay by interrupting their natural life cycle with the plow

Digging (or flooding) would of course achieve similar results; but in most environments human muscles did not alone suffice to break up enough land in a year to allow a family to subsist on the crop that could be harvested from only half of the cultivated area, while the rest was fallowed. Special soils and ecological conditions did allow some exceptions. The two most significant were (1) North China, where friable and fertile loess soil permitted human populations to subsist on crops of millet without the assistance of animal strength hitched to the plow; and (2) the Americas, where the high calorie yield per acre of maize and potatoes as compared with the Old World crops like wheat, barley, and millet, led to similar results even on soils less easily tilled than the loess of China.
³

One must admire the skill with which humankind discovered and exploited the possibilities inherent in remodeling natural landscapes in these radical ways, increasing human food supply many times over, even though it meant permanent enslavement to an unending rhythm of work. To be sure, the plow used animal strength to pull the share through the
soil, and the plowman’s life was generally less toilsome than the lot that fell to the rice farmer of East Asia, who used his own muscles for most of the tasks of water and soil engineering required to create and maintain paddy fields. But toil—persistent, unending, and fundamentally at odds with humankind’s propensities as shaped by the hunting experience—was nevertheless the lot of all farming populations. Only so could man the farmer successfully distort natural ecological balances, shorten the food chain, magnify human consumption and multiply human numbers until what had been a relatively rare creature in the balance of nature became the dominant large-bodied species throughout the broad regions of the earth susceptible to agriculture.