Decisive as cultural adaptation to differing natural environments was for humankind’s extraordinary expansion between 40,000 and 10,000 B.C., there was another factor of considerable importance. In leaving tropical environments behind, our ancestors also escaped many of the parasites and disease organisms to which their predecessors and tropical contemporaries were accustomed. Health and vigor improved accordingly, and multiplication of human numbers assumed a hitherto unparalleled scale.13
Humanity’s place within the balance of nature in tropical regions differed fundamentally from what developed in temperate and Arctic climatic zones. As we have just seen, in sub-Saharan Africa humankind continued to confront biological checks that remained powerfully effective even after human hunting skills had upset the older balances of nature among large-bodied creatures. But when human communities learned to survive and flourish in temperate climes, they faced a simpler biological situation. In general, lower temperatures meant less propitious conditions for life. As a result, the forms of plants and animals adapted to temperate and northern climates were fewer in number than those that pullulated in tropical climes. Consequently a less richly articulated web of life greeted human hunters when first they burst upon the scene. Moreover, temperate ecological balances proved to be much more easily disrupted by human agency. The initial absence or near-absence of organisms capable of living parasitically on or inside human bodies was a passing phenomenon. In time, biologically and demographically significant diseases developed among human communities of temperate climes too, as we shall soon see. But the vulnerability of ecological balances to human manipulation remained a permanent feature of the extra-tropical scene.
Thus humankind’s biological dominion in temperate climes assumed a different order of magnitude from the start. As a stranger and newcomer to temperate ecological systems, humanity was in a situation like rabbits met when introduced into Australia. Lacking both natural predators and natural parasites in the new environment, and finding, at least to begin with, abundant food, the rabbit population of Australia grew enormously and soon began to interfere with human efforts to raise sheep. Similar swarmings of imported forms of life—pigs, cattle, horses, rats, together with a broad spectrum of plants—occurred in the Americas when Europeans first arrived as well. But these initial runaway population explosions soon created their own correctives.14
In a long enough time perspective, perhaps the same will be true of humankind’s expansion into the diverse and novel ecological environments of the temperate world. But on the sort of time scale we are accustomed to, in which centuries and millennia rather than eons matter, ordinary biological adjustments among diverse species have not been enough to check the multiplication of humankind. The reason is that cultural rather than biological adaptations generated and sustained the entire adventure, so that, as one particular pattern of human exploitation of the environment began to encounter difficulties, thanks to exhaustion of one or another key resource, human ingenuity found new ways to live, tapping new resources, and thereby expanded our dominion over animate and inanimate nature, time and again.
Riches in the form of woolly mammoths, giant sloths, and other large and inexperienced animals that awaited human slaughter did not endure for long. Indeed, one calculation suggests that skilled and wasteful human hunters took a mere thousand years to exterminate most large-bodied game in North and South America. According to this vision of the American past, hunters gathered in large, organized groups along a moving frontier where large-bodied game could be found. Within a few years they so depleted the herds that they had to move on, ever southward, until most American species of big game had been completely destroyed.15 Such a catastrophic pattern could of course only arise when skilled hunters collided with totally inexperienced game animals. In the Old World no such dramatic confrontation ever occurred. There, hunting skills were applied more gradually to the large-bodied herds of the North, if only because with each advance northward, the hunters had to adjust to a harsher climate and more arduous winters. In the Americas, on the contrary, the movement was from north to south, from severity to mildness. The result was a far more sudden and widespread extermination of large-bodied game than occurred in the Old World.
Subsequent discoveries of new techniques allowed people to re-enact this frontier phenomenon of easy exploitation and rapid depletion of resources over and over again. Current oil shortages outside the Middle East are only the most recent example of humanity’s spendthrift ways. Yet as a result of the Stone Age occupancy of the temperate and sub-Arctic parts of the globe, humankind also entered into a far more enduring new pattern of co-existence with other forms of life—a pattern that was to play an important role in later history. Human distribution across diverse climatic zones created what may be called a parasitic gradient among the different communities that resulted. The general thinning out of the variety of life forms that took place as climates became colder and/or drier implies, after all, a diminution in the number and variety of parasitic organisms capable of afflicting human beings. Conditions for successful transfer from host to host became more difficult as temperatures (and humidity) dropped and as the seasons of warmth and sunlight shortened. The effect was to create a gradient of infection and infestation such that populations from warmer, wetter climes could travel to cooler and/or drier regions with little likelihood of encountering unfamiliar parasites, whereas infections and infestations lurking in southern and warmer or wetter lands constituted a standing threat to intruders from the cooler North or drier desert.
The gradient may be described conversely as follows: the farther human populations penetrated into cold and/or dry climates, the more directly their survival depended on their ecological relations with large-bodied plants and animals. Balances with minute parasitic organisms, so important in the tropics, became comparatively insignificant.
This difference has an important corollary. Nearly all microparasites are too small to be seen with the unaided human eye, and this meant that until the invention of the microscope and other elaborate aids to human powers of observation, no one was able to understand or do much to control encounters with such organisms. Despite the intelligence which served humankind so well in dealing with things it could see and experiment with, relations with microparasites remained until the nineteenth century largely biological, that is, beyond or beneath human capacity for conscious control.
In places where microparasites were less pervasive and significant, however, intelligence could play freely upon the parameters of human life that mattered most. As long as men and women could see food and foe, they could invent new ways to cope with both; and by so doing eventually became no longer the rare predator that a hunting mode of life alone allowed. Instead human numbers proliferated into millions in landscapes where only a few thousand hunters had been able to exist. Escape from the tropical cradleland, therefore, had far-reaching implications for humanity’s subsequent role within the balance of nature, giving a much wider scope to cultural invention than had been attainable within the tighter web of life from which naked humanity had originally emerged.
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 so
me 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.
Plagues and Peoples Page 5