Читать книгу The Power of Plagues - Irwin W. Sherman - Страница 21

Human history took off 50,000 years ago in what Jared Diamond, professor of geography and physiology at the University of California at Los Angeles School of Medicine, called “the Great Leap Forward.” Fifty thousand years ago H. sapiens used standardized stone tools that could be used for cutting, scraping, and grinding, as well as pieces of bone that could be fashioned into fishhooks and spears, needles, awls, harpoons, and eventually bows and arrows. These tools could also be used as weapons, and now humans could begin to hunt down and kill their animal prey at a distance. Not only did these early humans use the meat of animals for their nourishment, but they began to clothe themselves in the skins of these animals. Through the invention of rope it was possible to make snares and nets so that birds and fishes could also become part of the diet. All this attests to the fact that between 100,000 and 50,000 years ago there was a significant change in the cognitive capacity of the human brain without a significant change in its size. Coupled with this change in brain organization was the anatomical improvement of the voice box; now humans could not only speak but also begin to develop language.

For 99% of human existence we were hunter-gatherers, so why some 10,000 years ago did we settle down to become farmers? This change from hunting and gathering to farming has been termed the agricultural revolution, the time when humans domesticated plants and animals and exerted control over food production. Although the term “revolution” would seem to indicate that it appeared suddenly and dramatically, this was certainly not the case. The human control of food production was not discovered or invented, nor was it a conscious choice made by our ancestors “to farm” or “not to farm,” since at the time there would have been no farmers to serve as role models. No, domestication of plants and animals evolved as a consequence of human choice made without any awareness of its future long-term consequences.

Development of techniques and practices for agriculture and animal husbandry progressed step by step in sequential fashion. They were not developed over a short time, and not all the wild animals and wild plants that would eventually be domesticated in a particular region would be domesticated at the same time. Indeed, it probably took thousands of years to shift the human diet from wild foods alone to foods both cultivated and wild. The reason for this time lag is that food production evolved as a result of the accumulation of many separate choices, and there were trade-offs, especially in the allocation of time and effort.

Consider for the moment that you are a hunter-gatherer who has accumulated enough wisdom and technology to set up a small garden. Some of the choices you would be faced with are: Which plants should I grow? How much time should I spend planting instead of hunting or scavenging? What are the benefits of tending the garden over going out to hunt and gather wild plants? Perhaps your most important consideration might be which of the two, hunting or gardening, will save you from starvation in the future. It has been speculated by Jared Diamond that “all other things being equal, people seek to maximize their return of calories, protein or other specific food categories by foraging in a way that yields the most return with the greatest certainty in the least time for the least effort. Simultaneously they seek to minimize the risk of starving. … One suggested function of the first gardeners 11,000 years ago was to provide a reliable reserve larder as insurance in case wild food supplies failed.” Although the factors that contributed to the shift from hunting and gathering to farming still remain controversial with regard to their relative importance, one thing is certain: once there was a shift from nomadic hunting and gathering to more-sedentary food production, there could be no turning back.

At the end of the Pleistocene era (~11,000 years ago), the climate began to change: the glaciers had receded; the climate became milder and drier; and many large mammals had become extinct in Europe, Asia, and Africa. This led to a decline in the abundance of wild game, and hence the life of the hunter-gatherer became more precarious: to obtain the same amount of food as in the past would require the expenditure of greater amounts of time and energy. The reduction in the number and kind of game animals was coupled with a change in climate that favored plants with the potential for domestication, particularly the cereal grains. Now there were larger and larger areas with wild cereals, and these could be harvested with little difficulty, using stone and bone tools fashioned into sickles with flint blades. In addition to the technologies for harvesting cereals such as wild barley and wheat, the technologies needed for processing and storing these grains came into being. These technologies, which appear crude and simple by today’s standards, allowed for the first unconscious steps of plant domestication to take place. Tools included baskets to carry the grain from the field to the home base; mortars and pestles to remove the husks and to pulverize the grain; a technique of heating the grain to allow storage without sprouting; and the construction of underground storage pits, some of which were made waterproof by plastering. Coincident with farming, the density of the human population increased. It is not clear whether the rise in density of human populations led to the domestication of plants and animals or vice versa, but it is certainly true that with the availability of more and more calories it was possible to feed more and more people. Diamond, in his book Guns, Germs, and Steel, has observed that the adoption of food production is an autocatalytic process, a positive feedback, in which a gradual rise in population densities required that people obtain more food, and in turn those who took the steps to produce it were rewarded. Once “farming” began, people could become more and more sedentary—they settled down. In turn, birth spacing could be shortened, resulting in more births and larger families, who required still more food, and so on.

Farming populations became better nourished thanks to an increase in the availability of the number of edible calories per square mile, and eventually farmers replaced the nomadic groups of hunter-gatherers by converting them to engage in the practice of farming or by displacing them by the sheer force of greater numbers.

The life of nomadic hunter-gatherers was such that population levels were well below the maximum limit that would be imposed by their reproductive biology and the availability of food. What then limited their increase? The inability of the hunter-gatherer mother to carry more than a single child along with her normal baggage, coupled with her inability to nurse more than one child at a time, limited the practical interval between births to four years. It is likely that hunter-gatherers effectively spaced their children by means of lactation amenorrhea, sexual abstinence, infanticide, and spontaneous abortion. In contrast, once humans settled down, they were freed from the encumbrances imposed on the hunters and gatherers who had to carry their children around, so that now they could have as many children as they could bear and raise. Consequently, the birth interval for the “farmer” was reduced to two years. Agriculture also encouraged higher birth rates because additional children provided cheap labor. Further, farming had another advantage over hunting and gathering: more calories could be produced per unit land area and time expended. While 200 square miles could support 50 to 60 hunter-gatherers, more than 10,000 “farmers” could be supported on this same land area. The higher birth rate of the food producers, together with their ability to feed more people per square mile, allowed these “farmers” to achieve much higher population densities than those who were engaged in hunting and gathering.

Once agriculture and animal husbandry yielded a surplus of food that could be stored, there was a need for some members of the sedentary population to guard it. This was of course impractical for the nomadic hunter-gatherers since they would have to both hunt and gather and at the same time protect their bounty from others. But the availability of surplus stored food allowed members of the settled population to specialize: some became guards, and others were armed and served as soldiers who could group together to steal food from others. Perhaps this foreshadowed the origins of war. And when food production did not require every member of the settled community to directly work the land, then it became possible for some members of the group to engage in other activities. Once humans formed agriculturally based societies, those who chose to participate in such group activity may have been at a competitive advantage over those who did not. The latter likely became outcasts. Hence the agricultural revolution may have provided selective advantages to those who were in a controlled group, and superstition and organized religious practices were effective means for control by promoting group cohesion. It may be not be accidental that the first known and highly organized religions arose coincident with the agricultural revolution.

Some of the surplus food could be used to feed those “who provide religious justification for wars of conquest, artisans such as metalworkers who develop swords, guns and other technologies; and scribes, who preserve far more information than can be remembered accurately.” In time, political stratification would develop: heading the settled community would be the elite, consisting of hereditary chiefs (or kings) and bureaucrats. Under the appropriate circumstances these complex political units, which governed “the settled,” could also be mustered into formidable armies of conquest. Stored food and the land upon which it was grown became valuable resources that could be taxed, and surpluses could be traded for other goods; commerce and banking began to emerge. Thus, with larger populations family and inheritance schemes result, class structures with elaborate religious practices emerge, and writing is invented. Through agriculture and its prospect for increased food production, there was a population expansion that favored technological advances, as well as the development of cities (urbanization) and the rise of civilizations.

Another consequence of humans settling down was an increase in the amount of human disease. Agriculture by itself did not create new infections; it simply accentuated those that were already present or it converted an occasional event into a major health hazard. This was largely due to the fact that transmission of infectious agents becomes easier as individuals are crowded together; the practice of using human excrement (“night soil”) or animal feces (manure) as fertilizer allows for the transmission of infective stages; and finally, the closer association with domestic animals allows for their diseases to be transmitted to humans.