Читать книгу The Power of Plagues - Irwin W. Sherman - Страница 20

Today we speak of the problems associated with the population bomb—the unbridled growth of humans—that threatens our very existence. This growth in human populations cannot be calculated with any certainty until the middle of the 18th century, but we can make some educated guesses. Three hundred thousand years ago there were 1 million; 25,000 years ago that number had grown to 3 million; and 10,000 years ago the estimated human population was 5 million. By A.D. 1 it was 300 million. The phenomenal growth spurt in the human population coincides with the initiation of agriculture and the domestication of animals, which is generally dated to 8000 B.C. Between 8000 B.C. and A.D. 750, the population of the world increased 160 times to 800 million. Not only was the human population increasing, so too was overcrowding. For example, in 8000 B.C. human density was 0.2 people per square mile, but by 4000 B.C. it was 4 people per square mile.

What is the basis for this growth in the human population? The English clergyman Thomas Malthus (1766-1834) wrote An Essay on the Principle of Population in 1798, in which he stated that a population that is unchecked increases in geometric fashion. Malthus assumed that there would be a uniform rate of doubling, and this is of course naive, because it leads to impossibly large numbers. (By way of example, if you doubled a penny every day over a month, the final amount would be >$20 million.) It has been said that explosions are not made by force alone, but by a force that exceeds restraint. As Malthus correctly observed, there are factors that will eventually bring population growth to a halt; for example, restraint could result from the fact that the food supply increases only arithmetically. The consequences of unrestrained population growth, in Malthus’s words, would lead to “misery and vice,” or, in today’s vernacular, to starvation, disease, and war. These would tend to act as “natural restraints” on population growth. Thus, the Malthusian model suggested that a natural population has an optimal density.

If we were to make a graph plotting the human population on an arithmetic scale from 500,000 years ago to the present, we would find that the resulting curve suggests that the population remained close to the baseline from the remote past to about 500 years ago, and then it surged abruptly as a result of the scientific-industrial revolution (Fig. 2.5). More instructive, however, would be to plot the same data for a longer time period using a logarithmic scale, since this allows for more of the data points to be placed in a smaller space. This log-log plot reveals that the human population has moved upward in a stepwise fashion, and that there were three surges: those reflecting the development of tool making or the cultural revolution, followed by the agricultural, and finally by the scientific-industrial revolution. What were the checks on human growth rates that limited population size so that at equilibrium (the flat part of the logarithmic “curve”) there was a zero rate of change and the number of deaths equaled the number of births? Two kinds of checks occurred to set the upper limit (or the set point) for population growth: external or environmental factors (including limited food, space, or other resources) and self-regulating factors (such as fewer births, deliberate killing of offspring, or an increased death rate due to accidents or more-virulent parasites). For Malthus, disease and warfare as well as “moral restraint” (birth control) acted as “natural restraints”—the Four Horsemen of the Apocalypse: Disease, Famine, War, and the Pale Rider, Death. Indeed, it has been estimated that prior to the introduction of agriculture the earth could have supported a population of between 5 million and 10 million people who were engaged in hunting and gathering. Agriculture changed the environmental restraint so that the set point, or upper limit, of population size was increased.

Figure 2.5 A. Growth of the human population for the last 500,000 years. If the Old Stone Age (Paleolithic) were in scale it would reach 18 feet to the left. B. Log-log plot of the human population over the last million years.