The Power of Plagues. Irwin W. Sherman

Homo neanderthalensis. The Neanderthals, who previously have been depicted as brutish cavemen (Fig. 2.4a and b), have been “modernized” on the basis of a brain size slightly greater than our own, and they left evidence showing that they cared for the sick and performed ritual burials (Fig. 2.4c). Their stone tools, however, were cruder than those of Cro-Magnon man. Within a few thousand years the Cro-Magnons, with their superior weapons and other advanced cultural practices, had completely displaced the Neanderthals. In Africa there are skeletal remains that are more modern than those of Neanderthals, dating back 200,000 years. Thus, nearly a quarter of a million years ago, populations of H. sapiens lived in Africa, Asia, and Europe. Their migration into the Americas took place ~35,000 to 12,000 years ago when they crossed the land bridge at the Bering Strait from Asia into the Americas. Forty thousand years ago humans moved into Australia. Thus, over the past 5 million years, new hominid species have emerged, coexisted, competed, and colonized new environments, in some instances succeeding, in others becoming extinct. The fossil record is of necessity incomplete, and much will be learned from future anthropological digs, but what is certain is that we did not arrive by a straight-line descent from the apes; we are not the single topmost limb in the hominid evolutionary tree, but simply one of its many branches.

Figure 2.4b A 1953 B- grade movie poster representing a monster-like Neanderthal man. Courtesy of Popcorn Posters.

Figure 2.4c Neanderthal Family, Reconstruction. Ian Tattersall, American Museum of Natural History

      Hunter-gatherers, unable to preserve and store fruits, vegetables, and meat, were forced to roam over large distances in search of wild edible plants and to hunt down game animals and find sources of drinking water. Moving from place to place, these nomadic bands were not surrounded by heaps of rotting meat or feces, and exposure to parasite-infested waters was limited. Though the hunter-gatherers did come together in groups, the size of their populations was small, and so diseases of crowds requiring human-to-human transmission were absent. Based on what we know about modern hunter-gatherer societies like those in present-day New Guinea, the Australian aborigines, and the Kalahari bushmen, we believe our hunter-gatherer ancestors were a relatively healthy lot. Gradually, however, conditions would change as the size of human populations increased and people adopted sedentary habits—living for extended periods of time in permanent or semipermanent settlements. This would, over time, dramatically increase the incidence of human disease.

      The Road to Plagues: More Humans, More Disease

      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.

      The Effect of Agriculture

      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