Contemporary hunter-collectors in general lack effective chemical or mechanical means of preventing pregnancy—romantic folklore about herbal contraceptives notwithstanding. They do, however, possess a large repertory of chemical and mechanical means for inducing abortion. Numerous plant and animal poisons that cause generalized physical traumas or that act directly on the uterus are used throughout the world to end unwanted pregnancies. Many mechanical techniques for inducing abortion are also employed, such as tying tight bands around the stomach, vigorous massages, subjection to extremes of cold and heat, blows to the abdomen, and hopping up and down on a plank placed across a woman’s belly “until blood spurts out of the vagina.” Both the mechanical and chemical approaches effectively terminate pregnancies, but they are also likely to terminate the life of the pregnant woman. I suspect that only a group under severe economic and demographic stress would resort to abortion as its principal method of population regulation.
Hunter-collectors under stress are much more likely to turn to infanticide and geronticide (the killing of old people). Geronticide is effective only for short-run emergency reductions in group size. It cannot lower long-term trends of population growth. In the case of both geronticide and infanticide, outright conscious killing is probably the exception. Among the Eskimo, old people too weak to contribute to their own subsistence may “commit suicide” by remaining behind when the group moves, although children actively contribute to their parents’ demise by accepting the cultural expectation that old people ought not to become a burden when food is scarce. In Australia, among the Murngin of Arnhem Land, old people are helped along toward their fate by being treated as if they were already dead when they become sick; the group begins to perform its last rites, and the old person responds by getting sicker. Infanticide runs a complex gamut from outright murder to mere neglect. Infants may be strangled, drowned, bashed against a rock, or exposed to the elements. More commonly, an infant is “killed” by neglect: the mother gives less care than is needed when it gets sick, nurses it less often, refrains from trying to find supplementary foods, or “accidentally” lets it fall from her arms. Hunter-collector women are strongly motivated to space out the age difference between their children since they must expend a considerable amount of effort merely lugging them about during the day. Richard Lee has calculated that over a four-year period of dependency a Bushman mother will carry her child a total of 4,900 miles on collecting expeditions and campsite moves. No Bushman woman wants to be burdened with two or three infants at a time as she travels that distance.
The best method of population control available to stone age hunter-collectors was to prolong the span of years during which a mother nursed her infant. Recent studies of menstrual cycles carried out by Rose Frisch and Janet McArthur have shed light on the physiological mechanism responsible for lowering the fertility of lactating women. After giving birth, a fertile woman will not resume ovulation until the percentage of her body weight that consists of fat has passed a critical threshold. This threshold (about 20–25 percent) represents the point at which a woman’s body has stored enough reserve energy in the form of fat to accommodate the demands of a growing fetus. The average energy cost of a normal pregnancy is 27,000 calories—just about the amount of energy that must be stored before a woman can conceive. A nursing infant drains about 1,000 extra calories from its mother per day, making it difficult for her to accumulate the necessary fatty reserve. As long as the infant is dependent on its mother’s milk, there is little likelihood that ovulation will resume. Bushman mothers, by prolonging lactation, appear to be able to delay the possibility of pregnancy for more than four years. The same mechanism appears to be responsible for delaying menarche—the onset of menstruation. The higher the ratio of body fat to body weight, the earlier the age of menarche. In well-nourished modern populations menarche has been pushed forward to about twelve years of age, whereas in populations chronically on the edge of caloric deficits it may take eighteen or more years for a girl to build up the necessary fat reserves.
What I find so intriguing about this discovery is that it links low fertility with diets that are high in proteins and low in carbohydrates. On the one hand, if a woman is to nurse a child successfully for three or four years she must have a high protein intake to sustain her health, body vigor, and the flow of milk. On the other hand, if she consumes too many carbohydrates she will begin to put on weight, which will trigger the resumption of ovulation. A demographic study carried out by J. K. Van Ginneken indicates that nursing women in underdeveloped countries, where the diet consists mostly of starchy grains and root crops, cannot expect to extend the interval between births beyond eighteen months. Yet nursing Bushman women, whose diet is rich in animal and plant proteins and who lack starchy staples, as I have said, manage to keep from getting pregnant four or more years after each birth. This relationship suggests that during good times hunter-collectors could rely on prolonged lactation as their principal defense against overpopulation. Conversely, a decline in the quality of the food supply would tend to bring about an increase in population. This in turn would mean either that the rate of abortion and infanticide would have to be accelerated or that still more drastic cuts in the protein ration would be needed.
I am not suggesting that the entire defense against overpopulation among our stone age ancestors rested with the lactation method. Among the Bushmen of Botswana the present rate of population growth is .5 percent per annum. This amounts to a doubling every 139 years. Had this rate been sustained for only the last 10,000 years of the old stone age, by 10,000 B.C. the population of the earth would have reached 604,463,000,000,000,000,000,000.
Suppose the fertile span were from sixteen years of age to forty-two. Without prolonged nursing, a woman might experience as many as twelve pregnancies. With the lactation method, the number of pregnancies comes down to six. Lowered rates of coitus in older women might further reduce the number to five. Spontaneous abortions and infant mortality caused by disease and accidents might bring the potential reproducers down to four—roughly two more than the number permissible under a system of zero population growth. The “extra” two births could then be controlled through some form of infanticide based on neglect. The optimal method would be to neglect only the girl babies, since the rate of growth in populations that do not practice monogamy is determined almost entirely by the number of females who reach reproductive age.
Our stone age ancestors were thus perfectly capable of maintaining a stationary population, but there was a cost associated with it—the waste of infant lives. This cost lurks in the background of prehistory as an ugly blight in what might otherwise be mistaken for a Garden of Eden.
3
The Origin of Agriculture
The period from 30,000 to 12,000 years ago marked the climax of millions of years of slow technological evolution during which our stone age ancestors gradually perfected the tools and techniques for making a living by hunting large land animals. There are Old World habitation sites dating back hundreds of thousands of years at which archaeologists have found remains of a few pachyderms, giraffes, and buffalo, but these animals probably died natural deaths or were trapped or wounded by nonhuman predators. During this time our ancestors may have scavenged rather than hunted the meat from big game. But by 30,000 years ago the situation had changed, and bands of hunter-collectors in both the Old and New Worlds possessed the means for killing and butchering even the largest animals on a routine basis.
In Europe and Asia vast herds of reindeer, mammoth, horses, bison, and wild cattle grazed on lush grasses fed by glacial melt waters. The pursuit of these creatures came to dominate the food quest. Hunters rounded up their prey by setting fires, drove them over cliffs, and dispatched them with an arsenal of stone and bone projectile points, spears, darts, long knives, and bows and arrows. For thousands of years human predators and animal prey remained in ecological balance.
Then, about 13,000 years ago, a global warming trend signa
led the beginning of the terminal phase of the last ice age. The glaciers that had covered much of the Northern Hemisphere with mile-high sheets of ice began to back away toward Greenland. As the climate became less severe, forests of evergreens and birches invaded the grassy plains which nourished the great herds. The loss of these grazing lands in combination with the toll taken by the human predators produced an ecological catastrophe. The woolly mammoth, woolly rhinoceros, steppe bison, giant elk, European wild ass, and a whole genus of goats suddenly became extinct. While horses and cattle survived, their numbers in Europe sharply decreased. Other species like the saiga antelope and the musk ox survived only in scattered pockets in the far north. Scientists do not agree about the relative impact of the climatological changes and human predation in bringing about the extinction of these animals. Human predation definitely played a role because elephants and rhinos had managed to survive several earlier wanning trends caused by previous glacial retreats.
The collapse of the big-game hunting cultures in northern Europe was followed by the mesolithic period (or middle stone age), during which people obtained their proteins from fish, shellfish, and forest deer. In the Middle East (what is now southern Turkey, Iraq, Iran, Syria, Jordan, and Israel), where the age of the big-game hunters had come to an end much earlier than in the north, the pattern of subsistence became even more diversified. Here people turned from hunting giant wild cattle and red deer to preying on smaller species such as sheep, goats, and antelope and paid increasing attention to fish, crabs and other shellfish, birds, snails, acorns, pistachios and other nuts, wild legumes, and wild grains. Kent Flannery of the University of Michigan has called this system “broad spectrum” hunting and collecting. The retreat of the glaciers and the intensification of big-game hunting did not have precisely the same consequences in Europe and the Middle East, but both regions probably suffered similar environmental depletions which raised the costs of obtaining animal proteins. According to Karl Butzer, most of Turkey, northeastern Iraq, and Iran were treeless during the last ice age, and this would have facilitated the hunting of herd animals. True, the reforestation that occurred at the end of the glacial period was not as extensive as in Europe, but this may actually have made the ecological crisis in the Middle East more severe because of a deficit of both open-country and forest species.
Turning to North and South America, one can see the same process at work. The terminal phase of the last ice age represented the peak of specialized big-game hunting in the New World. At sites in Venezuela, Peru, Mexico, Idaho, and Nevada archaeologists have found beautifully crafted leaf-shaped projectile points, blades, and burins dating from 13,000 to 9000 B.C., some of which are associated with extinct species of antelope, horses, camels, mammoth, mastodon, giant ground sloths, and giant rodents. Between 11,000 and 8000 B.C. big-game hunters equipped with fluted and channeled points were active over a wide expanse of North America, but by 7000 B.C. predation and the climatological changes brought about by the receding glaciers had resulted in the total extinction of thirty-two whole genera of large New World animals including horses, giant bison, oxen, elephant, camels, antelope, pigs, ground sloths, and giant rodents.
Paul C. Martin of the University of Arizona has suggested that the ancestors of the American Indians killed off all of these large animals—called collectively the “Pleistocene Megafauna”—in one short burst of intense predation. Martin attributes this rapid extinction to the fact that the animals had never been hunted by human beings prior to the arrival of bands of Siberian migrants who crossed the Bering Straits land bridge 11,000 years ago. We now know, however, that the discovery of America by migrants from Asia took place much earlier—at least 15,000 and possibly even 70,000 years ago. Although Martin’s overall theory is thus disproven, his idea of rapid extinction deserves careful consideration. Using a computer program to simulate various kill rates practiced by a small initial human population, Martin has shown that all the big animals from Canada to the Gulf Coast could have been wiped out in three centuries if the hunters had permitted their own population to double each generation—a rate of growth well within the reproductive capacity of paleolithic hunters.
We introduce 100 Paleoindians at Edmonton. The hunters take an average of 13 animal units per person per year. One person in a family of four does most of the killing, at an average rate of one animal unit per week.…
The hunting is easy; the [band] doubles every 20 years until local herds are depleted and fresh territory must be found. In 120 years the Edmonton population grows to 5,409. It is concentrated on a front 59 miles deep at a density of 0.37 persons per square mile. Behind the front, the megafauna is exterminated. By 220 years, the front reaches northern Colorado … in 73 years, the front advances the remaining 1,000 miles [to the Gulf of Mexico], attains a depth of 76 miles, and reaches a maximum of just over 100,000 people. The front does not advance more than 20 miles in one year. In 293 years the hunters destroy the megafauna of 93 million animal units.
Martin’s scenario remains useful as an illustration of the vulnerability of large, slow-breeding species to hunter-collectors who decide to increase their kill rates as a result of reproductive pressures and threats to their standard of living. I suspect that the extinction was caused not by any sharp increase in the human population, but simply by an attempt to maintain dietary standards and low abortion and infanticide rates in the face of fewer numbers of prey animals.
After the decline of the New World big-game hunters, cultures appeared in the Americas whose subsistence systems resembled those of the Middle Eastern “broad spectrum” hunters and collectors. Details of the process of intensification and depletion are clearest in the remarkable studies carried out in the Tehuacán Valley under the direction of Richard MacNeish of the Peabody Museum of Archaeology. The Tehuacán Valley, a long, narrow depression located in the southeast part of the Mexican state of Puebla at an altitude of 4,500 feet, is surrounded by high mountains that give it a hot, dry climate. Here, during the Ajuereado period (7000–5000 B.C.), horses and antelope were hunted to extinction. The hunters then intensified the predation of jackrabbits and giant turtle, and these too soon became extinct. MacNeish estimates that at this time meat comprised 89 to 76 percent of the hunters’ total caloric intake at maximum and minimum seasons of the year. During the following El Riego (5000–3400 B.C), Coxcatlán (3400–2300 B.C.), and Abejas (2300–1850 B.C.) periods, the maximum-minimum seasonal calorie percentage of meat fell to 69–31, 62–23, and 47–15 percent, respectively. By about 800 B.C., when fully sedentary villages based on agriculture were finally established in the valley, the proportion of calories provided by animal proteins had fallen still farther and the difference in eating habits between hunting and non-hunting seasons had virtually disappeared. Eventually, as we shall see later on, meat in ancient Mexico was to become a luxury whose production and consumption was the occasion for some of the must brutal institutions in human history.
The implacable decline in the proportion of animal protein in the Tehuacán diet was the result of a continuous series of intensifications and depletions, accompanied by improvements in the technology of hunting. As each species was depleted, the hunters attempted to compensate for the declining return in the effort they invested by using more efficient hunting weapons and techniques. Lances, spear-throwers, darts, and finally the bow and arrow were pressed into service, all to no avail.
According to MacNeish’s estimates, the labor efficiency (calories obtained per calorie expended) of Ajuereado rabbit drives was 2.5:1. Lance ambushing started with an efficiency of 3.2:1 in the early Ajuereado period but fell to 1:1 in Abejas and then died out. Dart stalking of deer began at 7:1 but dropped to about 4:1 as the animal became less plentiful. Later the bow and arrow provided a new high of about 8:1 or 9:1, but by then game was so scarce that meat could contribute only insignificantly to the diet.
As they fought their long and futile delaying action against the consequences of the depletion of animal spec
ies, the people of Tehuacán gradually shifted their primary subsistence effort away from animals toward plants. Intensification of plant production resulted in a slowly increasing proportion of domesticated plants among the “broad spectrum” that was initially obtained entirely by collecting activities. By late El Riego times the hunting bands had succeeded in domesticating squash, amaranth, chili peppers, and avocados. They added maize and beans during the Coxcatlán period, and these crops steadily gained in importance as settlements increased in size and became more sedentary.
MacNeish estimates that the percentage calorie contribution of domesticated and/or cultivated plants was only 1 percent during the El Riego period, 8 percent during Coxcatlán, and 21 percent during Abejas. Even by the time the first permanent settlements appeared, domesticated and/or cultivated plants provided only 42 percent of the total caloric intake.
As in the case of hunting, intensification of farming gave rise to a series of technological advances. Horticulture, or rudimentary gardening, was followed by agriculture, which came to rely more and more on irrigation. The labor efficiencies of these different systems of food production advanced from 10:1 to 30:1 to 50:1. MacNeish does not discuss the possibility that successive declines in labor efficiency prompted the shifts to agriculture and irrigation. And I would not insist that such declines are always necessary to explain the shift to more efficient modes of farming. The decline in the production of animal protein could after all be compensated only by raising the output of plant proteins. The important point is, despite the fact that irrigation agriculture was five times more productive per man-hour than horticulture, the entire 9,000-year sequence of intensifications, depletions, and technological innovations resulted in an overall deterioration in nutritional status.
Cannibals and Kings Page 3