Hunting and Gathering versus Agriculture
|VIDEO 16-1: Hunting and Gathering Societies|
For most of their existence humans obtained food by hunting and gathering. Hunting refers to the capture of animals for use as food, and gathering refers to the collection of plant material for use as food. Hunting and gathering occur in relatively unmodified ecosystems. That is, hunters and gatherers obtain their food from the edible energy flows of natural ecosystems (See Video 16-1).
The fraction of total energy flows in natural ecosystems that can be eaten by people is relatively small. Humans cannot digest cellulose, which is the main component in plant cell walls. Therefore, humans eat only a small fraction of the plant species on Earth; and of these species, humans consume only small portions of the plants. The term vegetable is generally defined as the edible seeds, roots, stems, leaves, bulbs, tubers, or nonsweet fruits of herbaceous plants. The most commonly eaten portions include nuts, berries, and seeds. Other edible portions include roots such as carrots, tubers such as potatoes, and some types of leaves such as lettuce. Meat from animals is more digestible than vegetable matter, but the amount of food available from herbivores and carnivores generally is smaller than that available from plants due to losses associated with trophic inefficiencies.
FIGURE 16-2 Food Strategy and Population Density The density of hunters and gatherers is small relative to agriculturalists. The y-axis is in log 10, showing that traditional farming methods support 100–1,000 people per km2 compared to hunting and gathering, which support at most 1 person per km2. Source: Kaufmann, Robert K. and Cleveland, Cutler J. 2007. Environmental Science (McGraw-Hill, Dubuque, IA).
The low rate at which natural ecosystems generate edible energy influences many aspects of hunter–gatherer societies. Most notably, the number of people per unit area, or population density, of hunter–gatherer societies is low (Figure 16-2). On average, population density is less than 1 person per square kilometer (fewer than 3 people per square mile).Variations in population density among hunter–gatherer societies are determined in part by net primary production. In productive tropical rain forests the population density of hunter–gatherers can exceed 1 person/km2. Population densities often are less than 0.1 people/km2 (0.3 people/mi2) in less productive ecosystems, such as boreal forests.
Small population densities do not mean that hunter–gatherers live on the edge of starvation. Many hunter–gatherers are well fed because they are very efficient in getting food from the environment. Many hunter–gatherers spend only four to six hours per day obtaining food. As a result, many hunter–gatherer societies have considerable spare time. (How does this compare to the amount of spare time you have?) Some anthropologists call hunter–gatherers the original leisure society.
The relatively small fraction of the day that is spent obtaining food implies that hunting and gathering have a relatively high energy return on investment. Measurements indicate that the energy return on investment for hunting and gathering ranges between 10:1 and 20:1. This ratio implies that on average, a hunter–gatherer obtains between 10 and 20 kcal of food for every 1 kcal used to obtain food. Some hunting techniques, such as the capture of large marine mammals, can have an energy return on investment of well over 100:1.
Agriculturists obtain food by changing natural ecosystems in a way that increases the amount of edible energy generated. To do so, agriculturists replace species that produce relatively little edible energy, such as trees, with species that generate more edible energy such as cereals, which are cultivated members of the grass family whose seeds (grains) are eaten by people or domesticated animals.
|Figure 16-3 Nomadic Herding Goats gather for a drink at a borehole at the village of Shatabak in Garissa district, Kenya. Credit: Peter Smerdon, World Food Programme.||FIGURE 16-4 Traditional Farming A traditional farm in India. Credit: ricewisdom.org|
Agriculturists also change the characteristics of the species they use for food through domestication. Domestication modifies a species relative to its wild ancestors by selective breeding, in which only individuals with traits desired by agriculturists are allowed to reproduce. As such, people act in place of natural selection. For example, the wild ancestor of wheat produces its seeds at the top of a stalk that “shatters” spontaneously, thereby dropping its seeds to the ground. This makes it difficult to harvest. The domesticated version of wheat is bred from a form that does not drop its seeds and is therefore easier to harvest.
As you might expect, agriculture influences many aspects of agricultural societies. Replacing inedible species with edible species increases the amount of food that is available per unit area; therefore, the population densities of agricultural societies are much larger than those of hunter–gatherer societies. Even traditional farming methods can support 100–1,000 people/km2 (260–2,600 people/mi2) (Figure 16-2), which is two to three orders of magnitude greater than hunter–gatherers.
Relatively high population densities do not imply that agriculture is easy. The stereotype of a hardworking farmer is accurate. For reasons described in this chapter, farming requires a lot of time and effort. Therefore, the energy return on investment for agriculture can be relatively low, and often it is smaller than the energy return on investment for hunting and gathering. The transition from hunting and gathering to agriculture often is associated with smaller body size, poorer nutritional status, and increased disease. So why did hunter–gatherers switch to agriculture?
The Origins of Agriculture
Agriculture is a relatively recent innovation. Although humans have been around for nearly 4 million years, people started to practice agriculture between 5,000 and 12,000 years ago. Agriculture developed independently in at least nine areas (Figure 16-5). The earliest sites included southeast Asia (such as Papua New Guinea), where agriculture arose around the production of root crops, and the Fertile Crescent (Iraq, Syria, and Israel), where agriculture arose around wheat or barley. Later sites included sub-Saharan Africa and South America, where agriculture arose around the production of root crops.
|FIGURE 16-5 The Origins of Agriculture Agriculture started independently in at least nine sites over the previous 12,000 years. Redrawn from: J. Diamond, “Evolution: Consequences and Future of Plant and Animal Domestication,” Nature 418: 700–707; Figure from Kaufmann, Robert K. and Cleveland, Cutler J. 2007. Environmental Science (McGraw-Hill, Dubuque, IA).|
Why did agriculture originate in these geographically disparate areas? Some scientists assert that these areas were home to species that could be domesticated. Relatively few species can be domesticated. For example, the wild ancestors of almond and oak trees produce nuts with a bitter taste. Bitterness is controlled by a single gene in almond trees, so selective breeding was able to eliminate the bitter taste. On the other hand, the bitter taste in acorns is controlled by many genes, so oak trees cannot be domesticated (the people of Abu Hureyra worked hard to process acorns into a form that can be eaten). Similarly, zebras cannot be domesticated, whereas wild horses can be and were domesticated.
Given the opportunity, anthropologists and demographers suggest three reasons why a given society will adopt agriculture. One theory, the technical change hypothesis, holds that agriculture arose with increasing human technical capabilities. This hypothesis seems consistent with the observation that agriculture requires specialized tools and technologies. But there are some important contradictions. Some agriculturists use relatively simple tools, such as digging sticks, whereas some hunter–gatherer societies use sophisticated tools like poison arrows or complex fishing equipment.
FIGURE 16-6 The Coevolutionary Hypothesis The coevolutionary hypothesis postulates that agriculture originated with a positive feedback loop that includes human populations, edible plants, and cleared land. According to this hypothesis, people clear land, which opens space for edible plant species, whose seeds are contained in human fecal material. The local availability of plants enhances population growth, which opens more land and accelerates the process. Source: Kaufmann, Robert K. and Cleveland, Cutler J. 2007. Environmental Science (McGraw-Hill, Dubuque, IA).
Another explanation, the coevolutionary hypothesis, holds that agriculture coevolved with humans. According to this view, there is a positive feedback loop that includes the human population and the plants and animals it eats (Figure 16-6). The positive feedback loop works as follows. In the process of providing wood for fires and building materials, humans remove trees. Removing trees creates sunlit areas. Sunlit areas provide ideal habitat for edible plants. In addition, the organic wastes and human wastes generated by hunter–gatherers contain seeds of plants that they eat as well as nutrients that increase soil fertility. The combination of sunlight, seeds, and nutrients enhances the growth of edible plants near human settlements. The proximity and abundance of edible plants increases food supply and lets the local population grow. Population growth reinforces this process by speeding the rate at which trees are removed and the soil is fertilized.
A third explanation, the resource depletion hypothesis, holds that agriculture is a response to population growth and the best first principle. We can understand the role of resource depletion by examining the relationship between population size and the energy return on investment of hunting and gathering versus agriculture (Figure 16-7). Making the tools needed for hunting and gathering requires relatively little effort. And the costs of hunting and gathering also are relatively small so long as the human population is small. As such, hunting and gathering is the easiest way for small populations to obtain food. But as a hunter–gatherer population grows, it depletes the local supply of edible plants and animals. This forces them to look longer and walk farther to obtain food. Such efforts increase the cost and lower the energy return on investment for hunting and gathering.
FIGURE 16-7 The Costs of Hunting and Gathering versus Agriculture Hunters and gatherers invest relatively little in producing their tools, which implies that the cost of obtaining food is low initially. But as population density increases and local plants and animals have been collected, hunters and gatherers must walk long distances to obtain food, which raises the energy cost of obtaining food. Setting up an agricultural system requires a lot of initial effort. But increasing output from an agricultural field requires relatively little extra effort; so greater population density increases the cost of obtaining food relatively slowly. At some intermediate population size the cost of obtaining food from hunting and gathering exceeds the cost of agriculture, and people convert from hunting and gathering to agriculture. Data: C.A. Hastorf, “Changing Resources Use in Subsistence Agricultural Groups in the Prehistoric Mimbres River Vallery, New Mexico,” eds. T.K. Earle and A.L. Christenson, in Modeling Change in Prehistoric Subsistence Economies, Academic Press); Figure: Kaufmann, Robert K. and Cleveland, Cutler J. 2007. Environmental Science (McGraw-Hill, Dubuque, IA).
The energy return on investment for small-scale agriculture is low because of the high costs of establishing an agricultural system (the reasons are explained in the next section). But once such a system is established, relatively little effort is required to expand food production. As a result, the costs of obtaining food via agriculture rise slowly as population increases. Because the costs of agriculture grow less rapidly than those of hunting and gathering, the cost of obtaining food via agriculture for large populations is lower than hunting and gathering. Agriculture may have developed in hunter–gatherer societies whose population grew beyond the carrying capacities of their local ecosystems. Consistent with this hypothesis, there are historical examples in which an agricultural society returned to hunting and gathering when their population size shrank.
Regardless of the cause, the area occupied by agriculturists spread from each of the nine points of origination (Figure 16-5). In general, agriculture spread in an east–west direction, as would be dictated by the latitudinal bands in temperature, precipitation, and solar radiation that are described in Chapter 4. Like all species, domesticated plants and animals thrive under a relatively narrow range of environmental conditions. These conditions remain relatively constant moving east and west, compared to the greater changes in temperature, precipitation, and solar energy that occur in movement north and south.
Despite these difficulties, agriculture also moved north and south; hunters and gatherers now are found only where it is too cold for agriculture (such as northern Canada) or too dry for agriculture (such as central Australia), or where hunter–gatherers have been isolated from agriculturists, such as the interior portions of the Amazon rain forest (Figure 16-8). Nearly everywhere that agriculturists have encountered hunter–gatherers, hunting and gathering practices have been displaced.
There are two ways in which agriculturists can displace hunters and gatherers: the idea (and the domesticated plants and animals) of agriculture can spread from group to group, or the agriculturists themselves can expand their range. The historical record shows that agriculture spread largely through the movement of people. Specifically, agriculturists took over land used previously by hunter–gatherers either by spreading disease or by military conquest. Due to the coevolution of agriculturists and their diseases, hunter–gatherers often are more susceptible to the diseases that afflict agriculturists, such as chickenpox or smallpox. As a result, hunter–gatherer populations often suffer epidemics when first contacted by agriculturists. And in the case of violent conflict, hunter–gatherers usually retreat because their small numbers are no match for the higher population densities of agriculturists.
|FIGURE 16-8 Remaining Hunters and Gatherers The remaining populations of hunters and gatherers tend to live in areas too cold or too dry for agriculture. Or they live in remote areas where they have not yet come into contact with agriculturalists. Source: Kaufmann, Robert K. and Cleveland, Cutler J. 2007. Environmental Science (McGraw-Hill, Dubuque, IA).|