Areas of the world differed in the food resources available to foragers. In some, acquiring enough food to sustain a group was difficult, and groups had to move constantly. In others, moderate temperatures and abundant rainfall allowed for verdant plant growth; or seas, rivers, and lakes provided substantial amounts of fish and shellfish. Groups in such areas were able to become more settled. About 15,000 years ago, the earth’s climate entered a warming phase, and the glaciers began to retreat. As the earth became warmer, the climate became wetter, and more parts of the world were able to support sedentary or semi-sedentary groups of foragers.

In several of these places, foragers began planting seeds in the ground along with gathering wild grains, roots, and other foodstuffs. By observation, they learned the optimum times and places for planting. They removed unwanted plants through weeding and selected the seeds they planted in order to get crops that had favorable characteristics, such as larger edible parts. Through this human intervention, certain crops became domesticated, that is, modified by selective breeding so as to serve human needs, in this case to provide a more reliable source of food.

This early crop planting was done by individuals using hoes and digging sticks, and it is often termed horticulture to distinguish it from the later agriculture using plows. Intentional crop planting developed first in the area archaeologists call the Fertile Crescent, which runs from present-day Lebanon, Israel, and Jordan north to Turkey and then south to the Iran-Iraq border (Map 1.2). About 9000 B.C.E. people there began to plant seeds of the wild wheat and barley they had already been harvesting, along with seeds of legume crops, such as peas and lentils, and of the flax with which they made linen cloth. By about 8000 B.C.E. people were growing sorghum and millet in parts of the Nile River Valley, and perhaps yams in western Africa. By about 7000 B.C.E. they were growing domesticated rice, millet, and legumes in China; yams and taro in Papua New Guinea; and perhaps squash in Mesoamerica. In each of these places, the development of horticulture occurred independently, and it may have happened in other parts of the world as well.

Why, after living successfully as foragers for tens of thousands of years, did humans in so many parts of the world all begin raising crops at about the same time? The answer to this question is not clear, but crop raising may have resulted from population pressures in those parts of the world where the warming climate provided more food. More food meant lower child mortality and longer life spans, which allowed communities to grow. When population growth outstripped the local food supply, people had a choice: they could move to a new area — the solution that foragers had relied on when faced with the problem of food scarcity — or they could develop ways to increase the food supply to keep up with population growth, a solution that the warming climate was making possible. They chose the latter and began to plant more intensively, beginning cycles of expanding population and intensification of land use that have continued to today.

A recent archaeological find at Göbekli Tepe in present-day Turkey, at the northern edge of the Fertile Crescent, suggests that cultural factors may have played a role in the development of agriculture. Here, around 9000 B.C.E. hundreds of people came together to build rings of massive, multiton, elaborately carved limestone pillars and then covered them with dirt and built more. The people who created this site lived some distance away, where archaeological evidence indicates they first carved the pillars. The evidence also reveals that they ate wild game and plants, not crops. The project may have unintentionally spurred the development of new methods of food production that would allow the many workers to be fed efficiently. Indeed, it is very near here that evidence of the world’s oldest domesticated wheat has been discovered.

Whatever the reasons for the move from foraging to crop raising, within several centuries of initial crop planting, people in the Fertile Crescent, parts of China, and the Nile Valley were relying on domesticated food products alone. They built permanent houses near one another in villages surrounded by fields, and they invented new ways of storing foods, such as in pottery made from clay. Villages were closer together than were the camps of foragers, so population density as well as total population grew.

A field of planted and weeded crops yields ten to one hundred times as much food — measured in calories — as the same area of naturally occurring plants. It also requires much more labor, however, which was provided both by the greater number of people in the community and by those people working longer hours. Farming peoples were often in the fields from dawn to dusk. Early farmers were also less healthy than foragers were. Their narrower range of foodstuffs made them more susceptible to disease and nutritional deficiencies such as anemia.

Foragers who lived at the edge of horticultural communities appear to have recognized the negative aspects of crop raising, for they did not immediately adopt this new way of life. Instead farming spread when a village became too large and some residents moved to a new area. Because the population of farming communities grew so much faster than that of foragers, however, horticulture quickly spread into fertile areas. By about 6500 B.C.E. farming had spread northward from the Fertile Crescent into Greece, and by 4000 B.C.E. farther northward all the way to Britain; by 4500 B.C.E. it had spread southward into Ethiopia. At the same time, crop raising spread out from others areas in which it was first developed, and slowly larger and larger parts of China, South and Southeast Asia, and East Africa became home to horticultural villages.

People adapted crops to their local environments, choosing seeds that had qualities that were beneficial, such as drought resistance. They also domesticated new kinds of crops. In the Americas, for example, by about 3000 B.C.E. corn was domesticated in southern Mexico and potatoes and quinoa in the Andes region of South America, and by about 2500 B.C.E. squash and beans in eastern North America. These crops then spread, so that by about 1000 B.C.E. people in much of what is now the western United States were raising corn, beans, and squash. In the Indus Valley of South Asia, people were growing dates, mangoes, sesame seeds, and cotton along with grains and legumes by 4000 B.C.E. Accordingly, crop raising led to dramatic human alteration of the environment.

Certain planted crops eventually came to be grown over huge areas of land, so that some scientists describe the Agricultural Revolution as a revolution of codependent domestication: humans domesticated crops, but crops also “domesticated” humans so that they worked long hours spreading particular crops around the world. Of these, corn has probably been the most successful; more than half a million square miles around the world are now planted in corn.

In some parts of the world horticulture led to a dramatic change in the way of life, but in others it did not. Horticulture can be easily combined with gathering and hunting, as plots of land are usually small; many cultures, including some in Papua New Guinea and North America, remained mixed foragers and horticulturists for thousands of years. Especially in deeply wooded areas, people cleared small plots by chopping and burning the natural vegetation, and planted crops in successive years until the soil eroded or lost its fertility, a method termed “slash and burn.” They then moved to another area and began the process again, perhaps returning to the first plot many years later, after the soil had rejuvenated itself. Groups using shifting slash-and-burn cultivation remained small and continued to rely on the surrounding forest for much of their food.