Dirt. David R. Montgomery
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needs, our hunting and gathering ancestors were not passive inhabitants of the landscape. Despite their active manipulation, small human populations and mobile lifestyles left little discernable impact on natural ecosystems.
Transitions from a glacial to interglacial world occurred many times during the last two million years. Through all but the most recent glaciation, people moved along with their environment rather than staying put and adapting to a new ecosystem. Then, after living on the move for more than a million years, they started to settle down and become farmers. What was so different when the glaciers melted this last time that caused people to adopt a new lifestyle?
Several explanations have been offered to account for this radical change. Some argue that the shift from a cool, wet glacial climate to less hospitable conditions put an environmental squeeze on early people in the Middle East. In this view, hunters began growing plants in order to survive when the climate warmed and herds of wild game dwindled. Others argue that agriculture evolved in response to an inevitable process of cultural evolution without any specific environmental forcing. Whatever the reasons, agriculture developed independently in Mesopotamia, northern China, and Mesoamerica.
For much of the last century, theories for the origin of agriculture emphasized the competing oasis and cultural evolution hypotheses. The oasis hypothesis held that the postglacial drying of the Middle East restricted edible plants, people, and other animals to well-watered flood-plains. This forced proximity bred familiarity, which eventually led to domestication. In contrast, the cultural evolution hypothesis holds that regional environmental change was unimportant in the gradual adoption of agriculture through an inevitable progression of social development. Unfortunately, neither hypothesis provides satisfying answers for why agriculture arose when and where it did.
A fundamental problem with the oasis theory is that the wild ancestors of our modern grains came to the Middle East from northern Africa at the end of the last glaciation. This means that the variety of food resources available to people in the Middle East was expanding at the time that agriculture arose—the opposite of the oasis theory. So the story cannot be as simple as the idea that people, plants, and animals crowded into shrinking oases as the countryside dried. And because only certain people in the Middle East adopted agriculture, the cultural adaptation hypothesis falls short. Agriculture was not simply an inevitable stage on the road from hunting and gathering to more advanced societies.
The transition to an agricultural society was a remarkable and puzzling behavioral adaptation. After the peak of the last glaciation, people herded gazelles in Syria and Israel. Subsisting on these herds required less effort than planting, weeding, and tending domesticated crops. Similarly, in Central America several hours spent gathering wild corn could provide food for a week. If agriculture was more difficult and time-consuming than hunting and gathering, why did people take it up in the first place?
Increasing population density provides an attractive explanation for the origin and spread of agriculture. When hunting and gathering groups grew beyond the capacity of their territory to support them, part of the group would split off and move to new territory. Once there was no more productive territory to colonize, growing populations developed more intensive (and time-consuming) ways to extract a living from their environment. Such pressures favored groups that could produce food themselves to get more out of the land. In this view, agriculture can be understood as a natural behavioral response to increasing population.
Modern studies have shown that wild strains of wheat and barley can be readily cultivated with simple methods. Although this ease of cultivation suggests that agriculture could have originated many times in many places, genetic analyses show that modern strains of wheat, peas, and lentils all came from a small sample of wild varieties. Domestication of plants fundamental to our modern diet occurred in just a few places and times when people began to more intensively exploit what had until then been secondary resources.
The earliest known semiagricultural people lived on the slopes of the Zagros Mountains between Iraq and Iran about 11,000 to 9000 BC (or thirteen thousand to eleven thousand years ago). Surviving by hunting gazelles, sheep, and goats and gathering wild cereals and legumes, these people occupied small villages but made extensive use of seasonal hunting camps and caves. By 7500 BC herding and cultivation replaced hunting and gathering as the mainstay of their diet and settled villages of up to twenty-five households kept sheep and goats and grew wheat, barley, and peas. By then hunting accounted for only about 5 percent of their food. Why the big change, and why then and there?
The earliest evidence for systematic cultivation of grains comes from Abu Hureyra in the headwaters of the Euphrates River in modern Syria. The archaeological record from this site shows that cultivation began in response to a period when the drier conditions of glacial times abruptly returned after thousands of years of climatic amelioration. Abu Hureyra provides a unique record of the transition from the hunter-gathering lifestyle of the last glacial era to cereal-based agriculture. Moreover, evidence from the site helps explain why people adopted the labor-intensive business of agriculture. They were forced into it.
As glaciation ended, the Levant gradually warmed and received increasing rainfall. From about 13,000 to 11,000 BC open oak forest gradually replaced the grasslands of the glacial steppe. A core drilled from the bed of Lake Huleh in northeastern Israel shows that tree pollen increased from a fifth to three-quarters of all the pollen during this period. Abundant game and wild grains (especially rye and wheat) made for an edenic landscape with few people and lots of resources. Sedentary communities of hunter-gatherers began to take root in locations where resources were particularly abundant.
Then the world's climate reverted to almost full glacial conditions for a thousand years, from about 10,000 to 9000 BC, a period known as the Younger Dryas. Arboreal pollen dropped back to less than a quarter of the total amount of pollen, indicating a sharp decline in precipitation and a return to the steppelike conditions of the glacial climate. The forest retreated northward, away from the world's first settled community.
Abu Hureyra sat on a low promontory overlooking the Euphrates Valley, about 180 miles northeast of Damascus. Plant debris excavated from the site records the transition from foraging for a wide variety of wild plants to cultivation of a few crops by the end of the Younger Dryas. The earliest plant remains associated with settlement of the site include more than one hundred species of seeds and fruits from the marshes and forest of the Euphrates Valley. Abundant animal bones reveal substantial reliance on hunting, especially gazelles. Moreover, the site was occupied year-round. The people of Abu Hureyra were not nomadic hunter-gatherers. They permanently inhabited a defined territory around their village. A couple hundred people occupied Abu Hureyra by the time that the Younger Dryas ushered in a thousand years of cold, dry weather that dramatically altered plant and animal resources. Fruits and seeds of drought-sensitive plants disappeared from the diet. Wild lentils and legumes harvested from nearby woodland also disappeared. As eden dried out, food became scarce.
Why didn't they just move? Probably because Abu Hureyra was already one of the region's best sites. Surrounding areas experienced similar changes and offered even less sustenance. Besides, other people already occupied the next best land. People with rapidly disappearing food supplies usually do not welcome new neighbors. The people of Abu Hureyra had no place to go.
Out of options, they began to cultivate wild varieties of rye and wheat that survived the transition to a colder, more arid climate. Of the plants that survived, only cereals could be cultivated to produce food capable of storage for use throughout the year. Despite the worsening aridity, seeds of drought-intolerant weeds typical of agricultural fields increased dramatically during the Younger Dryas. At first, wild cereals were cultivated on hillsides using rain-fed agriculture. Within a few centuries domesticated varieties of rye appeared in the fields, as did legumes such as lentils.
The switch to cultivation required more time and energy to produce a calorie of food. It is not something that would have been undertaken lightly. The sedentary style of hunting and gathering practiced by the early inhabitants of Abu Hureyra left them susceptible to declining food availability as the climate changed. Once wild food sources were fully exploited the population was vulnerable to seasonal shortages brought on by increasing aridity. Begun out of desperation, agriculture expanded to include other crops such as barley and peas as the climate improved