was ejected before the rise of civilization?
Regardless of how we view such things, the changing climate of the last two million years rearranged the world's ecosystems time and again. The Ice Age was not a single event. More than twenty major glaciations repeatedly buried North America and Europe under ice, defining what geologists call the Quaternary—the fourth era of geologic time.
At the peak of the most recent glaciation, roughly 20,000 years ago, glaciers covered almost a third of Earth's land surface. Outside of the tropics even unglaciated areas experienced extreme environmental changes. Human populations either adapted, died out, or moved on as their hunting and foraging grounds shifted around the world.
Each time Europe froze, North Africa dried, becoming an uninhabitable sand sea. Naturally, people left. Some migrated south back into Africa. Others ventured east to Asia or into southern Europe as periodic climate upheavals launched the great human migrations that eventually circled the world.
Judged by the fossil evidence, Homo erectus walked out of Africa and ventured east across Asia, sticking to tropical and temperate latitudes about two million years ago just after the start of the glacial era. Fossil and DNA evidence indicates that the initial separation of Neanderthals from the ancestors of genetically modern humans occurred at least 300,000 years ago—about the time Neanderthals arrived in Europe and western Asia. After successfully adapting to the glacial climate of northwestern Eurasia, Neanderthals disappeared as a new wave of genetically modern humans spread from Africa through the Middle East around 45,000 years ago and across Europe by at least 35,000 years ago. People continued spreading out across the world when the Northern Hemisphere's great ice sheets once again plowed southward, rearranging the environments of Europe, northern Africa, and the Middle East.
During the most recent glaciation, large herds of reindeer, mammoth, wooly rhinoceroses, and giant elk roamed Europe's frozen plains. Ice covered Scandinavia, the Baltic coast, northern Britain, and most of Ireland. Treeless tundra stretched from France through Germany, on to Poland and across Russia. European forests shrank to a narrow fringe around the Mediterranean. Early Europeans lived through this frozen time by following and culling herds of large animals. Some of these species, notably wooly rhinos and giant elk, did not survive the transition to the postglacial climate.
Extreme environmental shifts also isolated human populations and helped differentiate people into the distinct appearances we know today as races. Skin shields our bodies and critical organs from ultraviolet radiation. But skin must also pass enough sunlight to support production of the vitamin D needed to make healthy bones. As our ancestors spread around the globe, these opposing pressures colored the skin of people in different regions. The dominant need for UV protection favored dark skin in the tropics; the need for vitamin D favored lighter skin in the northern latitudes.
Technological innovation played a key role in the spread and adaptation of people to new environments. Roughly 30,000 years ago, immediately before the last glaciation, the development of thin, sharp stone tools ushered in a major technological revolution. Then, about 23,000 years ago, just before the last glacial maximum, the art of hunting changed radically as the bow and arrow began to replace spears. Development of eyed needles allowed the production of hoods, gloves, and mittens from wooly animal hides. Finally equipped to endure the long winter of another glacial era, central Asian hunters began following large game across the grassy steppe west into Europe, or east into Siberia and on to North America.
Unglaciated areas also experienced dramatic shifts in vegetation as the planet cooled and warmed during glacial and interglacial times. Long before the last glacial advance, people around the world burned forest patches to maintain forage for game or to favor edible plants. Shaping their world to suit their 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
