Dirt. David R. Montgomery
increases soil erosion to well above natural rates, resulting in a fundamental problem. The United States Department of Agriculture estimates that it takes five hundred years to produce an inch of topsoil. Darwin thought English worms did a little better, making an inch of topsoil in a century or two. While soil formation rates vary in different regions, accelerated soil erosion can remove many centuries of accumulated soil in less than a decade. Earth's thin soil mantle is essential to the health of life on this planet, yet we are gradually stripping it off—literally skinning our planet.
But agricultural practices can also retard erosion. Terracing steep fields can reduce soil erosion by 80 to 90 percent by turning slopes into a series of relatively flat surfaces separated by reinforced steps. No-till methods minimize direct disturbance of the soil. Leaving crop residue at the ground surface instead of plowing it under acts as mulch, helping to retain moisture and retard erosion. Interplanting crops can provide more complete ground cover and retard erosion. None of these alternative practices are new ideas. But the growing adoption of them is.
Over decades of study, agronomists have developed ways to estimate soil loss for different environmental conditions and under different agricultural practices relative to standardized plots. Despite half a century of first-rate research, rates of soil erosion remain difficult to predict; they vary substantially both from year to year and across a landscape. Decades of hard-to-collect measurements are needed to get representative estimates that sample the effect of rare large storms and integrate the effects of common showers. The resulting uncertainty as to the relative magnitude of modern erosion rates has contributed to controversy in the last few decades over whether soil loss is a serious problem. Whether it is depends on the ratio of soil erosion to soil production, and even less is known about rates of soil formation than about rates of soil erosion.
Skeptics discount concern over erosion rates measured from small areas or experimental plots and extrapolated using models to the rest of the landscape. They rightly argue that real data on soil erosion rates are hard to come by, locally variable, and require decades of sustained effort to get. In their view, we might as well be guessing an answer. Moreover, only sparse data on soil production rates have been available until the last few decades. Yet the available data do show that conventional agricultural methods accelerate erosion well beyond soil production—the question is by how much. This leaves the issue in a position not unlike global warming—while academics argue over the details, vested interests stake out positions to defend behind smokescreens of uncertainty.
Still, even with our technological prowess, we need productive soil to grow food and to support plants we depend on—and our descendants will too. On the hillslopes that support much of our modern agriculture, soil conservation is an uphill battle. But there are some places where hydrology and geology favor long-term agriculture—the fertile river valleys along which civilizations first arose.
THREE
Rivers of Life
Egypt is the gift of the Nile.
HERODOTUS
FOUNDATIONAL TEXTS OF WESTERN RELIGIONS acknowledge the fundamental relationship between humanity and the soil. The Hebrew name of the first man, Adam, is derived from the word adama, which means earth, or soil. Because the name of Adam's wife, Eve, is a translation of hava, Hebrew for “living,” the union of the soil and life linguistically frames the biblical story of creation. God created the earth—Adam—and life—Eve—sprang from the soil—Adam's rib. The Koran too alludes to humanity's relation to the soil. “Do they not travel through the earth and see what was the end of those before them?…They tilled the soil and populated it in greater numbers…to their own destruction” (Sura 30:9). Even the roots of Western language reflect humanity's dependence on soil. The Latin word for human, homo, is derived from humus, Latin for living soil.
The image of a lush garden of Eden hardly portrays the Middle East today. Yet life for the region's Ice Age inhabitants was less harsh than along the great northern ice sheets. As the ice retreated after the peak of the last glaciation, game was plentiful and wild stands of wheat and barley could be harvested to supplement the hunt. Are vague cultural memories of a prior climate and environment recorded in the story of the garden from which humanity 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