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| B. Prehistory and the Great Ice Age |
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| The biological and cultural evolution of humankind unfolded against a complex backdrop of constant climatic change. For most of geological time, the world's climate was warmer and more homogeneous than it is today. The first signs of glacial cooling occurred in Antarctica about 35 million years ago. There was a major drop in world temperatures between 14 and 11 million years ago, and another about 3.2 million years ago, when glaciers first formed in northern latitudes. Then, just as humans first appeared, about 2.5 million years ago, the glaciation intensified and the earth entered its present period of constantly fluctuating climate. | 1 |
| Humans evolved during the period of relatively minor climatic oscillations. Between 4 and 2 million years ago, the world climate was somewhat warmer and more stable than it became in later times. The African savanna, where humans originated, supported many mammal species, large and small, including a great variety of the order Primates, to which we belong. | 2 |
| About 1.6 million years ago, at the beginning of the Pleistocene (commonly called the Great Ice Age), the world's climatic changes intensified. Global climates constantly fluctuated between warm and intensely cold. For long stretches of time, the northern parts of both Europe and North America were mantled with great ice sheets, the last retreating only some 15,000 years ago. While glaciers covered northern areas, world sea levels fell as much as 300 feet below modern shorelines, joining Alaska to Siberia, Britain to the Continent, and exposing vast continental shelves off ocean coasts. The glacial periods brought drier conditions to tropical regions. The Sahara and northern Africa became very arid, and rain forests shrunk. | 3 |
| Fluctuations of warm and cold temperatures were relatively minor until about 800,000 years ago. Since then, periods of intense cold have recurred about every 90,000 years, perhaps triggered by long-term changes in Earth's orbit around the Sun. Core samples taken from the sea floor tell us that there have been at least nine cold periods in the last 800,000 years, each of them characterized by a gradual cooling that took tens of thousands of years and a subsequent rapid warming up that saw glaciers retreat and world sea levels rise with remarkable speed. | 4 |
| Few of these oscillations are well documented, except for the last interglacial and glacial cycle, which began about 128,000 years ago. During the last interglacial, Europe, Eurasia, and North America were warmer than today. But a gradual cooling set in about 118,000 years ago, as the last glaciation, Würm, set in. (The term Würm comes from a river in the Alps where the glaciation was first identified.) Glacial conditions were especially intense about 75,000 years ago, when the archaic Neanderthal people were flourishing in Europe and when the Americas were still uninhabited. After a brief milder interval about 40,000 years ago, the cold intensified again, peaking about 18,000 years ago. A rapid amelioration began about 15,000 years ago, and the world's climate reached near-modern conditions by 6000 B.C.E. | 5 |
| By using pollen analysis, microscopic and statistical analyses of fossilized plant pollens found in geological deposits, scientists have chronicled dramatic changes in the world's environment during the Würm glaciation, changes that also took hold much earlier in the Ice Age. During the height of the Würm, most of western and central Europe and Eurasia was open steppe-tundra, while Scandinavia and much of Britain were under ice. The Balkans were joined to Turkey, the Sahara was extremely arid, and snow levels on the world's mountains were hundreds of feet lower. Rain forests shrank, and the Southeast Asian mainland was joined to the offshore islands. Only narrow straits separated the mainland from New Guinea and Australia, which formed a single landmass. Alaska and Siberia were joined by a low-lying land bridge. Most of Canada and the United States were covered with vast ice sheets, as far south as Seattle, the Great Lakes, and Nova Scotia. The 90,000 years of the Würm glaciation witnessed dramatic changes in human life, which unfolded in a world very different from our own. | 6 |
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