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The Pleistocene The Pleistocene is the fifth epoch of the Cenozoic; it started approximately 1.8 million years ago and ended 11,000 years ago. Due to the recent start and end of this epoch, a great deal is known about its geologic, climatic, and evolutionary history. During this epoch the polar ice sheets expanded and contracted -- the reason why the Pleistocene is commonly known as the Ice Age. Modern humans evolved and spread throughout the world during the Pleistocene. Although it's known as the Ice Age, this epoch was marked by glacial advances and retreats, and includes periods that were warmer than the present. Between about 1.8 and 1.0 million years ago, a complete cycle of glacial advance and melting took about 40,000 years. During much of the past one million years, the cycle was longer, completed about every 100,000 years. These dramatic oscillations in climate had severe impact on all forms of life. With glacial advances, sea levels dropped and land bridges were created; during glacial retreats, these bridges were once again submerged. The so-called Pleistocene "megafauna", such as the woolly mammoth and saber-toothed cats, evolved during these climatic fluctuations. Environmental fluctuation during the Pleistocene was apparently conducive to human evolution, which is characterized by an increase in the adaptability of the human lineage. However, one group of early human lineages, the robust australopiths, suffered extinction in the early part of the Pleistocene. This epoch is known for the radiation of early human species out of Africa, a trend that started in the late Pliocene. To learn about the different species that lived during the Pleistocene, visit the Hall of Human Ancestors, where each species is described. The end of this epoch is marked by the onset of the current interglacial. Most researchers consider this interglacial to be a new epoch, the Holocene; but other researchers feel that the division is arbitrary and that modern times should be treated as part of the Pleistocene. Indeed, there is no reason to doubt that modern-day warmth, and the retreat of glaciers to their present positions, is simply part of the glacial-interglacial cycle of the Pleistocene. |