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Pleistocene Epoch, third division of the Neogene Period of the geologic time scale (see Geologic Time). The Pleistocene Epoch (1.8 million years to 11,500 years before present) followed the Pliocene Epoch and is the epoch just previous to our current epoch, the Holocene Epoch (11,500 years before present to the present). Huge sheets of ice covered much of Earth’s surface during the Pleistocene Epoch. The ice changed the surface of the planet and helped provide a suitable climate for huge ice-age mammals. Animals such as the mastodon and saber-toothed tiger thrived during the Pleistocene, but became extinct at the end of the epoch as the final ice sheets retreated. The first humans appeared in the Pleistocene Epoch. British geologist Charles Lyell named the Pleistocene Epoch in 1839. The word Pleistocene comes from the Greek words pleistos “most” and kainos “recent”. In the 1820s Lyell studied fossils of the shells of snails and clams that were embedded in layers of sediment in Europe. By studying how these shells had changed over time, he was able to divide geologic history into pieces. The Pleistocene Epoch became one of these pieces. Another British geologist, Edward Forbes, helped popularize the name Pleistocene in the 1840s. Swiss geologist Ignaz Venetz-Sitten and Norwegian geologist Jens Esmark were the first to recognize that certain deposits in Earth’s surface from the Pleistocene Epoch had probably been created by glacial ice (see Glacier). Other geologists, including Swiss-born American scientist Louis Agassiz, expanded on these discoveries. Agassiz was the first to theorize that the Pleistocene was a time of massive glaciation. He believed that Earth had gone through an ice age. Modern geologists believe that glaciers occupied only small areas of the planet at any one time, and that they melted at different times and in different places, so the term ice age is somewhat misleading. Agassiz used a layer of rock in a tiny town called Vrica, in Italy, to designate the beginning of the Pleistocene. That layer is about 1.80 million years old. Agassiz thought that the layer of rock corresponded to the beginning of a worldwide ice age, but further research showed that glaciers actually began advancing about 2.58 million years before present. In the 1990s many geologists began suggesting that the designation of the beginning of the Pleistocene should be pushed back to that date. The end of the Pleistocene Epoch is more straightforward: Geologists designate the boundary between the Pleistocene and the current epoch, the Holocene, as 11,500 calendar years before present. No rock layer or other geological feature marks this date. Instead, scientists use absolute dating methods such as radiocarbon dating to determine whether a rock layer falls within the Holocene Epoch. More from Encarta
Geologists have studied the Pleistocene extensively because it is so close in time to the present. Studying the Pleistocene can help scientists understand how the oceans, ecosystems, and atmosphere behaved before humans began interfering with these natural systems. The Pleistocene Epoch had cold periods of time, called glacial stages, and warmer episodes, called interglacial stages. Temperatures during glacial stages averaged 5 to 7 Celsius degrees (9 to 13 Fahrenheit degrees) cooler than current average temperatures. The interglacial stages were generally shorter than the glacial stages and had temperatures similar to or slightly above current temperatures. Geologists can determine the length and frequency of glacial cycles by examining samples of deposits on the floor of the deep sea. The composition of seawater changes with the amount of glacial ice in the oceans. The bodies of tiny sea creatures, called foraminifera, provide information about the composition of the seawater around them. When foraminifera die, their bodies sink to the seafloor and become fossils. Each layer of foraminiferan fossils represents a different period of time. Geologists take samples that reach deep into the layers deposited on the ocean floor. The changing composition of the foraminiferan skeletons corresponds to the amount of glacial ice in the oceans when the creatures lived. Other clues about the climate of the Pleistocene are trapped in the ice of Antarctica and the Arctic region. During glacial stages, seawater froze into huge glaciers, lowering the level of the oceans. Sea level dropped by as much as 150 m (490 ft), as water from the oceans froze into glaciers and ice sheets. The continental shelves, or the edges of a continent that extend out into the ocean, became dry land as the ocean levels dropped. The newly uncovered land quickly dried and added dust to the atmosphere. Some of the dust settled in the Arctic and Antarctic regions, where new layers of ice covered and trapped it. During interglacial stages, increased plant and animal growth changed the levels of carbon dioxide and methane in the atmosphere. Bubbles of air from the ancient atmosphere remain trapped in the ice of the polar regions, where scientists can now sample them. A glacial cycle is made up of one glacial stage and one interglacial stage. Geologists have used deep-sea cores and polar ice cores to determine that the longest glacial cycles of the Pleistocene lasted about 100,000 to 130,000 years. Many glacial cycles were shorter than this, with several occurring in the time it took for one long cycle. At least 25 glacial cycles have occurred in the last two million years.
Less than 20,000 years ago, the Laurentide Ice Sheet covered almost all of the area of present-day Canada that lies east of the Rocky Mountains. The southern edge of the ice sheet crossed the geographic region that is now the northern United States, from Montana to North Dakota and south central Iowa, east to southwest Ohio and New York. A large part of the region now comprising Alaska and portions of the Yukon and the Northwest Territories were ice-free. The Cordilleran Ice Sheet covered much of the Rocky Mountains from well into the Canadian Rockies to as far south as the northern parts of present-day Washington and Idaho in the United States. Another ice sheet covered much of the land that is currently northern Europe, from southern England across northern Germany into Poland and eastern Russia. Ice also covered parts of Siberia and the Barents Sea. Smaller ice sheets and ice caps existed at high altitudes in most of the world’s mountain regions. Glacial cycles altered Earth’s landscape significantly. Erosion caused by ice, wind, and water leveled mountains and cut valleys. Melting glaciers deposited huge amounts of rocks and dirt far from their origins. Geologists use aerial photography and satellite imagery (see Remote Sensing) to map the movements of ancient glaciers. Glaciers leave behind telltale features such as cirques, which are depressions at the heads of U-shaped valleys, and vast plains littered with glacial debris. Geologists use radar to look below surface soil for evidence of glacial activity that more recent events might have covered. The weight of the ice pushed down pieces of Earth’s crust during each glacial stage. During the following interglacial stage, the piece of crust would gradually recover and rise back into place. In parts of Canada and Scandinavia, the crust is still rising from the last glacial stage. Huge glacial lakes appeared in North America as the continental ice sheets melted away. The largest of these glacial lakes was Lake Agassiz, which covered parts of the geographic area presently consisting of the states of North and South Dakota and Minnesota in the United States, and the provinces of Saskatchewan, Ontario, and most of Manitoba in Canada. The Great Lakes of central North America are remnants of glacial lakes that the Laurentide Ice Sheet created as it disappeared. Fishing boats towing deep nets off of eastern North America and in the North Sea in the 1800s often brought up wood, peat, and animal remains, indicating that these submerged areas were once dry land. Submersible vehicles have allowed scientists to study caves deep under oceans. Features such as stalactites and stalagmites, which can only form above water, provide confirmation that sea level was lower during parts of the Pleistocene. The lower sea level and changing patterns in ocean currents throughout the world modified the weather and climate of many areas. Even in areas untouched by glaciers, permafrost formed far closer to the equator than it does today. In regions closer to the equator, increased rainfall created lakes in what are now desert areas in North America, Africa, and Australia. In South America, cooler, drier temperatures in what is now rain forest along the Amazon River turned some areas into grassland. Many modern soils upon which present-day humans depend for food are a product of the latter part of the Pleistocene and the Holocene. The glaciers plowed up rocks and dirt as they advanced. Wind took the finer particles of silt and spread them into today’s topsoil.
The changing global temperatures and weather patterns of the Pleistocene Epoch affected animals and plants all over the world, even in areas untouched by ice. Some animals and plants moved from one continent to another, and some species became extinct. Peat bogs that have preserved ancient plants and fossils of plants provide evidence of the characteristics of Pleistocene plant communities. Pollen grains, spores, seeds, leaves, twigs, and mosses all allow scientists to compare Pleistocene plants with modern ones. The remains of simple animals provide additional information about climate and climatic change. Because different beetle species are especially well suited for either warm or cool climates, the presence of fossils of a particular type of beetle can provide scientists with clues to the climate of the region. Fossil algae reveal much about water acidity or alkalinity, water temperature, and the speed of water movement. The Ocean Drilling Program, which collects samples from the seafloor, has collected enough data to show that the distribution of marine organisms changed significantly during the Pleistocene Epoch. Invertebrates—animals without backbones, such as shellfish and insects—and plant communities survived the glacial cycles of the Pleistocene relatively unscathed. Some animal and plant groups, such as the beetles, moved vast distances but underwent little or no evolution. Pleistocene mammals, on the other hand, underwent important changes, probably because climate changes affected mammals more than they did invertebrates. Many mammals have evolved significantly since the Pleistocene. Some changes in familiar animals include greater numbers of species of mice and rats and the appearance of modern species of the dog family. Many mammalian species have become extinct since the Pleistocene. A few of the spectacular mammals that disappeared during the last 20,000 years include the woolly rhinoceros, the giant ground sloth, the saber-toothed tiger, the giant cave bear, the mastodon, and the woolly mammoth. These animals existed at the same time as early humans, and drawings of them exist on cave walls in Europe. Recent theories suggest that these huge mammals could not reproduce quickly enough to replace the number of animals that humans killed for food, and were therefore driven to extinction by human hunting. Humans continued to evolve during the Pleistocene Epoch (see Human Evolution). Two genera, Australopithecus and Homo, existed during the early Pleistocene. The last australopithecines disappeared about one million years before present. Several species of the genus Homo existed during the Pleistocene. Modern humans (Homo sapiens) probably arose from Homo erectus, which are thought to have evolved from Homo habilis. Paleontologists have found fossils that support the transition between Homo erectus and Homo sapiens dating from about 500,000 years before present to about 200,000 years before present. Anatomically modern humans (Homo sapiens) arose from an earlier human species that lived in Africa. A likely ancestor, known as Homo ergaster, evolved by about 1.9 million years ago. This ancestor arose from an earlier Pleistocene species in Africa, perhaps one known as Homo rudolfensis. Anatomically modern Homo sapiens appear to have evolved by 130,000 years ago, if not earlier. For a time our species also coexisted in parts of Eurasia with another species of Homo, Homo neanderthalensis, until around 28,000 years ago (see Neandertals). Since then only our species has survived.
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