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A geochronological division of geological time, an epoch of the Quaternary period following the Pliocene. During the Pleistocene, large areas of the northern hemisphere were covered with ice and there were successive glacial advances and retreats. The Lower Pleistocene began c 1.8 million years ago, the Middle Pleistocene c 730,000 years ago, and the Upper Pleistocene c 127,000 years ago; it ended about 10,000 years ago. Most present-day mammals appeared during the Pleistocene. The onset of the Pleistocene was marked by an increasingly cold climate, by the appearance of Calabrian mollusca and Villafranchian fauna with elephant, ox, and horse species, and by changes in foraminifera. The oldest form of man had evolved by the Early Pleistocene (Australopithecus), and in archaeological terms the cultures classed as Palaeolithic all fall within this period. By the mid-Pleistocene, Homo sapiens evolved in Africa and Europe. Homo sapiens spread to Asia and the Americas before the end of the epoch. There were mass extinctions of large and small fauna during the Pleistocene. In North America more than 30 genera of large mammals became extinct within a span of roughly 2,000 years during the late Pleistocene. Of the many causes that have been proposed by scientists for these faunal extinctions, the two most likely are changing environment with changing climate, and the disruption of the ecological pattern by early humans. The Pleistocene was succeeded by the Holocene or present epoch.