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The earlier of the two epochs of the Quaternary Period, from about 2 million to 10,000 years ago. The Pleistocene Epoch was characterized by the formation of widespread glaciers in the Northern Hemisphere and by the appearance of humans. Mammals included both small forms, such as saber-toothed tigers and horses and giant ones, such as mammoths and mastodons. Almost all the giant mammals, including woolly mammoths, giant wolves, giant ground sloths, and massive wombats disappeared at the end of the Pleistocene and the start of the Holocene. See Chart at geologic time.