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The Pleistocene is an epoch of the longer Neogene period. It extends from 1,808,000 to 11,550 years ago, when the Earth warmed up from its most recent glaciation. In terms of human progress, the end of this epoch is also the boundary between the Old Stone Age (Paleolithic) to the Middle Stone Age (Mesolithic). This is when modern humans emerged and basically took over the planet.
Like the rest of the Neogene, the Pleistocene was a relatively cold time. The world experienced a cycle of glaciations, with highs like the current climate and lows where much of modern-day Canada, Europe, and Asia were under thousands of feet of ice. The epoch featured many large mammalian fauna, such as mastodons, mammoths, cave bears, and many others. These are called megafauna, and it is thought that humans made most of them extinct when they spread across the globe 100,000 to 30,000 years ago. The decline in large animal fossils synchronizes nicely with human migration patterns. Most efforts to blame the extinction on other causes, such as "hyperdisease," have been relatively futile.
During the major periods of glaciation, glaciers were 0.9 to 1.8 miles (1.5 to 3 km) thick, similar to Antarctica today. This locked up massive amounts of water, leading to temporary sea level drops of 328 feet (100 m). The sea level drop opened up some tracts of land currently underwater, such as the North Sea (called Doggerland), the Bering Strait (Beringia), and the area around Indonesia (Sundaland). All of these regions were eventually inhabited by humans, and helped make it possible for ancient peoples to colonize Australia from Asia.
During the Pleistocene, the higher latitudes had more and larger lakes due to glacial runoff and decreased evaporation from low temperatures. Lake Agassiz, a prehistoric lake in present-day Canada, was larger than any contemporary lake, including the Caspian Sea. When the glaciation ended, it may have drained into Hudson Bay in as little as a year, increasing world sea levels by as much as 3.2 feet (1 meter). This is one of many events cited as possible inspiration for biblical flood stories.