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This question has been hotly debated for over a century, especially now with the recent interest in the last Ice Age.According to established theory, sometime during the Pleistocene, around 13,000 years ago, the last great eastern ice sheet, the Laurentian, reached down only to Wisconsin and was already receding.Since 1955 the estimate for the age of the Earth has been based on the assumption that certain meteorite lead isotope ratios are equivalent to the primordial lead isotope ratios on Earth.In 1972 this assumption was shown to be highly questionable.Paleo Indian had immigrated across the Bering Sea by the land bridge of the Aleutian Island chain (known as Beringia) from Siberia into North America and followed a corridor south between the retreating ice sheets, perhaps to hunt wooly mammoth.A more recent theory propose prehistoric Solutreans of Ice Age France also sailed west to America across the Atlantic Ocean along the south ridge of the polar ice cap more than 18,000 years ago.

Sensing an easy target, they closed in for the kill.The Chicago region in particular was once under the southern edge of the Wisconsinan ice sheet.When this massive ice sheet started to retreat 18,000 years ago, it depressed the land and created a widen basin: Much of the city of Chicago lies on beach and lake sediments deposited by Lake Michigan and its predecessor glacial Lake Chicago.Archaeologists called these presumed pioneers the Clovis culture, after distinctive stone tools that were found at sites near Clovis, New Mexico, in the 1920s and 1930s.As caches of Clovis tools were uncovered across North America over subsequent decades, nearly all archaeologists signed on to the idea that the Clovis people were the first Americans.

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