 The first is the "short chronology theory", that the first migration occurred after the Last Glacial Maximum, which went into decline after about 19,000 years ago, and was then followed by successive waves of immigrants.The second theory is the "long chronology theory," which proposes that the first group of people entered the Americas at a much earlier date, possibly before 40,000 years ago, followed by a much later second wave of immigrants.The Clovis First theory, which dominated thinking on New World anthropology for much of the 20th century, was challenged by the secure dating of archaeosites in the Americas to before 13,000 years ago in the 2000s.