From the point of view of native or indigenous peoples in the United States, and for that matter, the western hemisphere, 1491 was the final year in which their cultures and civilizations were allowed to progress in an undisturbed way, free of widespread European contact, conquest and genocide. While there was undoubtedly some random contact with Europeans before that year, especially via Greenland, these contacts were insignificant in terms of cultural impact.

In 1492, Columbus's arrival would disrupt and shatter native cultures, as historians such as Howard Zinn have documented. This process would continue for centuries. 

In 1491, native cultures in the United States would have been unaware of Christianity, guns, smallpox, European concepts of private ownership of land and racial hierarchy, and unaware that a change was coming that would completely upend their cultures. While the West routinely celebrates the arrival of Columbus as opening the New World to the Europeans, native cultures look at it as the year that brought disaster to their continent, and 1491 as the last year of prosperity for them. The land that comprises the US would be left largely uncolonized for more than a century, but after 1491, the end for them of life as they knew it was in sight.

1491 is the last year before Columbus arrived in the Caribbean, thus launching a long series of European explorations in the New World.  Charles C. Mann's book 1491 describes the importance of this year in early American history.  Before Columbus, native groups in the Americas had a very elaborate culture.  They actively practiced agriculture using companion planting and selective forest burning techniques that have become popular in recent years.  The native groups in this country also had expansive trade networks--there are conch shells from the Gulf of Mexico found in burial sites in New England.  Finally, the Aztec Empire controlled most of Mexico and Central America through trade, war, and tribute.  Historians are quickly growing to appreciate the world that existed before Columbus, as there has long been a bias that has stated that American history really began with the coming of Europeans.  The field of history that studies Pre-Columbian societies is rapidly growing.