Please explain the significance of the year 1491 had as it pertains to the history of the United States.

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.