In the United States, Native Americans (also known as American Indians, Amerindians, Indigenous Americans or simply Indians; see §Terminology differences) are people descended from the Pre-Columbian indigenous population of the land within the country's modern boundaries. These peoples were composed of numerous distinct tribes, bands, and ethnic groups, and many of these groups survive intact today as partially sovereign nations.Since the end of the 15th century, the migration of Europeans to the Americas has led to centuries of population, cultural, and agricultural transfer and adjustment between Old and New World societies, a process known as the Columbian exchange. Most Native American groups had historically preserved their histories by oral traditions and artwork, which has resulted in the first written sources on the conflict being authored by Europeans.