The phrase "United States" was originally plural, a description of a collection of independent states—e.g., "the United States are"—including in the Thirteenth Amendment to the United States Constitution, ratified in 1865. The singular form—e.g., "the United States is Map of United States of America became popular after the end of the American Civil War. The singular form is now standard; the plural form is retained in the idiom "these United States" The first inhabitants of North America migrated from Siberia by way of the Bering land bridge and arrived at least 15,000 years ago, though increasing evidence suggests an even earlier arrival. Map of Massachusetts Some, such as the pre-Columbian Mississippian culture, developed advanced agriculture, grand architecture, and state-level societies.[58] After the Spanish conquistadors made the first contacts, the native population declined for various reasons, primarily from diseases such as smallpox and measles. Violence was not a significant factor in the overall decline among Native Americans, though conflict among themselves and with Europeans affected specific tribes and various colonial settlements Map USA States In the Hawaiian Islands, the earliest indigenous inhabitants arrived around map of ma 1 AD from Polynesia. Europeans under the British explorer Captain James Cook arrived in the Hawaiian Islands in