The United States of America, commonly known as the United States or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territories, 326 Indian reservations, and some minor possessions. At 3.8 million square miles, it is the world's third- or fourth-largest country by geographic area. The United States shares significant land borders with Canada to the north and Mexico to the south as well as limited maritime borders with the Bahamas, Cuba, and Russia. With a population of more than 331 million people, it is the third most populous country in the world. The national capital is Washington, D.C., and the most populous city is New York City. Paleo-Indians migrated from Siberia to the North American mainland at least 12,000 years ago, and European colonization began in the 16th century. The United States emerged from the thirteen British colonies established along the East Coast.