The Americas, or America, also known as the Western Hemisphere and the New World, comprise the totality of territories in North America and South America. Along with their associated islands, they cover 8.3% of the Earth's total surface area (28.4% of its land area). The topography is dominated by the American Cordillera, a long chain of mountains that run the length of the west coast. The flatter eastern side of the Americas is dominated by large river basins, such as the Amazon, Mississippi, and La Plata. Since the Americas extend 14,000 km (8,700 mi) from north to south, the climate and ecology vary widely, from the arctic tundra of Northern Canada, Greenland, and Alaska, to the tropical rain forests in Central America and South America.Humans first settled the Americas from Asia between 42,000 and 17,000 years ago. A second migration of Na-Dene speakers followed later from Asia. The subsequent migration of the Inuit into the neoarctic around 3500 BCE completed what is generally regarded as the settlement by the indigenous peoples of the Americas.The first known European settlement in the Americas was by the Norse explorer Leif Ericson. However the colonization never became permanent and was later abandoned. The voyages of Christopher Columbus from 1492 to 1502 resulted in permanent contact with European (and subsequently, other Old World) powers, which led to the Columbian exchange. Diseases introduced from Europe and Africa devastated the indigenous peoples, and the European powers colonised the Americas. Mass emigration from Europe, including large numbers of indentured servants, and forced immigration of African slaves largely replaced the indigenous peoples.Decolonization of the Americas began with the American Revolution in 1776 and Haitian Revolution in 1791. Currently, almost all of the population of the Americas resides in independent countries; however, the legacy of the colonisation and settlement by Europeans is that the Americas share many common cultural traits, most notably Christianity and the use of Indo-European languages; primarily Spanish, English, Portuguese, French and to a lesser extent, Dutch. The population is approaching 1 billion, with over 65% of them living in one of the three most populous countries (the United States, Brazil, and Mexico). The most populous cities are São Paulo, Mexico City, New York City, Buenos Aires and Los Angeles.