The Americas (also collectively called America) comprise the totality of the continents of North and South America. Together, they make up most of the land in Earth's western hemisphere and comprise the New World.
Along with their associated islands, they cover 8% of Earth's total surface area and 28.4% of its land area. The topography is dominated by the American Cordillera, a long chain of mountains that runs the length of the west coast. The flatter eastern side of the Americas is dominated by large river basins, such as the Amazon, St. Lawrence River / Great Lakes basin, 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 West Africa devastated the indigenous peoples, and the European powers colonized the Americas. Mass emigration from Europe, including large numbers of indentured servants, and importation 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 colonization 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 over 1 billion, with over 65% of them living in one of the three most populous countries (the United States, Brazil, and Mexico). As of the beginning of the 2010s, the most populous urban agglomerations are Mexico City (Mexico), New York (U.S.), Sao Paulo (Brazil), Los Angeles (U.S.), Buenos Aires (Argentina) and Rio de Janeiro (Brazil), all of them megacities (metropolitan areas with ten million inhabitants or more).