The Americas, or America, also known as the New World, are the combined continental landmasses of North America and South America, in the Western Hemisphere.”continent n. 5. a.” (1989) Oxford English Dictionary, 2nd edition. Oxford University Press ; “continent1 n.” (2006) The Concise Oxford English Dictionary, 11th edition revised. (Ed.) Catherine Soanes and Angus Stevenson. Oxford University Press; “continent1 n.” (2005) The New Oxford American Dictionary, 2nd edition. (Ed.) Erin McKean. Oxford University Press; “continent [2, n] 4 a” (1996) Webster’s Third New International Dictionary, Unabridged. ProQuest Information and Learning ; “continent” (2007) Encyclopædia Britannica. Retrieved 14 January 2007, from Encyclopædia Britannica Online. 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 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 40,000 BCE and 15,000 BCE. 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. Beginning with the American Revolution in 1776 and Haitian Revolution in 1791, the European powers began to decolonise the Americas. 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, and Portuguese. More than 900 million people live in the Americas, the most populous countries being the United States, Brazil, and Mexico, the most populous cities being São Paulo, Mexico City and New York City.