The Americas, or America, are lands in the Western Hemisphere that are also known as the New World. Comprising the continents of 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. Extending 14,000 km (8,699 mi) in a north-south orientation, the climate and ecology varies strongly across the Americas, from arctic tundra of Northern Canada, Greenland, and Alaska, to the tropical rain forests in Central America and South America. When the continents joined 3 million years ago, the Great American Interchange resulted in many species being spread across the Americas, such as the cougar, porcupine, opossums, armadillos and hummingbirds.