the Mackenzie River drains waters from the Arctic Great Lakes of Arctic Canada, as opposed to the Saint-Lawrence River that drains the Great Lakes of Southern Canada into the Atlantic Ocean. The Mackenzie River is the largest in Canada and drains . The largest river basin in South America is that of the Amazon, which has the highest volume flow of any river on Earth. The second largest watershed of South America is that of the Paraná River, which covers about 2.5 million km2. Ecology North America and South America began to develop a shared population of flora and fauna around 2.5 million years ago, when continental drift brought the two continents into contact via the Isthmus of Panama. Initially, the exchange of biota was roughly equal, with North American genera migrating into South America in about the same proportions as South American genera migrated into North America. This exchange is known as the Great American Interchange. The exchange became lopsided after roughly a million years, with the total spread of South American genera into North America far more limited in scope than the spread on North American genera into South America. Countries and territories There are 35 sovereign states in the Americas, as well as an autonomous country of Denmark, three overseas departments of France, three overseas collectivities of France, and one uninhabited territory of France, eight overseas territories of the United Kingdom, three constituent countries of the Netherlands, three public bodies of the Netherlands, two unincorporated territories of the United States, and one uninhabited territory of the United States. Demography Population In 2021 the total population of the Americas was about 1.03 billion people, divided as follows: North America: 596.6 million (includes Central America and the Caribbean) South America: 434.3 million Largest urban centers There are three urban centers that each hold titles for being the largest population area based on the three main demographic concepts: City proper A city proper is the locality with legally fixed boundaries and an administratively recognized urban status that is usually characterized by some form of local government. Urban area An urban area is characterized by higher population density and vast human features in comparison to areas surrounding it. Urban areas may be cities, towns or conurbations, but the term is not commonly extended to rural settlements such as villages and hamlets. Urban areas are created and further developed by the process