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The resulting Andes Mountain chain isolated the South American interior, trapping sea creatures in a pool of water that began to slowly change from saltwater to fresh. Over time, the resulting pool of freshwater began to flow eastward toward the Atlantic, forming the thousands of rivers of the Amazon basin, and the Amazon River itself. Those plants and animals that could adapt to their new freshwater environment not only survived, but proliferated under millions of years of the tropical, equatorial sun. Many of today's Amazon creatures are reminders of the continent's saltwater beginnings. Offshoot freshwater species of manatees, dolphins, crabs, manta rays, and other critters make their home in the thousands of rivers of the Amazon Basin.
Humans entered the South American continent approximately 15,000 years ago to find Earth's largest rainforest teeming with the most diverse forms of life on the planet. Flooded much of the time, the Amazon Basin's thousands of ever-shifting rivers ensured limited human habitation. That is the case in the Beni area of Bolivia, where the 2000-mile long Mamore River, the longest tributary on Earth, floods annually and keeps the region wild. Those people that do live in the Amazon Basin have adapted well. Their main mode of transport is the dugout canoe, and they find plenty of food in the animals, fish, and fruit of the surrounding rainforests and rivers.
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