Most of us are familiar with Darwinian evolution. Most people accept the fact that life evolved from the ocean. The trail led from fish to amphibians to reptiles, to more evolved animals like mammals.
What many people don't know, is that some life evolved back into rivers, lakes, and the ocean. Millions of years ago, land-based animals entered the water, eventually to become dolphins, whales, manatees, seals and sea lions, and other marine mammals. The dolphin family tree actually goes back 20 to 40 million years!
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