RedOrbit: About 10 million years into the current Cenozoic Era, or roughly 56 million years ago, during a climate that was hot and wet, two groups of mammals moved from land to water. These were the cetaceans, which include whales, dolphins and porpoises, and the sirenians, with its sea cows, manatees and dugongs.
Over time, their bodies began to adapt to their new environment. They lost their hind limbs, and their forelimbs began to resemble flippers. Their nostrils moved higher on their skulls. The cetaceans...