About 50 million years ago, dog-like mammals returned to the seas, eventually evolving into whales and dolphins. Around then, too, an early cousin of elephants took the plunge, giving rise to manatees and dugongs.
About 20 million years later, bearlike mammals also waded back into the sea, evolving into seals, sea lions and walruses.
Each of these marine species adapted to the aquatic life in its own way. Manatees and dugongs slowly graze on sea grass. Seals and their relatives dive deep underwater after prey, but still haul.