Around 400 million years ago, the ancestor of all four-limbed creatures took its first steps onto dry land. Fast-forward about 350 million years, and a descendant of these early landlubbers did an about-face: It waded back into the water. With time, the back-to-the-sea creatures would give rise to animals vastly different from their land-trotting kin: They became the magnificent whales, dolphins, and porpoises that glide through the oceans today.
