Every now and then life throws up an evolutionary backslider: a creature who, having successfully evolved into a new way of life, turns around and evolves right back into something that resembles its primitive ancestor.
The ancestors of tree kangaroos (inhabitants of PNG and far North Queensland) evolved from mammals who came down from the trees and adapted to life on the ground. Then, at some point, they began to re-adapt themselves to life in the trees, taking their big, ungainly, land-adapted feet with them.
Whales have taken things a step further. The ancestors of whales were land dwelling mammals with limbs and toenails, or possibly hoofs. Life had emerged from the ocean 400 million years ago, and was ticking along nicely for more than 300 million years before these ancestral whales decided it would be a bright idea to crawl back into the primal soup. At this point, they grew their limbs back into flippers and went back to live in the ocean like a bunch of fat, bloated fish.
How, whales, is this a positive lifestyle choice? You are now WARM BLOODED creatures who live in a FREEZING OCEAN. You're the size of a TRUCK and you eat things that are the size of a FULL STOP. You breathe AIR and live UNDER WATER. And you wonder why Japanese people want to kill you!