Answer: Cetaceans

Whales, and their cetacean cousins the dolphins and porpoises, are quite a huge curiosity in the animal kingdom. Truly, there are small curiosities here and there like fish that lose their eyesight or insects that hibernate for decades, but the cetaceans are a curiosity oh so much larger and complex.

What makes them so interesting? The cetaceans, from the tiniest porpoise to the most massive Blue Whale, are descended from land-dwelling mammals. They breathe air, they have limb-like bone structures in their fins, and the vertical movement of their spines is more closely matched to the movement found in mammals that run on land than to fish that swim in the water.

For the better part of the 19th and 20th centuries, it was a mystery as to how water-dwelling mammals even came about; most theories centered on identifying relatives via skeletal or dental similarities in the fossil record. The point of divergence from other species was murky at best though.

New fossils discovered in Pakistan in the late 1970s, however, painted a new picture of the cetacean family tree and it is now accepted that the family is closely related to prehistoric artiodactyls and their closest living relative outside of the ocean is the modern day hippopotamus.