If you take a look at a whale or dolphin–any whale or dolphin anywhere on Earth–it will follow certain rules. It will have a blowhole. It will not have legs. It will have horizontal tail flukes it raises and lowers to swim (as opposed to fish that bend from side to side). And it will have been born tail first (as opposed to the head-first position of land mammals). These sorts of rules set whales and dolphins off from other living animals, but only if you look at life as it is today. If you move back in time–especially back between about 50 million and 40 million years ago–the rules collapse.

As I detailed in my book, At the Water’s