A Whale of an Argument

When Charles Darwin wrote the first edition of On the Origin of Species, he speculated about how animals like whales might have evolved. We had some whale fossils that had been known in the 1800s and some in the early 1900s as well, but all of these were definitely fully aquatic fossil whales with no hints of them potentially having come from animals that lived on land. Even the best paleontologists of the early twentieth century, like George Gaylord Simpson, had no idea how whales were related to other mammals or where they came from.

Molecular biologists thought that they might be able to answer this question: they compared the proteins that we find in the blood of whales with the proteins of other groups of mammals and they proposed, based on similarity, that whales might be descended from animals called artiodactyls. These are hoofed mammals that have an even number of toes—animals like pigs, deer, goats, and hippos. But paleontologists favored a different group of animals, an extinct group of animals called mesonychids, as potentially giving rise to whales because mesonychids had very similar skulls and teeth.

For well over forty years, paleontologists and molecular biologists disagreed about which group of these animals most likely gave rise to whales. But then in 2001, two different groups of scientists found some early fossil whales that included the ankle bones of some of these early whales with a skull.

Why in the world would we look at ankle bones? Artiodactyls, these even-toed, hoofed animals, have a very distinctive ankle bone that we call an astragalus. Looking at an astragalus from a modern cow, you can see that on both ends there is a shape like a pulley; we often call this a double-pulley astragalus. Artiodactyls are the only animals, living or fossil, that have ankles like this. The first time we found definitive, nice, complete ankle bones with early whales, they looked a lot like this. So finally the paleontologists said, “Okay molecular biologists, you had it right. It looks like whales are actually descended from hoofed animals, called artiodactyls.”