What springs to mind when you think of a whale? Blubber, blowholes and flukes are among the hallmarks of the roughly 80 species of cetaceans (whales, dolphins and porpoises) alive today. But, because they are mammals, we know that they must have evolved from land-dwelling ancestors.
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About 375 million years ago, the first tetrapods—vertebrates with arms and legs—pushed themselves out of the swamps and began to live on land. This major evolutionary transition set the stage for all subsequent groups of land-dwelling vertebrates, including a diverse lineage called synapsids, which originated about 306 million years ago. Though these creatures, such as Dimetrodon, looked like reptiles, they were actually the archaic precursors of mammals.
By the time the first mammals evolved 200 million years ago, however, dinosaurs were the dominant vertebrates. Mammals diversified in the shadow of the great archosaurs, and they remained fairly small and secretive until the non-avian dinosaurs were wiped out by a mass extinction 65 million years ago. This global catastrophe cleared the way for a major radiation of mammals. It was only about 10 million years after this extinction—and more than 250 million years since the earliest tetrapods crawled out onto land—that the first whales evolved. These earliest cetaceans were not like the whales we know today, and only recently have paleontologists been able to recognize them.
For more than a century, our knowledge of the whale fossil record was so sparse that no one could be certain what the ancestors of whales looked like. Now the tide has turned. In the space of just three decades, a flood of new fossils has filled in the gaps in our knowledge to turn the origin of whales into one of the best-documented examples of large-scale evolutionary change in the fossil record. These ancestral creatures were stranger than anyone ever expected. There was no straight-line march of terrestrial mammals leading up to fully aquatic whales, but an evolutionary riot of amphibious cetaceans that walked and swam along rivers, estuaries and the coasts of prehistoric Asia. As strange as modern whales are, their fossil predecessors were even stranger.
Pioneers who cleared land in Alabama and Arkansas frequently found enormous round bones. Some settlers used them as fireplace hearths; others propped up fences with the bones or used them as cornerstones; slaves used the bones as pillows. The bones were so numerous that in some fields they were destroyed because they interfered with cultivating the land.
In 1832, a hill collapsed on the Arkansas property of Judge H. Bry and exposed a long sequence of 28 of the circular bones. He thought they might be of scientific interest and sent a package to the American Philosophical Society in Philadelphia. No one quite knew what to make of them. Some of the sediment attached to the bone contained small shells that showed that the large creature had once lived in an ancient sea, but little more could be said with any certainty.
Bry’s donation was soon matched, and even exceeded, by that of Judge John Creagh from Alabama. He had found vertebrae and other fragments while blasting on his property and also sent off a few samples to the Philadelphia society. Richard Harlan reviewed the fossils, which were unlike any he had seen before. He asked for more bones, and Creagh soon sent parts of the skull, jaws, limbs, ribs, and backbone of the enigmatic creature. Given that both Creagh and Bry said they had seen intact vertebral columns in excess of 100 feet in length, the living creature must have been one of the largest vertebrates to have ever lived. But what kind of animal was it?
Harlan thought the bones were most similar to those of extinct marine reptiles such as the long-necked plesiosaurs and streamlined ichthyosaurs. He tentatively assigned it the name Basilosaurus. He wasn’t certain, though. The jaw contained teeth that differed in size and shape, a characteristic of mammals but not most reptiles. Why did the largest fossil reptile that ever lived have mammal-like teeth?
Harlan traveled to London in 1839 to present Basilosaurus to some of the leading paleontologists and anatomists of the day. Richard Owen, a rising star in the academic community, carefully scrutinized every bone, and he even received permission to slice into the teeth to study their microscopic structure. His attention to such tiny details ultimately settled the identification of the sea monster. Basilosaurus did share some traits with marine reptiles, but this was only a superficial case of convergence—of animals in the same habitat evolving similar traits—because both types of creature had lived in the sea. The overall constellation of traits, including double-rooted teeth, unquestionably identified Basilosaurus as a mammal.
A few years later, a scientist handling a different specimen with his colleagues pulled out a bone from the skull, dropped it, and it shattered on the floor. When the unnerved scientists gathered the fragments, they noticed that the bone now revealed the inner ear. There was only one other kind of creature with an inner ear that matched: a whale.