In the 1980s Gingerich turned his attention to Wadi Hitan. Along with his wife, paleontologist B. Holly Smith, and their Michigan colleague William Sanders, he began looking for whales in formations some ten million years younger than the beds where he'd found Pakicetus. The trio excavated partial skeletons of fully aquatic whales like Basilosaurus and the smaller, 16-foot Dorudon. These had large, dense auditory bullae and other adaptations for underwater hearing; long, streamlined bodies with elongated spinal columns; and muscular tails to drive them through the water with powerful vertical strokes. The area was teeming with their skeletons. "After a short time in Wadi Hitan you think you're seeing whales everywhere," Smith says. "And after a little more time you realize you really are. We soon understood that we'd never be able to collect all the whales, so we started mapping them and excavating only the most promising specimens."
It wasn't until 1989, however, that the team found the link they were seeking to the whales' terrestrial ancestors, almost by accident. Near the end of the expedition Gingerich was working on a Basilosaurus skeleton when he uncovered the first known whale knee, on a leg positioned much farther down the animal's spinal column than he had expected. Now that the researchers knew where to look for legs, they revisited a number of previously mapped whales and rapidly uncovered a femur, a tibia and fibula, and a lump of bone that formed a whale's foot and ankle. On the last day of the expedition Smith found a complete set of slender, inch-long toes. Seeing those tiny bones brought her to tears. "Knowing that such massive, fully aquatic animals still had functional legs, feet, and toes, realizing what this meant for the evolution of whales—it was overwhelming," she remembers.
Though unable to support a Basilosaurus's weight on land, these legs weren't completely vestigial. They had attachments for powerful muscles, as well as functional ankle joints and complex locking mechanisms in the knee. Gingerich speculates that they served as stimulators or guides during copulation. "It must have been hard to control what was going on down there on that long, snakelike body, so far from the brain," he says.
Whatever Basilosaurus actually did with its little legs, finding them confirmed that the ancestors of whales had once walked, trotted, and galloped on land. But the identity of these ancestors remained unclear. Certain skeletal features of archaic whales, particularly their large, triangular cheek teeth, strongly resembled those of mesonychids, a group of hoofed Eocene carnivores. (The massive, hyena-like Andrewsarchus, probably the largest carnivorous mammal that has ever lived on land, may have been a mesonychid.) In the 1950s immunologists had discovered characteristics in whale blood that suggested a descent from artiodactyls, the mammalian order that includes pigs, deer, camels, and other even-toed ungulates. By the 1990s molecular biologists studying the cetacean genetic code concluded that the whale's closest living relative was one specific ungulate, the hippopotamus.