Resource: Evolving Ideas: How Do We Know Evolution Happens?

Mammals evolved on land more than 200 million years ago. So how did the world's largest mammals -- whales -- end up back in the water? That's the question Dr. Philip Gingerich, a paleontologist at the University of Michigan, is trying to answer. Gingerich became intrigued when he found what looked like the fossil skull of an early wolf in Pakistan in 1978. But closer examination of this wolflike skull revealed it contained an ear structure resembling that of modern-day whales! This was the first fossil ever found that supported one of Darwin's most controversial ideas -- that whales had descended from land mammals.

Whales are so different anatomically from any other mammals that they make up a separate branch of mammal evolution. Had Gingerich discovered the beginning of that branch? The skull he unearthed was located among land mammal fossils, not in a marine layer of rock. Gingerich named the creature Pakicetus, meaning "whale from Pakistan." Was Pakicetus the land mammal whose descendants became modern whales?

Gingerich wanted to go back to Pakistan to find the animal's legs. However, war in the area kept him from returning. Instead, he went to a place called Zeuglodon Valley, or Valley of the Whales, in Egypt. Here, in the middle of the Sahara, hundreds of whale skeletons lie buried in sandstone. Gingerich's excitement turned to disappointment when he found that most of the skeletons were of Basilosaurus, a known aquatic whale ancestor. But Gingerich kept on digging. A few days later he made a new discovery -- Basilosaurus had legs. Even though Basilosaurus was fully aquatic, it still had vestiges of its terrestrial past. Ten million years of whale evolution had passed between Pakicetus and Basilosaurus, and yet whales still had hind legs and feet. Now the challenge for Gingerich and his colleagues was to fill in the gaps of whale evolutionary history.

Since Gingerich's early discovery in Pakistan, evolutionary biologists have found a series of what they call transitional fossils. These fossils link earlier groups of organisms with more recent groups, often combining traits of each. Some such fossils include 55-million-year-old land-dwelling mesonychids, walking whales called Ambulocetus that could also swim, and Rodhocetus, which were mostly aquatic animals but could probably walk a little on land.

In this way, Gingerich and other evolutionary biologists are slowly piecing together the whale family tree. Unfortunately, because only a small proportion of organisms ever become fossils, it is unlikely that every transition in the evolution of whales or any other group of animals will be recovered. Nonetheless, scientists have uncovered enough fossils to illustrate many important evolutionary transitions, including those between fish and amphibians, reptiles and mammals, and dinosaurs and birds.

Gingerich discusses specifically only the whale fossils found in Egypt's "valley of the whales," but he points out that this now desert area was once a sea. Discuss the other kinds of fossils, and the types of rocks that must have been present for him to make this conclusion.

Examine the cladogram of whales and their ancestors presented in this video. Note that this diagram does NOT show modern whales evolving from any specific fossil form, but form the common ancestors of known fossil species and modern animals. Discuss the important difference between this view of evolutionary history, and the old view -- which often attempted to identify specific fossils as THE ancestors of a living form.

Discuss the similarities and differences between whale flippers and flukes and fish fins, using the scientific terms "homologous structures" and "analogous structures" in the discussion.