1. Whale Evolution

2. ANCIENT LAND ANIMALS MAKE THEIR WAY BACK TO SEA The for-runner of whales originated on land. They reversed the course of evolution form the original land animals that had crawled out of the sea millions of years previously.

3. Evidence: this back-to-the-water evolution is recorded by a profusion of intermediate fossils that have been uncovered over the past two decades.

4. Fossil Record 1978, paleontologist Phil Gingerich discovered a 52- million-year-old skull in Pakistan that resembled fossils of creodonts -- wolf-sized carnivores that lived between 60 and 37 million years ago, in the early Eocene epoch. The skull also had characteristics of Archaeocetes, the oldest known whales. The new bones, dubbed Pakicetus, had features that were transitional between terrestrial mammals and the earliest true whales. One of the most interesting features was the ear region. –In whales, it is modified for directional hearing underwater. –In Pakicetus, the ear region is intermediate between that of terrestrial and fully aquatic animals.

5. Pakicetus

6. A more recent form, called Ambulocetus, was an amphibious animal. Its forelimbs with fingers and small hooves. The hind feet were clearly adapted for swimming. It could get around effectively on land and could swim by pushing back with its hind feet and undulating its tail, as otters do today.

7. Rhodocetus shows evidence of an increasingly marine lifestyle. Its neck vertebrae are shorter, giving it a less flexible, more stable neck. The ear region of its skull is more specialized for underwater hearing. Its legs are disengaged from its pelvis, symbolizing the severance of the connection to land locomotion.

8. 40 million years ago, Basilosaurus was swimming the ancient seas, propelled by its sturdy flippers and long, flexible body. It still retained small, weak hind legs -- baggage from its evolutionary past -- even though it could not walk on land.

9. Gradual Change Each fossil whale shares new, whale-like features with the whales we know today. The fossil record shows the gradual accumulation of these aquatic adaptations that led to modern whales.