The First Whale
Officially considered the first whale, wolf-sized Pakicetus lived about 45 million years ago in a river delta in what's now Pakistan. In many ways it was still accustomed to living on land -- after all, it still had feet -- but other traits tell of adaptation to submergence.
The ears of Pakicetus seem attuned to detecting underwater vibration, and its eyes were positioned close together on the top of its head, as is common in animals that spend much of their time submerged, looking at prey above. Its bones were also unusually heavy, making them useful as ballast.
People who study the evolution of whales think Pakicetus may have lived on land but hunted in water, ultimately sending its descendants on a completely aquatic journey.
Image: Carl Buell