The Ear-Bone Tell

If you saw Pakicetus alive, you would never guess it was a whale. It was a wolf-sized, hoofed runner, far more at home on land than in deep water. So how do we know it belongs on the whale family tree? The proof hides inside its skull. Its ear bones were already shaped to channel sound the way every modern whale's ears do — an unbroken thread linking this little land animal to the great cetaceans.

Step by Step

Over roughly 10 million years, Pakicetus's descendants moved fully into the water — and almost every stage left a fossil. Read in order, they form one of the most complete transitions in all of evolution:

• Pakicetus: a four-legged runner near the water's edge.
• Ambulocetus: webbed feet for both walking and swimming.
• Rodhocetus: shrinking legs and a stronger swimming tail.
• Basilosaurus: a long, fully aquatic giant.