The Fossil Trail

Whale fossils form one of the clearest evolutionary sequences ever found. Ambulocetus, from about 49 million years ago, had webbed feet and could both walk on shore and swim. Basilosaurus, from around 40 million years ago, was fully aquatic and serpent-like — yet it still carried tiny, useless hind legs, leftovers from its land-walking past.

Living whales keep small pelvic bones buried in muscle, attached to nothing structural. Evolution does not wipe the slate clean. It leaves receipts.