Of hippopotami and whales

All the mammals that existed in the early Tertiary period (65-50 million years ago) lived on land. Therefore the aquatic cetaceans (toothed and baleen whales) that began to appear 50 million years ago in the Eocene must have evolved from terrestrial animals. What remains unclear is which group of terrestrial animals returned to the water and exactly how this transition was achieved.

Two separate discoveries of ancient whale fossils in Pakistan; one described in 20 September Nature by Thewissen and colleagues and the second in 21 September Science by Philip Gingerich and colleagues, suggest that the ancestral group of the cetaceans is most probably the artiodactyls. The artiodactyls are even-toed ungulates — hoofed mammals that include cows, hippos, giraffes and camels — with highly distinctive ankle bones.

Gingerich, working in Egypt in the early 1990s, unearthed a fossil whale with leg, foot and toe bones, named Basilosaurus. But close examination...