Nature & Beasts
Why is a whale's nearest living relative the hippopotamus?
- Whales evolved from hoofed land mammals
- They both swim
- They are the same size
The answer is Whales evolved from hoofed land mammals. Whales came from land, and the fossils show the whole walk back in.
Because whales are land mammals that went back to the water, and the group they came from is the even-toed hoofed animals - the same branch that includes hippos, cattle and deer. The molecular evidence said so in the 1990s and the fossils then obliged.
The sequence is one of the best-documented transitions anywhere. Pakicetus, at about 50 million years ago, was a wolf-sized animal that ran on land and had an ear bone found nowhere but in whales; Ambulocetus swam like an otter; Rodhocetus had shrinking legs; Basilosaurus, fully aquatic, still carried tiny hind limbs a few centimetres long. Modern whales retain a vestigial pelvis, and embryos briefly grow hind limb buds that are then reabsorbed.
Living whales carry the land inheritance plainly. They breathe air, are warm-blooded, nurse their young on milk, and have a spine that flexes up and down like a running mammal rather than side to side like a fish. The blowhole is a nostril that migrated to the top of the head, and it is possible to trace its journey across the skull through the fossil sequence.