Around 50 million years ago, in what's now Pakistan, a small four-legged mammal called Pakicetus was hunting fish in shallow rivers. It looked like a cross between a wolf and a deer. It had working ears, working legs, and no swimming ability to speak of. It was also, genetically, the direct ancestor of the blue whale.

The whale's evolution from land mammal to 30-meter ocean giant is one of the best-documented transitions in the fossil record. About 49 million years ago came Ambulocetus — 'walking whale' — which had webbed feet and hunted like a crocodile in estuaries. By 40 million years ago, Basilosaurus had lost its rear legs almost entirely, keeping only tiny useless remnants. By 35 million years ago, whales were fully ocean creatures.

The evidence is written into modern whale anatomy. Whale embryos grow four legs in the womb, then reabsorb the back pair. Adult whales still carry the bones of their lost hind legs — shrunken remnants floating detached inside their bodies, serving no anatomical purpose except to confuse anyone who doesn't believe in evolution.

Whales still breathe air through what used to be a nose, now migrated to the top of their heads. Their closest living land relative is the hippo. Their genes for leg development are still present, just permanently switched off. Occasionally the switch malfunctions, and a whale is born with visible back flippers — a living echo of 50 million years ago.

The largest heart ever to beat belonged to an animal whose ancestors jogged.