The bone that proved whales used to walk

A blue whale has no legs, no hips you would notice, and no reason to be on land. It also has, buried in the wall of its body, the remains of a pelvis.

That is not a curiosity. It is a receipt.

The sequence, with dates

The fossils come mostly from what is now Pakistan, India and Egypt — Eocene coastline, then as now the right place to be buried in fine sediment. The Smithsonian dates the sequence like this:

48m

years ago — Pakicetus, four-legged, running on land, already carrying a whale inner ear

47m

years ago — Ambulocetus, shorter legs, hands and feet enlarged into paddles

46m

years ago — Maiacetus, further out to sea, nostrils migrating back from the snout

37m

years ago — Dorudon, fully aquatic, hind limbs reduced to almost nothing

Eleven million years, start to finish. It is a short walk for something that ends in a blue whale.

What actually changed

Four things, in roughly this order, according to Berkeley's summary of the fossil series.

The ear went first. Pakicetus was a land animal that would have looked something like a long-snouted dog, but its skull already resembles a whale in the inner ear region. Hearing adapted to water before the body did.

Then the nostrils walked backwards. In the early forms they sit at the front of the snout, as they do on a dog. Across the sequence they migrate up and back until, in a modern whale, they are a blowhole on top of the head, above the eyes.

Then the limbs swapped jobs. The forelimbs stopped carrying weight and became steering surfaces. The hind limbs shrank — Berkeley describes them becoming almost nonexistent, then tiny, then vestiges inside the body wall. The 1990 paper in Science that described what was left of them in Basilosaurus is titled "Evidence of Feet in Whales."

Then the tail took over. It got longer and more muscular and grew horizontal flukes. Horizontal, not vertical like a fish — because the animal inherited a spine that bends up and down, the way a running mammal's does. A whale swims by galloping.

The argument that would not close

For most of the twentieth century the fossils showed that whales came from land mammals but not which ones. The favoured answer was the mesonychians, an extinct group of hoofed carnivores. It was a reasonable guess and it was wrong.

Molecular work on living animals kept pointing somewhere else — at the artiodactyls, the even-toed hoofed mammals: deer, camels, pigs, hippos. That answer had a problem. The fossils did not obviously agree, and the two lines of evidence sat there contradicting each other.

The ankle

In September 2001, Thewissen, Williams, Roe and Hussain published skeletons of early Eocene pakicetids in Nature. The skeletons included the astragalus — the ankle bone.

The artiodactyl astragalus has a shape found in no other group of mammals: a pulley at both ends, so the joint rolls in two places instead of one. Berkeley's description of the later basilosaurid version is worth reading twice: a pulley and a hooked knob pointing up towards the leg bones, as in artiodactyls.

It was there, in the whales. The paper concluded that cetaceans are more closely related to artiodactyls than to any mesonychian, and the disagreement between the molecules and the bones closed.

The whale's closest living relative is the hippopotamus.

Why this one is worth knowing

Not because it is strange, though it is. Because of the shape of the argument.

Two independent methods — reading DNA in living animals, and digging up bones from 48 million years ago — were asked the same question and gave different answers. Nobody split the difference. People went and found more ankles.

That is the part worth carrying around. A disagreement between two good sources is not a problem to be smoothed over; it is an instruction about where to dig next.