EVOLUTION × LIFE

Whales Return to the Sea

~50 to 38 million years ago

Phanerozoic · Cenozoic · Paleogene · Eocene

Whales descend from small hoofed land mammals, and their closest known relative outside the group is a cat-sized wader called Indohyus: its ear bones are built like a whale's, and its limb bones are unusually dense, the ballast of an animal that spends its time in water. Within about twelve million years the lineage had produced fully aquatic animals with hind legs reduced to stubs.

Why it matters

This is one of the best documented major transitions in the fossil record, and it runs the other way: a land vertebrate going back to the water its ancestors had crawled out of. Followed step by step, it shows what such a change actually costs, since the ear, the limb bones and the diet were all remade, and the earliest of those changes were under way while the animal still walked.

Dating & uncertainty

The transition is documented by a series of fossils from India, Pakistan and Egypt spanning roughly 50 to 38 Ma. Individual sites are dated with differing precision, so the twelve million years is a bracket rather than a stopwatch, and the earliest steps rest on ears, teeth and limb bones rather than on complete skeletons.

Sources

1. Thewissen et al. (2007), Nature · Whales originated from aquatic artiodactyls in the Eocene epoch of India
2. Gingerich et al. (2001), Science · Origin of whales from early artiodactyls: hands and feet of Eocene Protocetidae from Pakistan
3. Antar et al. (2023), Communications Biology · A diminutive new basilosaurid whale reveals the trajectory of the cetacean life histories during the Eocene
4. Thewissen et al. (2009), Evolution: Education and Outreach · From land to water: the origin of whales, dolphins, and porpoises

Earth history as one day

23:44:0815 min 52 s before midnight

if Earth’s 4.54 billion years were compressed into a single day