Two trees, one shore

What the picture shows

1. The sea invents, the land multiplies. Every deep branch starts in the sea, and no animal body plan is known to have begun on land. Yet most known species now live on land, where mountains, deserts and islands keep splitting populations apart.
2. Ashore, the food web is built from the bottom. Plants come first, then plant- and debris-eaters, then hunters. Going back, the sea is already full, so nearly every returnee is a predator; the few plant-eaters stay in the shallows.
3. Extinctions empty the sea’s top floor, and land refills it. Ichthyosaurs arrive about 4 million years after the Great Dying. Penguins arrive about 4 million years after the asteroid, whales and sea cows within 20.
4. Breeding is the last tie, in both directions. Amphibians and land crabs still breed in water; sea turtles, seals and penguins still breed on land. Only lineages that evolved live birth at sea, such as whales, sea cows and ichthyosaurs, cut the cord.

How to read it

• A lineage through time. Vertical joins are where lineages split; † marks extinction.
• A lineage moving onto land.
• A lineage moving into the sea. Arrows leave from the nearest relatives that stayed behind, at the time of the move; the split itself was often earlier.
• Kin: cousins on opposite sides of the shore.
• Form: the same shape or sense, evolved twice.
• Role: the same job in the ecosystem.
• No counterpart on the other side of the shore.
• The five great mass extinctions. The coloured band on top is the standard geological time scale.

Dates are rounded. Fossil and molecular estimates differ, most of all for the mammal splits before the asteroid. This is a selection of lineages: crocodiles, sea snakes, primates and many others are left out.