You are in the riverbanks of Greenland, 370 million years ago.

The air was thick and humid, smelling of sulfur and decaying algae. As you stepped onto the muddy bank, the wet silence was broken by a wet, scraping sound—like a heavy bag of sand being dragged over damp wood.

Emerging from the shallow water was a creature roughly a meter and a half long, a bizarre patchwork of two worlds. The Ichthyostega hauled itself forward. It didn't walk; it crutched. Using its robust, five-fingered front limbs, it lifted its heavy anterior body and swung its mud-caked chest forward, while its paralyzed hind limbs trailed behind.

The creature stopped, pausing on the shore to sun itself and keep its damp skin hydrated, resting its heavy head and belly on the wet ground, before oblivious to your presence…