For billions of years, land on Earth was uninhabitable. But in the seas, predation allowed species to thrive before — and after — two mass extinctions.

Sprawling moss, towering trees, flying insects, limbed amphibians: Early species vied for domination as the land went from hostile to hospitable.

After Earth's third mass extinction, mammals' surviving ancestors ruled the supercontinent Pangea. But lizards soon ushered in the age of reptiles.

Evolution. Competition. Mass extinction. Three fundamental rules have driven the rise and fall of life on Earth for over four billion years.

The formation of continents with varied environments allowed for an explosion of biodiversity — and turbo-charged the evolution of mighty dinosaurs.

The dinosaurs met their end with a cataclysmic asteroid impact. Rising from the ashes, birds reinvented themselves into a dynasty 10,000 species strong.

Emerging from the dinosaurs' shadows, mammals went from underdogs to global power, with game-changing adaptations that would conquer land, air and sea.

As the Ice Age thawed, humans rose above the rest. But the possibility of a sixth mass extinction now looms: Has our ingenuity caused our downfall?