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?
Sprawling moss, towering trees, flying insects, limbed amphibians: Early species vied for domination as the land went from hostile to hospitable.
On the unforgiving frontier of climate change, polar bears, walruses, seals and penguins find their icy Edens in peril.
