They were strange days at the beginning of the age of mammals. The planet was still hungover from the astonishing disappearance of its marquee superstars, the dinosaurs. Earth’s newest crater was still a smoldering system of hydrothermal vents, roiling under the Gulf of Mexico. In the wake of Armageddon our shell-shocked ancestors meekly negotiated new roles on a planet they inherited quite by accident. Before long, life settled into new rhythms: Earth hosted 50-foot-long boas sliding through steam-bath jungles, birds grew gigantic in imitation of their dearly departed cousins, and mildly modern mammals we might squint to recognize appeared. Within a few million years, loosed from under the iron heel of the vanished giants, they began to experiment. Early whales pranced across a Pakistani archipelago on all fours, testing out life in the water. The first lemur-like primates leapt from the treetops, and hoofed things of all varieties dashed through the forest.
But the most striking feature of this early age of mammals is that it was almost unbelievably hot, so hot that around 50 million years ago there were crocodiles, palm trees, and sand tiger sharks in the Arctic Circle. On the other side of the blue-green orb, in waters that today would surround Antarctica, sea-surface temperatures might have topped an unthinkable 86 degrees Fahrenheit, with near-tropical forests on Antarctica itself. There were perhaps even sprawling, febrile dead zones spanning the tropics, too hot even for animal or plant life of any sort.