We humans are the inheritors of a dynasty that has reigned over the planet for nearly 66 million years, through fiery cataclysm and ice ages: the mammals. Our lineage includes saber-toothed tigers, woolly mammoths, armadillos the size of a car, cave bears three times the weight of a grizzly, clever scurriers that outlasted Tyrannosaurus rex, and even other types of humans, like Neanderthals. Indeed humankind and many of the beloved fellow mammals we share the planet with today—lions, whales, dogs—represent only the few survivors of a sprawling and astonishing family tree that has been pruned by time and mass extinctions. How did we get here?