After the dinosaurs died out, Earth was by no means empty. There were still some little resourceful critters around who, without big predators to hunt them down, survived and thrived. Who were these scrappy creatures? Early mammals, our ancestors! In the Cenozoic Era, mammals rose to dominance and spread over the globe, resulting in woolly mammoths, saber-toothed tigers, and eventually all of humankind. In this adventure, we meet the three kinds of mammals - monotremes, marsupials, and placental mammals - as well as the other amazing ancient beasts they shared the Earth with during the Cenozoic Era.