15 million years after the end of the Age of Dinosaurs, Earth has recovered from the massive meteor impact. Rainforests dominated the early Eocene, and the planet has a warm, tropical climate. It is this age that the mammals begin to thrive and spread, taking over niches left vacant by the death of the dinosaurs. But it is the birds that dominate this period in Earth's history, and they are almost all killers. But in the water, the first evidence of the mammal's destiny has appeared!