The Good Dinosaur (2015)

                            [ 66 million years ago ]

If you really want to see how the theory begins, you must start here and learn the story of a shy Apatosaurus, named Arlo, who is stranded far from home. But he meets a cave boy, Spot, and they work their way home.

This film sets the stage for the Pixar cinematic universe, showing us an alternate history of Earth, one in which dinosaurs are never killed by the asteroid's impact. This allowed dinosaurs to advance to a level close to humans, foreshadowing the capabilities of animals in the future. However, we also see dinosaurs struggle, especially in the new world, where humans are succeeding.