Jon Negroni has a theory about Pixar films. They are all part of the same universe, connected to each other. This goes way beyond seeing a character from one movie in the background of another (often as a toy). This is a unified theory he explains in a timeline. The first Pixar event in the timeline is the movie Brave, which explains the origin of animals and inanimate objects that talk. He explains that and then goes on.
Centuries later, the animals from Brave that have been experimented on by the witch have interbred, creating a large-scale population of animals slowly gaining personification and intelligence on their own.
There are two progressions: the progression of the animals and the progression of artificial intelligence. The events of the following movies set up a power struggle between humans, animals, and machines. The stage for all-out war in regards to animals is set by Ratatouille, Finding Nemo, and Up, in that order. Notice I left out A Bug’s Life, but I’ll explain why later.
The way he connects each Pixar film, from Cars to Up! to Monsters, Inc. will blow your mind …but not as much as how the end connects to the beginning. Read the whole theory at Negroni's blog. Link -via Geekosystem