The Pixar Theory

Brave

Brave is the first and last movie in the timeline. Obviously, this movie about a Scottish kingdom during the Dark Ages is the earliest time period covered by the Pixar films, but it's also the only Pixar movie that actually explains why animals in the Pixar universe behave like humans sometimes.

In Brave, Merida discovers that there is "magic" that can solve her problems but inadvertently turns her mother into a bear. We find out that this magic comes from an odd witch seemingly connected to the mysterious will-of-the-wisps. Not only do we see animals behaving like humans, but we also see brooms (inanimate objects) behaving like people in the witch's shop.

We also learn that this witch inexplicably disappears every time she passes through doors, leading us to believe that she may not even exist. Don't get ahead of me, but we'll come back to Brave. Let's just say, for now, the witch is someone we know from a different movie in the timeline.

[Some of you pointed out that the animals in Brave gradually regress back into an animal state, disproving the idea that this is the source of animals acting like humans. My rebuttal is simple. They regress because the magic wears off. Over time, their intelligence grows naturally.]

Centuries later, the animals from Brave that have been experimented on by the scientist 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.