There's an interesting theory floating around about the Pixar films. I'll explain the theory to you and link you to longer articles/videos about this theory if you are interested. If you don't care--just stop reading here. I should also point out that it is a theory which means it's just a fun way to link the entire thing together for fun using easter eggs hidden in the films and imagination.
Brave is the first and (arguably) the most important movie in the timeline, because it is the beginning and it's characters are the most important. This is set in Scotland during the Dark Ages which is way before anything else in the Pixar universe. Brave is the only movie to explain why all of these animals can talk--magic. We assume these kinds of things but no other movie explicitly tells us how any animals can naturally start behaving like humans. Animals behaving like humans is started by the wood carving witch whose workshop is filled with easter eggs from other movies and is connected to the will-o-the-wisps. We'll get back to Brave in a bit.
So centuries after the witch has been experimenting on personifying animals, we have movies like Up, Ratatouille, and Finding Nemo. In Ratatouille, the animals with human personalities are starting to branch out of their animal colonies/groups/what have you and doing distinctly human activities--like cooking. The villian of Ratatouille (Chef Skinner) disappears--taking with him the fact that animals are starting to attain human characteristics. They can't speak to humans yet, of course, but that is remedied soon enough in Up. Up and Ratatouille both occur in roughly the same time period. So it is possible that Charles Muntz, the villain of Up, heard of this rumor and decided to get the jump on using these animals of growing intelligence for his own personal gain. These animals are set free to roam the earth with (possibly) a growing animosity towards humans.
In the beginning of Up, Carl is being forced to give up his house to urban development via a corporation. This corporation is, arguably, Buy-n-Large which is the corp that runs EVERYTHING in the Wall-E universe. They control the government, the people, the planet... everything. This corporation is alluded to in Toy Story 3 and a short article in Finding Nemo. In Finding Nemo, we have a boat load (haha) of fish trying to save this one teensy fish with a bad fin from the humans. The article that references BNL in this movie is talking about the beatiful underwater world--and people are invading it and antagonizing the ever-growing population of intelligent animals.
And then, there is the growth of Artificial Intelligence. For this, we begin with The Incredibles which is set in the eighties-ish time period. Syndrome uses technology to destroy all the supers--this technology learns from everything done to it and advances itself--soon enough, the killbot turns on its owner because it has learned that it does not have to take orders from Syndrome. Years later, the Toy Story world begins. These toys are being powered by the "Zero Point Energy" that Syndrome used to power his lasers. The toys (and AI in general) are completely reliant on humans and they are getting tired of being tossed around and discarded. This leads to the Artificial Intelligent machines developing a human corporation where it uses and controls humans with a faceless corp that slowly takes over everything. There's an industrial revolution and what goes best with an industrial revolution? Pollution. Who hates pollution? Animals. So the animals rise up against the humans to stop them from polluting... and who saves the humans? The machines. The Earth soon becomes unfit for humans and animals alike, so all humans are put on a space ship and sent off the earth to 'save humanity.' But the humans bred on that ship just sit and are given things by machines, making humans completely depandant on the machines because they are too big to do it themselves. Oh how the tables have turned. Machines are thriving on earth by themselves--creating the world for Cars. Originally, the machines intended to fix the planet... but machinery used up all the resources and eventually they ran out which created the world for Wall-E where he is the only machine left to clean up after everyone.
So Wall-E liberates humans again and replants the last of plant life which grows into a big tree--that looks a lot like the central tree in A Bugs Life. The explaination for why bugs are plentiful and humans are non-existant is that the bugs bounced back a lot faster--like the little cockroach that keeps Wall-E company. Humans are still scarce, though bugs live and thrive in their once-great cities. The bugs are a lot different than the animals from before because they seem to be completely steeped in human culture even though there are few humans around. So humanity rebuilds itself and lends a helping hand to machines and animals. What comes next? The super-species of Monsters. Humans could have possibly died out after the rebuilding of the earth, which would explain the energy crisis in the time period of Monsters, Inc. So what do t hey do? They create time-travel technology to go back in time to the peak of human civilization to collect the screams of children.
So Boo knows about the monster world after Monsters, Inc. She becomes obsessed with finding Sully and remembers that doors are how they travelled. Who else travels and disappears in and out of doors? That's right--the witch from Brave. She figures out how to travel through time and goes back to what she thinks is the source--the will-o-the-wisps. In her wood carving woodshop, you can see a carving of the Planet express truck as well as an etching of Sully.
The article where this theory is found is located here:
Although my explaination of the theory is long, this article provides a lot more details. Trust me.
The video that inspired the article can be found here: