#1 Axiom
Because the machines tip everything out of balance, Earth becomes an unfit planet for humans and animals, so the remaining humans are put on Axiom (or Noah's Ark if you want to carry on the Biblical theme where Wall-E is basically Robot Chosen One and his love interest is aptly named Eve) as a last-ditch effort to save the human race.
#2 Lazy Humans
On Axiom, the humans have no purpose aside from having their needs met by the machines. The machines have made humans dependent on them for everything because that is how they were treated as "toys." It's all they know.
#3 Where are the humans?
Meanwhile on Earth, machines are left behind to populate the world and run things, explaining human landmarks and traditions still being prominent in Cars. There are no animals or humans in this version of Earth because they're all gone, but we do know that the planet still has many human influences left.
In Cars 2, the cars go to Europe and Japan, making it plain that this is all taking place on Earth as we know it. So what happened to the cars? We've learned by now that humans are the source of energy for the machines. That's why they never got rid of them.
#4 Energy Crisis
In Wall-E, they point out that BNL intended to bring the humans back once the planet was clean again, but they failed. The machines on Earth eventually died out, though we don't know how.
What we do know is that there is an energy crisis in Cars 2, with oil being the only way society trudges on despite its dangers. We even learn that the Allinol corporation was using "green energy" as a catalyst for a fuel war in order to turn cars away from alternative energy sources. That "clean" fuel could have been used to wipe out many of the cars, very quickly.
#5 Wall-E
Which brings us back to Wall-E. Have you ever wondered why Wall-E was the only machine left? We know that the movie begins 800 years after humans have left Earth on Axiom, governed by the Auto Pilot (another A.I. reference).
Could it be that Wall-E's fascination with human culture and friendship with a cockroach is what allowed him to keep finding fulfillment and the ability to maintain his personality? That's why he was special and liberated the humans.
He remembered the times when humans and machines lived in peace, away from all of the pollution caused by both sides.
#6 The Tree
After Wall-E liberates the humans and they rebuild society back on Earth, what happens then? During the end credits of Wall-E, we see the shoe that contains the last of plant life. It grows into a mighty tree. A tree that strikingly resembles the central tree in A Bug's Life.
That's right. The reason no humans show up in A Bug's Life is because there are not a lot left. We know because of the cockroach that some of the insects survived, meaning they would have rebounded a bit faster, though the movie had to be far enough in the timeline for birds to have returned as well, though they're noticeably less intelligent than the bugs.
#7 Bug Cities
There's something strikingly different about A Bug's Life when compared to other Pixar portrayals of animals, which leads me to believe it takes place in the future. Unlike Ratatouille, Up, and Finding Nemo, the bugs have many human activities similar to what the rats in Ratatouille were merely experimenting with.
The bugs have cities, bars, advertisements, their own machines, know what a Bloody Mary is and even have a traveling circus. This all assumes that the movie is in a different time period.
The other factor that sets A Bug's Life apart from other Pixar movies is the fact that it is the only one, besides Cars and Cars 2, that does not revolve (or even include) humans.
#8 Monsters Inc.
So what happens next? Humanity, machines, and animals grow in harmony to the point where a new super species is born. Monsters. The monsters civilization is actually Earth in the incredibly distant future.
Where did they come from? It's possible that the monsters are simply the personified animals mutated after the diseased earth was irradiated for 800 years.
Whatever the reason, these monsters seem to all look like horribly mutated animals, only larger and civilized. They have cities and even colleges, as we see in Monsters University.
#9 Gather Energy
The monsters are going back in time. They're harvesting energy to keep from becoming extinct by going back to when humans were most prominent. The peak of civilization, if you will. Though a lot of time has passed, animosity towards humans never really went away for animals/monsters.
Monsters must have relied on anti-human instincts to believe that just touching a human would corrupt their world like it did in the past. So they scare humans to gather their energy until they realize that laughter (green energy) is more efficient because it is positive in nature.
#10 More on Energy Crisis
In Monsters Inc., they have an energy crisis because they are in a future earth without humans. Humans are the source of energy, but thanks to the machines, again, the Monsters find a way to use doors to travel to the human world. Only, it's not different dimensions.
That said, Monsters Inc. is so far the most futuristic Pixar movie. By the end, humans, animals, and machines have finally found a way to understand each other and live harmoniously.
#11 Look Familiar
We even see a connection between A Bug's Life and Monsters Inc. via the trailer we see in both movies. As you can see, the trailer looks exactly the same, except the one in A Bug's Life is noticeably older and more decrepit, while the one in Monsters Inc. (where Randall is sent via a door) has humans and looks newer.
Look at the picture above. On the left is the trailer from A Bug's Life and the one on the right is from Monsters Inc. The one on the left looks older and more rundown. Even the vegetation is noticeably dryer and there's less of it. The trailer on the right has humans and the frame even includes tall grass and a tree hanging overhead.
#12 Boo
And then there's Boo. What do you think happened to her? She saw everything take place in future earth where "kitty" was able to talk. She became obsessed with finding out what happened to her friend Sully and why animals in her time weren't quite as smart as the ones she'd seen in the future.
She remembers that "doors" are the key to how she found Sully in the first place and becomes
#13 One in the same?
A WITCH. Yes, Boo is the witch from Brave. She figures out how to travel in time to find Sully, and goes back to what she believes is the source: The will-of-the-wisps.
They are what started everything, and as a witch, she cultivates this magic in an attempt to find Sully by creating doors going backwards and forwards in time.
[Just to clarify: The theory is that Boo discovered a way to use doors to travel through time on her own, possibly by developing magic on her own. She probably went back in time to the Dark Ages to get more magic from the will-o-wisps.]
#14 How do we know?
In Brave, you can briefly see a drawing in the workshop. It's Sully. We even see the Pizza Planet truck carved as a wooden toy in her shop, which makes no sense unless she's seen one before(and I'm sure she has since that truck is in almost every Pixar movie).
You remember Merida opening doors and the witch constantly disappearing? It's because those doors are made the same way from Monsters Inc. They transport across time and that is why Merida could not find the witch later in the movie.
But wait. How did Boo travel in time in the first place, and why is she obsessed with wood? Boo must have discovered that wood has been the source of energy all along, not just humans. The machines and monsters in Monsters Inc. use doors because they're made of wood and found a way to use that energy to travel in time.
#15 The Search for Sully
Obsessed with finding Sully, Boo traveled across the Pixar universe using doors. So Boo went back to the Dark Ages, probably because she could use plenty of wood there for her experiments or to study the will-o-wisps. We know that her first encounter with Mor'du ended with her turning him into a monstrous bear, but he regresses.
She probably wanted to turn him into a bear because Sully resembles a bear, and she is still trying to figure out where Sully comes from.
Does Boo ever find Sully? I like to think so. He surely reunited with her at least once as a child at the end of Monsters Inc., but eventually, he had to stop visiting.
But her love for Sully is, after all, the crux of the entire Pixar universe. The love of different people of different ages and even different species finding ways to live on Earth without destroying it because of a lust for energy.
