This is long, and complicated, but awesome. The original theory is not updated so I added some of the newer movies in, which are thesupercarlinbrothers ideas. You can find their channel on youtube.
It starts out with The Good Dinosaur. All of the following things would have happened if the asteroid didn't hit earth. The dinosaurs still would've got exterminated eventually, just not at that exact moment.
Next is Brave. The witch turns Merida's mom into a bear. She still has human features, but is an animal. This could have happened with other animals, but when they permanently turn into one, perhaps they stick with those features. Now those are two big "if"s, but if they are true, it is the base of the theory.
Next is Finding Nemo, a few hundred years later. (Remember not all animals are evolving at the same rate.) Humans have not been treating animals good. This is the first time we see them do something about it. Marlin finds his son and takes him back. Animals are starting to rise up more.
In Finding Dory there is lots of pollution, and only one species can live off that.....
Rats! Ratitoille! This is when the humans find out that animals are evolving. Remy wants to cook, which only humans do. When humans find out rats are evolving, the news is crazy, and the knowledge travels to explorers all over the world like...
Charles Months! Adventure is out there. We see that not all animals are evolving at the same speed. Dogs like Doug are getting smarter but they still need technology to help them talk. Charles Months sees this and invents the leash so dogs can evolve faster. Eventually it will become natural for them. In the end the dogs are set free so that technology gets out, along with the smart dogs. In the beginning the company, Buy-n-Large, or BnL, took Carl's home. BnL has an important part in the future. We see this company in the next few movies.
In Toy Story 3 we see BnL batteries in Buzz. Not all toys need batteries, but the ones that do use those. Those batteries have an effect on all the other toys, until all the toys are alive. They also use human emotions to come alive. I will explain further later.