Being a fan of Science fiction literature and FMA (The perfect combination for killing brain cells. XD), I realized a connection...
Well, first off, let's start with the butterfly theory. It was mentioned in one of Ray Bradbury's short stories from R is for Rocket, called "A Sound of Thunder".
The theory, and the story itself, explained how destructive it would be if time travelling ever became possible.
The thing is, if you went back into the prehistoric era and stepped on a bug, all the future offspring of that bug would instantly cease to exist, and that's a lot of bugs, not to mention THEIR offspring. All those bugs lost might cause a single bird to starve to death. Think about that bird's entire offspring, and all of the animals that eat that bird, and their offspring. The sudden absence of all these animals could cause, further in the future, for a caveman to starve to death. Now, what if said caveman was the ancestor of Martin Luther King Jr.? Mother Theresa? George W. Bush?
How does this connect to the FMA movie? You see, the Elric brothers were meant to stay in Amestris. They were never meant to go on our side of the gate. They went against the laws of nature, just like a time traveller would do. If this actually happened in real life, what kind of impact would they make on the world? If the Elrics spent the rest of their lives on our side of the gate, how many bugs do you think they stepped on?
Really makes you think, doesn't it? ;D