So my theory is:

A long time ago, there was a world that had a tragic ending. Maybe some sort of environmental apocalypse.
Then a scientist or group of scientists find out about the parallels universe and tried to cross over, but it was too late for their people and almost all die.

The few survivors became the observers of the others universes. Fearing that the other worlds would make the sames mistakes, the intervening once and a while.

I belive Sam Weiss was one of those "First people" but he prefers to live a more normal life.

Great theory! I can see that...

Or maybe they want on of the universe to destroy the otther so they can re create their world...

So in here are my predictions for the future of Fringe:
1.- Fauxlivia will betray Walternative triying to protect her baby.
2.- Peter will use the machine, becomes evil for a while, but ultimately he finds a way to free him self.
3.- Walter VS Walternative ends with both realizing they are being manipulated by thrid party (the first people)
4.- Olivia does something badass to save the Universe (probably really piss off about the baby)
5.- Peter and Olivia break up because they feel like they were manipulated to be together.
6.- The war is over with the other side.