Now humans by nature were, still are, and always will be chaotic creatures. Before the appearance of humans on the Planet, the Lifestream was like a great flowing river, wrapped around and within the Planet itself. Now with humans a part of the Planet, parts of the Lifestream began to diverge into smaller streams. Naturally, the Cetra were concerned, but not overly so, because they understood that with the additions of humans on the Planet, there would be a slight increase of chaos which would cause these small diversions. They also knew that eventually these smaller streams will merge back into the main Lifestream itself. (When that would happen of course, no one knows.) Such was the way of things, and the Cetra was a patient race who would wait and watch as they have done since the beginning of their race.
Then the being known as Jenova came, whom the Cetra called the Calamity from the Heavens. The Planet cried out with fear and pain, and the Cetra went to war with the intruder. It was with great cost that the Cetra eventually contained the intruder, but perished in the process. The Planet itself was left with a wound that could not be healed for ages to come.
Jenova was a creature full of chaos, and even though it was eventually sealed, that chaos became a part of the Lifestream as well. With the increased chaos and the lack of ministrations of the Cetra, the Lifestream shattered. Instead of one main river and a few small, diverging streams, the Lifestream had split into hundreds of smaller rivers, with each smaller river diverting into even smaller streams. The rivers and streams themselves were not constant, but joined, diverged, and rejoined with each other so often that each stream no longer resembled each other in any way or form except for the constant image of the Planet itself...