All the timelines of our universe can be represented as an immense tree. At the base of this tree is a single point; this point is the creation of the universe. Every time a choice is made, or chance affects an outcome, this tree branches, creating a new timeline for every possible outcome of every possible event.
One of these timelines follows a perfectly straight path along the middle of the tree, from creation to the end. On this timeline, and this timeline alone, exists a world known as The Academy. In this Academy are trained the Dimensional Guides, persons with the power to travel between and along all timelines save their own.
The most important duty of these Guides is to watch over the entirety of our universe’s timelines, ensuring that no malicious persons gain the ability to travel through time and affect a timeline besides his or her own. This is accomplished in part through the monitoring of the Great Tree, a holographic projection in the heart of the Academy.
Every branch of the tree is another timeline, and each timeline’s current location in time is indicated by a shining point. Branches beyond the timeline are estimates by the Watcher, leader of the Academy, an immortal with limited foresight. If abrupt changes occur to the tree, Dimensional Guides are sent to ascertain the cause. If the cause is temporal in nature, such as a time machine or a wormhole, the Guides travel back in time to prevent the change and remove the cause entirely.
The most shocking change the Guides ever saw occured in our timeline. Our light reached a certain point on the timeline, and whole branches began to vanish. Worse, the vanishings were, more often than not, of the best possible timelines for our world.
The branch in which humanity cures all diseases, including cancer and the common cold? Erased.
The branch in which the distribution of food is adjusted to end world hunger? Erased.
The branch in which society is pulled out of its darkness and the world comes to true peace? Erased.
Naturally, this greatly alarmed the Guides. They sent a team to our timeline, hoping that the cause was temporal, hoping that it would be a cause they could fix.
They entered our timeline at the moment the first branch vanished, and held themselves still in time, so they could examine the entire world at that moment to find the cause.
Carefully they searched the world, but found no temporal disturbances. They began again, this time searching for acts of destruction, wondering what could be so horrible as to end those good timelines.
They saw murder; but that was not new. They saw slavery; again, it had existed for some time. It had to be something new, the first occurence of some unspeakable crime.
After several passes, they finally found it. They were horrified, repulsed even… but this was not a problem they could solve.
The Guides returned to the Academy and told the rest of their colleagues what had happened to cause the erasure of so many shining timelines.
“The people of Earth,” they said, “now find it acceptable to murder their unborn children.”