The primary reasoning behind alternate timelines is that for every event that occurs in our timeline, other realities form in which the alternatives occur. Let’s said that Joe turned left while going to work this morning. Alternate realities with their own “Joe” could have one where he turned right and was late to work, or continued straight ahead and died in a car crash in the intersection.
Ideally, a graph of the timelines should follow a model of fractal geometry. Large branches sprout smaller branches, and those would have even smaller branches, ad infinitum. However, there have been gaps found in this perfect fractal model. When Project Bifrost attempted to map branches produced by differences in the lives of a number of historical figures, they found realities where those people had not existed at all. Instead, a temporal “stand-in” had replaced them in that alternate reality.
A classic illustration of this is the life of Abraham Lincoln. Recon teams sent to various branches formed during the Civil War discovered the existence of a second “Abraham Lincoln” in those realities where Lincoln’s personal history varied. Abe II’s history closely followed the Honest Abe we are all familiar with, but photographs from that time show him as an entirely different person.
This case and others like it have helped to form the Theory of Prime Reality. It is not very popular among the more practical-minded scientists at the Project, since it seems to imply such a thing as Destiny exists. It states that there is actually one “true” reality that weaves through the branches of time and it is found through those people whose lives do not produce temporal copies of themselves, but stand-ins.