Free Will.

[Human No More]

One theory is that for every decision, every chance occurrence, there is a parallel universe where the other occurred, to the point that some might just be slightly different, while some might be entirely different, from one where I am a bit taller, to one where we are all Na'vi, to one where life never evolved.
In that theory, we still have free will, but we end up in the appropriate situation influenced by the decision made, while if we made another choice, there would have been another outcome.