Biggest.
The question leads straight to existentialism. However, it doesn't stop at mere philosophy. History, technology, lineages, etc all likely come into play, as well as a great deal of spiritualism.
Quantum Mechanics imposes some heavy consequences on the idea of absolute reality. It may not be quite as flexible as many people like to suggest it is, but it leaves a lot of open territory, particularly as it pertains to "souls" - or 'the whole being greater than the sum of its parts.'
The deeper we go, the more unrealistic it all becomes.
QM is unique in that cause and effect do not have to occur sequentially in time. A coherent system, such as a Bose-Einstein Condensate, can be hit by energy and reveal a set of states. From that set of given states, there are a limited number of possible states that can precede and proceed it. The implications for the galactic scale should be intriguing to many, as this means there is the potential to define exactly how much information about a system at a given distance interval and energy "interactivity" has become coherent and subject to QM… But I digress…
Imagine that a being of substantially higher order decides to 'create' humanity. This resolves a planet to a state at which the desired life form exists, and all past states are simply probabilistic and are determined at the time of their polling. This would, for example, explain why the isotope composition in igneous rock does not match up with its age. QM doesn't proceed the same way classical physics does, and therefor none of the implied events of geology needed to actually occur in 'real time' as we would expect.
Thus, the Earth can be a few thousand years old and a few billion years old at the same time, as many of the details were resolved at the moment of creation, acting both forward and backward in time to create a largely consistent (logically) universe… Within QM. This explains why random evolution can act in impossible ways, such as gene combinations across the whole of a species as if each member of a species mated with each other one to create new proteins and tissue groups in order to 'make it in time' for us to exist.
Of course, this has vast implications for the concept of aliens and what technology may have existed at the dawn of creation … And why it is no longer around (or why/how it has been squandered away).
This is the Unified Conspiracy Field Theorem. The conspiracy theory to contain and combine them all into a cohesive thesis. Also known as insanity. Speaking of which, I have tea at home and need to go drink it.