Aaron, Jacob, MIB, Bad Twin and some rules by NoFaith

MIB and Jacob are the same person. That person is Arron. Please give this a read before you sigh (I usually shy away from these sort of theories) :).

* Who's Who? *
The MIB"s name is unknown. Similar to the "narrator" in Fight Club, the two main characters are actually the same person. I am not saying, however, that one of them is (Fight Club spoiler) imaginary.

Richard Malkin (Claire's psychic) told us that "darkness" surrounds her baby and that SHE MUST be the one to raise the child. The child must not be raised by another. In the original timeline, Arron is not raised by Claire, so I will assume that the original timeline Arron represents the "dark" version (MIB). Arron's "Bad Twin" is the version of himself that grew up without Claire"s influence.

In the Alt timeline, it seems that Claire will keep Arron, which will fulfill Richard Malkin's requirements that Claire raises the child herself. This version of Arron will be the "non darkness" (light) version of Arron. At some point in the future, the timelines will merge and both version of Arron will find themselves on the island, travel back in time, and begin the circle of events we"ve watched over the past 6 seasons. I don't what the details of the merge will be and it's not important for the fun of this theory :).

* Multiple Versions *
We've seen multiple people exist in the same timeline before. Miles and baby Miles were both in 1977. In "A Shape of Things to Come", Ben is seen looking at his body in the desert in deleted Scene "Desert Stash". So far in Lost, there is no reason to believe there is a rule that would prevent multiple version of a person existing at the same time.

As for the appearance of MIB and Jacob being different from each other, I am going to say that this was a necessity to show the story on screen. We do know that MIB can and has taken on multiple forms in the past, so it"s not too far of a stretch to say that the forms we saw having the conversation on the beach may not be their (original) form. I would actually say that the form of Jacob is Arron's adult form and that MIB is in the form of someone who previously died on the island. With MIB taking Locke's form and essentially being stuck in that form (according to Ilana), MIB appears to exist as "consciousness without a body".


* The Rules and the Beach Scene *
MIB: "Do you have any idea how badly I want to kill you?"
Jacob: "Yes."

We learned in the episode "Dr. Linus" that Richard Alpert cannot kill himself because he was touched by Jacob. I think that, more specifically, it is against the "Laws" of the "Universe/Time Travel/Whatever" for people involved in time travel to kill themselves or even die before their major actions in the timeline are complete. We've seen many Losties have unsuccessful murder or suicide attempts over the past few seasons -- Michael (gun, car). Jack (jump off bridge), Hurley (jump off cliff).

This theory was inspired by "Evidence...Jacob & MIB are a single entity... by Maxx Hitpoints" and "There is only one time line by Daniel Terenzio", both found here at DarkUFO. This is the first theory I've posted after my many years of lurking. It may not be the most original theory, but I think it's fun to consider. At least I didn't say that MIB is Annie :).