Where does it all start? [Only a Theory]
Sooooooo.... many viewers believe that the bomb detonating in 1977 created the universe we see in the flash-sideways. In other words, a branching of universes happened at the time of the incident: one where it didn't work and everything that we were told had happened had, indeed, happened; and the other where the island was destroyed and history starting in 1977 was altered.
This means that both 'universes' shared an identical past up to the point of detonation! Well, this is the reasoning I use to disprove that theory! Follow me.
Let's start by naming our original universe/timeline U0, the flash-sideways show UA.
Assumption: The detonation of the bomb created the UA.
-Corollary: Both universes share an identical past up to the point of detonation of the bomb.
Proof of corollary: In the U0, Christian shows Sun and Frank a picture of the DHARMA recruits, indicating that they were at least in U0's past. If the detonation of the bomb created UA, then it follows they were also in UA's past, therefore U0 and UA share a common past up to the detonation.
WHAT WE KNOW
-Ben was still recovering when the bomb went off on Day 3 of the Oceanic 6's arrival (Whatever Happened, Happened, Dead Is Dead). The incident happened on Day 4. It's not clear whether Ben was returned to DHARMA (Widmore tells him soon, but how soon) and made it on the sub.
-Roger was still in the camp a mere hour or two (estimate) before the bomb detonated (The Incident). He wasn't with his son and it doesn't give him much chance to evacuate.
-We are told that Roger and Ben did spend time on the island as part of DHARMA in UA. This is irrelevant as it coincides with what we already knew of U0. What is relevant is what Roger says to Ben:
"Imagine how different our lives would have been if we'd stayed. [...] Who knows what you would have become?", (Dr. Linus).
This suggests that they left out of choice and not through an evacuation and leaves the possibility of having stayed on the island, which is contrary to the bomb sinking the island. According to what they know, the island had at least a short term future when they left.
Given that both Ben and Roger are alive and seem to have left the island out of choice at a time where the island was still in good health (figuratively), this contradicts the corollary that U0 and UA share a common past and therefore contradicts the starting assumption.
--PROPOSED ALTERNATIVE THEORY--
I propose an alternative theory in which not only is the bomb not, itself, directly responsible for the island sinking and also aligns with the whatever happened, happened school of thought.
Let's start with what we know.
WHAT WE KNOW
-Year when the statue was destroyed (Ab Aeterno) in a storm that brought the Black Rock inland through the statue itself.
-Corollary: Sawyer, Jin, Juliet and Miles jumped as far back as 1867 and possibly earlier as proven by the full-body statue (This Place Is Death, LaFleur).
Pre-1867 (let's call it Tx):
-there is a random rope sticking from the ground (This Place Is Death),
and since Sawyer let go of it before the next flash (LaFleur) the rope remained in Tx.
-The well that Locke used to lower himself down to the Dharmacakra chamber was filled by DHARMA times (LaFleur).
-Corollary: The well was dug prior to the DHARMA Initiative and after Tx - no time known.
At any given point:
-A giant Tawaret statue has been on the island for a long time
-An elaborate Temple occupies a section of the island
-Ruins have been found on different occasions on the island dating back to 1867 and Tx.
--The dense presence of cultural symbols and artifacts suggests the presence of one or more civilization dating possibly as far back as Egyptian times, at least significantly further back than 1867.
Here, I posit a theory suggesting there is a branching happening at Tx - when the rope was left into the ground.
I assume that everyone is aware of the butterfly effect (with respect to time travel) that the smallest change can alter history, as small as a rope sticking from the ground as opposed to no rope. But how do I think it happened?
Assumption: Tx is significantly earlier than 1867. The presence of the statue only proves it was earlier, it does not suggest it was soon before.
Assumption: Tx happens during or between the presence of civilization(s) on the island.
Once Sawyer's group left the rope in the ground, it was laying there for a member of an existing group to find it and try to dig it out. They eventually found the cave, and possibly the Dharmacakra wheel, and ultimately released the smoke-monster/MIB -- and... maybe Jacob? (no confirmation)
This would result in the conflict between Jacob and MIB taking form and controlling all aspects of life on the island from this point forward, leading to Hanso and Richard ending up on the island, the conflict with the US army and DHARMA, the bomb creating the incident, leading to the Swan issue with Desmond and Oceanic 815 crashing and everything we've seen since the beginning of the show.
The absence of time travel, the rope is never left in the ground. Preventing the escape of the smoke monster, there would never have been this conflict and ultimately no time travel. Ben would never have been recruited by the Others and Roger angered by the hostiles - they may have left earlier. DHARMA would have created the incident as normal that would have possibly resulted in the island sinking.
This opens the possibility that an earlier civilization had imprisoned the smoke-monster (with the help of Jacob?).
There may obviously be some holes in this theory, but hey, I'm just having fun!