I was under the impression that historically, it is the same as the main timeline up through the 70s, and the point of divergence is the incident at the hatch where the nuke was set off, presumably resulting in the island sinking later on. That would mean that any flashbacks we saw from before the 70s (like say, Locke's childhood, or Ben's birth, arrival on the island and first meeting with Richard, or the Ford family's murder-suicide) would have been identical in the sideways timeline, for example. The changes would include things like Jacob not being there to touch Sawyer at his parents' funeral and send him on the path to being a con man, for example. However that would mean that all of the adventures of Sawyer, Juliet & co with the Dharma initiative actually happened. Dr. Ethan Goodspeed, who saw to Claire in the hospital, WAS born to Roger & Amy Goodspeed, on the island, and delivered by a blonde mechanic named Juliet, who was from an alternate timeline. A young Charles Widmore fought mysterious time travelers on that same island, and his would-be paramour encountered an absent-minded physicist who instructed her on the care and feeding of a hydrogen bomb, and later shot that same man in her camp as he talked with Alpert, and so on.

The reason I bring this up, is that would mean that Ben was shot by an Iraqi on the island as a boy, and survived because the Dharma chief of security and a new arrival at the island brought him to the Hostiles, who took him to their temple to heal his injury, and make him one of them forever, and corrode his innocence. So how did any of that affect Benjamin Linus PhD, a History teacher at a Southern California High School? You would think something like would have made a difference. Especially as the paralells with his story arc in Sideline and his own regrets and remorse in the real timeline are so close. Either Ben is the same man in both worlds, or the world where he made his sacrifices for Alex is irrelevant. Or maybe the point is to show how the island DID make him a jerk - that the sideline diverged earlier, and the nice Dr Linus is a result of his never having been exposed to the island & the Temple and all the rest of those influences.

It's stuff like this that makes me wonder if maybe Smokelocke isn't closer to being right than we might think, or that Jacob is just as much of a villain as he is. Look at what his touch as wrought: He protected Kate from the consequences of one of her first criminal enterprises, and look at how she turned out (though there would not seem to be much of a difference in the Sideline). He helped nudge Sawyer down the path of revenge, plunged Hurley back into the middle of a war zone, got Sayid widowed and was there at the start of the Kwons' rather unhappy marriage, and Jack's self-destructive and obsessive career. Was he deliberately wrecking their outside lives so they would have no options other than the island?