Since Season 2 we have seen a mysterious "Him" spoken of by various people who were already on the island before the castaways arrived there. Sometimes when the Others mention "Him", it seems as though they are talking about Alvar Hanso, like when Tom in his Zeke persona makes reference to "a great man". At other times it sounds as if they are talking about Ben, as when Tom is talking to Ethan in the Staff station, about what a certain "He" is expecting. Then sometimes Ben talks about a mysterious, all-knowing, unforgiving "Him", that has to be Jacob. But regardless of who all these different "Hims" might be, when they are referring to Jacob, how far back does Jacob's sacred "Him-ness" with the DIs/Others go?
In Ben's FBs, Jacob seems to have something to do with Ben's apparitions, and his being chosen as the next project manager of Mysterious Island. If that is the case, then Jacob had to be on the island when the DI was there, most likely before. It would be kind of weird for him to show up somewhere in the middle of the DI's preparations. So from that, we could assume that Jacob was there even before the DI/HG unloaded their first crates of scientific equipment and pitched their tents on the beach of their shiny new magical research island. And if Jacob was there pre-DHARMA, I wonder how and when he revealed himself to them! Did they wander into his lair during a jaunty jungle recon hike and shine an evil flashlight into his cabin? Or did the blasting to create their subterranean stations awaken a dormant Jacob-Monster? However they met up, I hope that we get to see it!
Maybe Jacob is one of those fabled island volcano gods and they woke him up and he spewed fire and ash all over the DI's work until they managed to somehow keep him contained and under control. Which might be why Miss Olivia told her class that the volcano hadn't erupted in a long time -- "...because now we've got that evil volcano-man locked up safely in a rickety old cabin on the hill (with his Howard Hughes urine bottles!) and he won't bother us any more." Or so they hoped. At any rate, we need to know how long the DI knew about Jacob, and whether or not they tangled with Him as well, when they were scuffling with the Natives and causing their own inevitable Purge.
UPDATE: On Memphish's blog, Lost2010 said this about the DHARMA relationship with the Hostiles: "My guess is that the Dharma Initiative showed up and the "hostiles" welcomed them with open arms and showed them all the island had to offer. Then Dharma basically turns around and takes over their island and treats them as if they were the interlopers." I really like this idea, and it may tie into the DIs indeed communicating with Jacob early on, and then getting him P.O.'d when they started bringing all their nasty technology to upset the delicate balance of the island's properties.