In one of his early conversations with Jack, Ben said that he had been on the island his whole life. He appears to be much older than 30-ish, which would mean that, if we take him at his word, he pre-dates DHARMA's arrival on the island.

So, where did he come from?

Is it possible that Ben is somehow related to the skeletal "Adam & Eve" discovered in the cave early in S1?

Maybe he was orphaned and wandered the island alone, socially regressed and surviving only by submitting to his primitive instincts.....until DHARMA came.

There are already numerous threads on this subject.