Contrary to what I’ve still sometimes seen on web pages and blogs, we now pretty much have all the pieces of the puzzle to make sense of the presence of polar bears on the island.
(1) Charlotte visited a site in Tunisia where a skeleton of a polar bear, wearing a Dharma Initiative logo on its collar, was unearthed.
(2) The site where the donkey wheel is that moves the island is freezing cold.
(3) When Ben moved the island, he ended up at a different time and in the Tunisian desert.
All this adds up to the Dharma Initiative having used a polar bear to move the island at some point. I don’t think there are any loose ends, although the fact that Dharma rather than the Others appear to have moved the island at some point is striking. But we’ve been getting indications that the Dharma Initiative and the Others may just be human factions vying for the island. Neither seems to be the “original inhabitants” that have been mentioned, who are presumably Richard Alpert’s own people.