They don’t spoon feed you the answers, but >!The polar bears were imported by the Dharma Initiative for their experiments with the electromagnetic energy. They used a variety of animals for this. At the start of season 3 Kate and Sawyer are kept in their bear cages before Sawyer is sent to work on Jacob’s desired runway, which the second plane lands on. They sent one back in time to the place where you chronologically first see adult Charlotte, digging in the desert for a collar. This is the same place Ben teleports to after moving the island. The polar bears escaped or were let free after the Others chemical attack on the Dharma Initiative.!<