Polar Bears came to the island by the Dharma Initiative. They were studying them in cages and somehow got out to run free. I'm assuming they swam from the smaller island with the zoo to the bigger one. Polar Bears are great swimmers and can swim great distances. It appears that one somehow turned the donkey wheel (or will turn the wheel) because it winds up dead in Tunisia. I'm assuming the finding of the bones from the polar bear is how Widmore knew where the exit was when John returned from the island.


"Why did Danielle Rosseau stay in the island for 18 years and know nothing about the black cloud? "
She does know about the smoke monster. She was the one who told the losties that it was a security system of sorts. She first saw the thing when she first crashed on the island. Her crew was taken down a hole by smokey and changed by the experience.