To the people who responded before, here's my take.
Jacob said that they were brought to this island because their life's are incomplete. The island universe is the alternate universe, and the other one is the real universe. They acquired the memories and completed their life's on the island and brought it to the real universe, and they finally became complete.
And the explanation of the island is:
ITS AN ALTERNATE UNIVERSE. DIFFERENT LAWS OF PHYSICS. .TU