Horizon on the Middle of Nowhere

In the far future, humans abandon the devastated Earth and move to the upper world "Tenjou." But Tenjou and the law of causation collapse by a war and humans returns to the Earth, which has turns into an inhabitable planet except for Shinshuu area. Shinshuu is too small to accommodate all the humans, so they duplicate the area and create "Juusou Sekai." In order to rebuild Tenjou and the law of causation, the returned humans start reproduction of history from B.C. 10,000. When the reproduction c...