High Concept

A meteor carrying organisms crashed into the oceans of Earth causing the temperatures to rise, polar ice caps to all melt, super volcanoes erupted and massive earthquakes happen around the globe. The organisms caused new life to be created  on earth and greatly accelerated the evolution of the old life on earth, even human life. The little bit of humanity that survived had to create their floating fortresses before the waters rose and all the land fell to the depths of the ocean floor.