Atlantis

Atlantis is the name of a continent located in the North Atlantic Ocean and represents one of the earliest and most highly advanced cultural societies on Earth. During the Great Deluge (Great Flood), all of Atlantis sunk beneath the surface. Fortunately, Atlantean scientists predicted the Great Flood long before it happened so they were able to take precautions. These precautions involved human citizens of Atlantis injecting themselves with a serum that would allow them to evolve and adapt to their surroundings at an accelerated rate.

Those that adapted to their surroundings and the oncoming flood became merpeople. Those that didn't evolve.. died ordinary humans.