Life has progressed a great deal, and as it did, extinction was a constant.  Once, life was naught but single-celled creatures, too simplistic to develop. An extinction forced them to become multi-cellular creatures, eventually developing into fish.  Fish had flippers, and were unable to use the tools necessary for true development.  They were culled once again, and in order to survive, they had to develop into dinosaurs.  Dinosaurs had the potential to develop, but they grew too large and powerful, and fought with each other.  They were killed, and replaced by smaller creatures called mammals.  Mammals developed into humans, and humans advanced, but they became too violent and disparate.  Mammals were subject to extinction, and humans were forced to band together and work to survive.