The storyline, simplified, is: Precursors created Forerunners and Humans, Precursors decided the Forerunners weren't adapt to take over the responsability of the Mantle, Forerunners rebelled and hunted down Precursors until a few last of them were driven off the galaxy boarders, Human expanded hunted by the Flood, Human-Forerunner war, Human were victorious but they had to fight also the Flood (for wich they found a "cure"), so the Forerunners stroke back and won, Humans de-evolved, Forerunners-Flood war, Halos fired.