the forerunners thought that they were to inherit the mantle from the precursors but it was revealed by the Primordial(or timeless one) that the mantle was meant for the humans and "they would be tested by floods". the primordial also revealed that the flood and the precursors are the same being, so i think that the flood is an early form of the precursor species because the rings were fired so they would have to evolve again from scratch. And one more interesting fact: the flood have created the first utopian society(disturbing!!). they all work in unison for the common good of their species. no conflict among one another, no wealth, no poverty. utopia