First, Precursors were too massive to be Human. Second, Precursors supposedly 'crafted' the Forerunners Third, Precursors were so little in number after the Precursor-Forerunner War. Fourth, the last Precursor was possibly absorbed by the Flood, possibly turning into the first Gravemind in the Milky Way Galaxy. Fifth, all Precursor architecture was almost impossible to destroy. This was NOT the case for the Forerunners. They tested Halo on a single planet with Precursor buildings. These buildings were completely demolished by the test because Halo destroyed them at a Molecular level. Sixth, because of Fifth, when the Forerunners activated the Halo Array to purge the Galaxy of Flood, all Precursor architecture was destroyed. COMPLETELY. Seventh. The Precursors were considered to be the most Intelligent beings in the galaxy, surpassing even the Forerunners. And if Four actually happened, then it explains why the Gravemind knows so much. Each Gravemind is reborn with the memories of the past ones, thus containing the flow of knowledge a Gravemind can know. Eighth, if Humans WERE Forerunners - WHICH they're NOT, however they could've been 'crafted' like the Forerunners from the Precursors - the Gravemind would not only have his Precursor side rage, but his Flood side too - both hate the Forerunner line or any who the Forerunners left behind for. This would be a theory why Gravemind despises Humankind above the others. [Edited on 02.23.2011 12:05 PM PST]