- last post: 01.01.0001 12:00 AM PDT
here's my theory
Gravemind was an experiment by the forerunners to make some kind of super bio weapon, perhaps to fight the prophet's race (who i'm pretty sure are faking the religious crap to loot the forerunners or something like that). Something went wrong and Gravemind (who was just an infection form at the time) absorbed a forerunner leader, and because of the wealth of knowledge he has, turns into a "brain cocoon" like keyes. This would also explain how keyes didn't just turn into a combat form flood. Keyes had all of earth's defensive plans and stuff. Over a period of 10,000 years or so, Gravemind matured and eventually hatched. He was blessed with several powers including the ability to make clones of his original form (infections). To contain him and his army, the forerunners built halos 1-7 and made it so only earth was outside the blast radius. Infants of forerunners were sent to earth with AI's to care for them and then self terminate so the children could'nt know about their past. These children evolved over the next 90,000 years into modern humans. Meanwhile, most life has to reevolve, and no life is on the ring to sustain the flood. Gravemind sends them into a hibernation-like state until they are released, which happens when keyes and company release the installation 04 flood.
I pay many thanks to the other theories who sparked these ideas and if i remembered any of your user names i'd cite you down here.