Maybe the Vex (meld of organic and machine) are the ultimate result of the Fallen tinkering with SIVA. What could have happened: * Humanity created SIVA * Rasputin locked away SIVA. * The Iron Lords tried to liberate SIVA, getting decimated in the process. They locked it away again. * The Fallen discovered SIVA. * The Guardians wiped out SIVA once and for all. * The Fallen looked elsewhere for a way to meld themselves with machine. * The Fallen found another form of nanotech, which they used to self-augment. * Over some centuries, they evoloved into the Vex. * As Vex, they gained the ability to warp through time. They came back here. * In encountering the splicer Fallen, we are seeing the crude prototypes of the Vex species.