Romeo:

In Ghosts of Onyx, it says that the Forerunners would not escape to the Ark, but rather to a Shield World. Where the Ark acted as the sword, cutting down all in it's path, the Shield planet would defend the Forerunners from their own offensive. We also hear that the Forerunners failed to escape to the Shield World when they activated the Ark, consiquently destroying themselves. I don't think the Forerunners really escaped to Earth, otherwise we would be tremendously advanced. Rather, I think that the Forerunners were humans (or something very similar) from a different planet, who started out much as we did. As time passed, they decided to play god, with abysmal results. In turn, they were forced to destroy themselves. Now, humanity rises from a different corner of the galaxy, following the footsteps of their ancestors. Slowly but surely, they begin to fall to the same mistakes that the Forerunners did, and will therefore be responasble for the death of their race, and every other in the known galaxy. Just as the Flood and the remainder of the Covenant arrive on Earth to deal the final blow, the Arbiter and the Spartan Blue team pull them back from the gates of hell. But will they succeed in halting the onslaught? Or are we doomed, as Cortana says, to destroy ourselves?

 The Only proof I have to back my allegations is the logic (Forerunners were wiped out. Therefore they can't have moved to Earth. Also, we'd be too advanced if they did). I also rely upon the novel Ghosts of Onyx, by Eric Nylund (terrific read, I highly recommend it) for further insight. Lastly, I also use tidbits of details from Halo 1 and 2 (such as when 343 Guilty Spark says your using Stage II armor, instead of the recommended XII. This would imply the Forerunners had a simliar beginning to humanity, or that humanity used Forerunners ideas. Either way, it means that humanity has much in common with the Forerunners).Of course, I could be astronomically incorrect, and most likely, I am.

I would like to remind you that we dont the span of the forerunners civilisation...So we may be also on par with them regarding advancement...