User blog comment:Commdor/Game Informer ME3 Preview (Feb. 2012)/@comment-3053284-20120110110727/@comment-1900300-20120110123601

No the Reaper was killed 37 million years ago, while the Protheans only came into promenance 500,000 years age. If the Protheans had the technological capablities to kill a Reaper 37 million years ago the Reapers would have wiped them out already. And the Reapers dont just leave them in a state where they can rebuild again, they wipe you out completely.

It should be noted that as far as we know, the Protheans were the only creatures to reverse engineer relays, so they had the greatest under standing of Reaper technology. And they had centeries before the Reapers had killed them all. That would have been time for remote pockets to hold out and study the enemy as best they could.