John Anthony West said that the Egyptian civilization is a legacy from something older. He said this precisely because of the fact that the pyramids of Giza are the oldest and the most sophisticated. Apparently, around 11,600 years BP something catastrophic occurred on Earth that wiped out the megafauna and countless other species. The legends of Atlantis and the multiple legends of floods seem to be in agreement. Now, something catastrophic could have wiped out an older civilization and the remnants we see are either the legacy of it or constructed by the survivors. That being said, I do not rule out the possibility of alien intervention.