Natural regeneration and human migration are part of the answer. Ancient civilizations were local, feeding on particular ecologies. As one fell, another would be rising elsewhere. Large tracts of the planet were still very lightly settled. A fast film of the earth from space would show civilizations breaking out like forest fires in one region after another. Some were isolated and spontaneous; others were carried from place to place across the centuries, sparks on the cultural wind. A few flared a second time in a good place after a long fallow, rekindling from old coals.
