Fantastic food for thought!
One could almost wonder if we reached our modern period in time before (perhaps more than once) and all the evidence eventually returned to nature. We only see parts of what lasted for longer, to leave clues to it, but after a collapse, if nothing is maintained it would eventually be nothing more than a story, if people chose to pass on that story. Perhaps this devolution of buildings and other things is one of the things that leads to a collapse of civilisation. They would be the first things to return to the earth leaving us speculating over the wrong period of time when things were built to last.

Learning a bit about ancient Sumaria was the first thing that really got me wondering about civilization cycles and whether we'd always be doomed to make the same mistakes as we don't retain the history to learn from.