i think it is reasonably possible that a relatively advanced human civilization (e.g., concrete- and steel-making) lived, say, 20000 years ago and left no traces. that is tens of thousands of years after humans became anatomically modern, but still twice as old as the oldest ruins of human cities found. cities made with buildings like our modern ones would simply not survive in any form whatsoever over that time scale, but stone buildings likethe pyramids would remain recognizable.