Prehistoric Civilizations

The notion of limiting historical study to a roughly 5000-year span, out of a possible few million years of human existence, and to only those few world cultures that left written records, is hardly representative. Likewise, the whole period of human evolution during the last few million years represents just a few moments in the grand history of the Earth.

Can it be completely ruled out that in the vast periods on the geologic time scale there existed sophisticated civilizations on Earth that matched or even surpassed our current one?
In due time, the tectonic plate movements on our planet erase pretty much every trace of the past. Could it be that the life as we know it is just the most recent growth on a rinsed-out up Petri dish, one in a long string of natural experiments on Earth? Or, possibly, are we the first tenants, with many others to come after our demise?

"Namýšľali sme si, že sme veční. A pritom tu ostanú tu len mravce a lišajníky. A ryby. Potom, za pár miliónov rokov vylezie z mora nejaký dôstojnejší nasledovník človeka. Nová rasa, možno nejaké chodiace delfíny. A celkom isto tu, na tomto mieste, bude jedného dňa namiesto mňa sedieť mladá delfínka a bude práve raňajkovať toasty, keď jej ktosi telepaticky oznámi tú novinu. A zasa prídu noví nájomníci. Nesmierny cyklus civilizácií za hranicami nášho chápania..." [link]