Who's to say we didn't exist back then? Several jumps back in historical origins of man have already been made. I don't see them ending anytime soon. Also, why do you think it was a human civilization?
Cormac is correct, and questioning science or fundamental facts thereof does not alter reality. As all extant textual evidence shows us, Plato is the origin of the Atlantis fable. It is exists in no similar form in any source prior to Plato's Timaeus and Critias. That said, when one posits Atlantis scenarios that lie outside the details of Plato's fable, that individual has left the source material and has descended into make-believe. It is not a constructive approach.