If an ancient civilization invented time travel and accidentally eliminated themselves when going back in time we would never know they existed because they didn't anymore.
Discussion
The downdraw is that any ancestors of that civilisation we wouldn't know either because they wouldn't exist and if they were our ancestors we wouldn't have this conversation.
Exactly. By this paradox there is no limit to how many past supercivilizations the earth could have had, because it never had any of them after all.
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