Just imagine all the civilizations that could have arisen, fallen, and all remains disappeared it that time frame (our current civilization is at best only 6K old, although Hindu literature would put that date back an additional 16K).
If each civilization lasted just 10K and it took an additional 10K for all remains to subsume to Nature, then dividing by 20 we get something like 17 prior civilizations. No need for Aliens to explain the oddities and anomalies found in various places.
Would you believe what an alien told you anyway?