An Ancient Aliens segment talks about arm made of silver, which was said to be like a modern prosthetic. This got me wondering, what if Human civilization is far older than is currently known? That there was one equal to or greater than our own. This could explain the various mysteries without requiring an "aliens did it" explanation.
Some examples could be:
* Pyramids around the world could have been from commonplace long distance travel.
* The above mentioned prosthetic arm.
* Indian texts describing nuclear power being used in weapons or power plants.
* The Sphinx aligning with the constellation Leo, which requires it to be older than general accepted.
* Atlantis being said to have been a major power but little if anything remains.
What else could there be?