My guess is that complex civilizations have existed on this planet so long prior to the time most people now think they did, that they would fall down in utter astonishment if they found out about what really has gone before.

Maybe as far as 35,000 years back. Maybe 50,000 years back. Maybe even 500,000 years back.

But that's just my best guess. It remains sheer conjecture. I very much doubt that people will ever get it all figured out or even half figured out. I think what we know now is just fragments of all that has gone before.

Every age has had its ruling set of "informed" authority figures at the top of the society who thought they had it pretty well all figured out. They didn't, they don't now, and they won't in the future, in my opinion....but that's not to say it's not worth continuing to investigate and gather further fragments of information to add to our existing body of knowledge, assumption, and conjecture. It's a very interesting matter, if one keeps an open mind and is prepared to admit one might not know it all.