Can't say I've ever thought about it. Did you have anything particular in mind?

Conspiracies?

Never happen, just don't tell the Highlanders and the shifting allegiances;  the Yorks and Lancasters; don't mention the Knights Templar;

heck... HISTORY is nothing more than a retelling and rehashing of conspiracies!  Without conspiracies you'd HAVE no history.

I knew what you meant Andrew. I think the examples you quote are due to the improved communications in the last century. It's quite possible there were just as many of these theories in the past but they would probably have been more localised. That's my theory anyway.

As Hagar said, communication drastically expanded from the end of the 19th century on: intercontinentally, globally and even extraterrestrially (ours -- not neccesarily extraterrestrial aliens).  Relative to what Felix posted, without communication, history would be as the sound of that falling tree with no one around to hear it; we'd be all the more ignorant than we are.
Unfortunately, knowledge seems too often abused. As they say, "History is written by the winner." Of course, this is assuming they could write. On that matter, oral accounts can quickly flaw by simple misconceptions whereas fiction presented as fact is more often written intentionally -- yet it, too, is affected by misconceptions. In my great ignorance, I may be providing such an example now.

In a round-about sorta way, the Salem Witch Trials were a conspiracy theory.  Doesn't get much older than that.  lol