jesus christ, even in the old ages its a myth people thought it was flat. people had the same mental capacity as now pretty much, 
Capacity? Probably. Though now we see the Flynn Effect, i.e. an increase of about 15 IQ points in the general population in the past century or so corresponding with a rise in technology; tech is complex and forces us to think about things in complex ways and engage in more complex tasks.
Still, there's an upper limit to how well we can deconstruct and analyze complex situations, and most conspiracy theorists vastly overestimate how good they are at doing this (because they vastly underestimate complexity).