I’ve directed readers to Jason Colavito’s blog many times before, but I don’t believe I’ve included this specific essay: How David Childress Created the Myth of a Smithsonian Archaeological Conspiracy.
Jason makes a good case for the modern origin of this oft-repeated point of conspiracist dogma. I’m not claiming (and neither would Jason, I presume) that Childress is the explanation for every thread along these lines, but it seems pretty clear he’s a major fountainhead.