@Setanta,
There is also the issue of co-evolution. Back in high school, our German class compared fairy tales between cultures. It turned out that the storylines in fairy tales are more or less the same all over the world, even between cultures that have minimal to no contact with one another.
Copying and pasting cannot explain the similarities there. The explanation, rather, is that an unbounded number of storylines compete for people who remember and re-tell them. A process akin to Darwinian selection kicks in, and after a while, all but the most memorable storylines go extinct. The remaining ones are the same the world over, because people's memories and gossip revolve around the same plots the world over.
Religious narratives being a special case of fairy tales, the similarities between them seem unsurprising. And although copying and pasting played a role, the convergence would have happened even without them.