A few points...
First things first. Which group are we talking about when we say "discovered America"? After all, the FIRST people to discover America are all the Southern and Northern American Indians.
Europeans is a different story. The conventional way was of course Columbus. However, vikings following in the footsteps of Lief Ericsson did in fact 'find' America around 1000AD, a good 500 years before Columbus. Phoenicians, if they did reach America, were still 20000 years too late. The Indians got there first when they crossed the Baring Sea's land bridge.
With regards to Ancient cultures: Pyramids developed from Ziggurats? The Great Pyramid of Cheops was built around 2700BC, around 1000 years before the Ziggurat of Ur which is Babylonian in origin. It would rather be the other way around.
However how they built those structures and now that they LAST is amazing! (I just saw the Mummy Returns
) Not to mention the geographic and geometric accuracy!
I truly wonder about whether there was a great ancient civilization. I'd say if it was anything near the technological capacity of ours, we would have found MANY ruins, not just bits and pieces. You can't hide something THAT big for THAT long!
Re Dark Ages: The effect of the Dark Ages wasn't much. I personally used to think it was, yet the developments of the Arabs (who kept certain sciences alive during the Dark Ages/Medievel times) were all researched during the Renaissance. However, I'd say given our present rate of technological change, we would have been years ahead if all civilization was continuous.
I would love right now to be either fighting Kilrathi (WC) or Minbari (B5)...or the occasional Borg or Klingon! Not to mention getting my hands on an Orion slave girl! MUAHAHAHA