The fundamental problem with the tech tree is that it always flows forward. It doesn't know "dark ages" where knowledge and tech are lost. There have been many instances in history where higher developed civilizations were destroyed by lower developed civilizations. The European dark ages came to be because of the collapse of the Roman Empire. The Roman empire fell partially due to severe population shrinkage, population that was replaced by less developed "barbarians" with high populations and no loyalty towards Rome. These "barbarians" spent centuries catching up to what the Roman Empire had already achieved in the tech tree.
History channel documentary on the Dark Ages:
(bad picture quality unfortunately but an awesome documentary)