How did all the great civilizations come to an end?

[krishna L]

I have been wondering about this for quiet sometime, how did the great civilizations come to an end ?...Egytian, Roman, Greek, Chinese, Indian.how ? These civilizations built some of the most astounding structures, and some of those structures are impossible to replicate even today, most notably , The Pyramids. These great civilizations all pretty much came to an end more or less at the same time, is wht many believe. What I am really get at is, the Egyptians built the pyramids, thousands of years ago, yet the egyptians today do not know how they were built (Nobody does, all the babble about using ramps and wooden logs are just mere babble). If the egyptians today do not know for sure how they were built, that means there was a break in the human civilization by thousands of years. All humans were probably wiped out, and somehow from distant lands, humans probably evolved again. So many unexplained things, nobody really knws the answer, Nazca lines, Pyramids, Stone henge, crop circles.