re: Ancient civilizations. Y or N?

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The history that has been lost, dwarfs what we know.

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Definitely, look at the Black Plague and how it setback development and advancement.

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The Black Death was a bubonic plague pandemic occurring in Europe from 1346 to 1353. One of the most fatal pandemics in human history, as many as 50 million people[2] perished, perhaps 50% of Europe's 14th century population

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The Spanish flu killed up to 100 million, although the population had grown significantly by then. Plagues, comet/asteroid impacts, the Earth's magnetic field reversing, Mass Coronal ejection events, there are many ways that a civilization could have been wiped out or moved back to start. Also, many early civilizations were established along coastlines in areas that are now underwater.