Nasa's Most Disturbing Hypothesis Is that We Would Not Be Earth's First Advanced Civilization.

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Certain plastics, resistant industrial waste or unusual patterns in isotopes and metals would be clues to differentiate an artificial origin from a common geological event. The idea that another intelligent species could have prospered on Earth before we posed an obvious challenge to what we believe we know about our past. Under this hypothesis, an earlier civilization would have achieved advanced technological development, capable of altering i…