Posits that there were previous civilizations on Earth before humans. Some scientists are conducting research to see if we could detect changes in things like atmospheric carbon, plastics, and nuclear fallout since it’s highly likely no other physical evidence would exist e.g. buildings and artifacts.
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The idea that life originated on another planet and was transported to Earth. It’s difficult to prove or disprove in a similar way to the Silurian hypothesis, evidence is difficult to obtain—artifacts from civilizations don’t last that long, evidence of life is not visible on any likely originating planet within our solar system.
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The conceptualization of long periods and how they fit into our lives which was originated from studying geological strata.