We get hit by an Asteroid. We are well over due for a large Asteroid impact. Astronomers have tracked many Asteroids in close Earth orbit.
There was an advanced ancient civilization before us. Earth is 4,500,000,000 years old that means and Humans in one form or another have only been around for 2,000,000 years which means there has been more than enough time for other sentient species to have come and gone.
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All your base are belong to us. Been gaming since 1979. Xbox Live Account Sophitia99