nuclear bombs, atomic warfare, war ingeneral, disease, global warming, any f that, will destroy civilization and the human race will eaither not make it or will be completely stunted. kind of like in the time machine by hgwells.
i think..

anyway, it will desolate and ancient civilizations will regn and new species will develop to cope with the different world

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Marijuana: It grows naturally
on Earth; doesn't outlawing something in nature seem...unatural?