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Interesting, how did the Roman Empire suppress advanced civilizations developing in China and America? Assuming that Asian cultures outside of the influence of the Roman Empire already had nuclear weapons, how did the Empire survive their incursions? When do you presume will the descendants of advanced south American cultures return from Mars to punish us for what we did to the environment?
But to this day, there is no land transport connection between North and South America! Corn eventually made it from South to North America, but the potato was stuck in Peru until the Spaniards showed up on very nice boats.
China was doing great until Mongke Khan showed up.