Lost human cilvilizations?

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The thing that always makes me laugh about these wild theories is that there is this assumption that humans were stupid in the past. There is no real difference in intelligence between a human in 2600 BC and one today, the only differences appear from compounding technological progress. How did they build the Pyrimids? By using fairly standard / simple tools like ramps, levers, rollers, chisels, saws, and a ton of human labor.