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Hypothesis: 4 billion years ago, Venusians realized they had a climate change problem and colonized Earth. Now, Earthlings realize we have a climate change problem and are talking about colonizing Mars. Rinse and repeat. washingtonpost.com/climate-solutions/2020/03/20/runaway-greenhouse-effect-turned-venus-into-hell-could-same-thing-happen-here/ sarahkaplan48
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The colonists all died, having failed to terraform the environment fast enough. But the microbes they brought with them survived, eventually evolving into the biosphere we see today.