If another advanced civilization lived on Earth before us, would we know?

Brian Stelter is the chief media analyst for CNN Worldwide and the lead author of the Reliable Sources newsletter. When astrobiologist Adam Frank walked into the office of NASA scientist Gavin Schmidt, he had what he believed was the weirdest question: do aliens experience climate change? It took just moments for Schmidt to out-weird him. Frank, a University of Rochester professor who looks for signs of life in the universe, was digging into whether advanced civilizations would inevitably trigger climate shifts, just as humans are doing now. He was looking at other planets, he told Schmidt that day back in 2017, “because, of course, we know that there have been no other civilizations on Earth.” Schmidt immediately cut him off. “How do you know that?” he asked. It was a record scratch moment, Frank said, and it started the two scientists off on an investigation.