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

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. If an advanced civilization had lived on Earth millions of years ago, powered themselves with energy and wiped themselves out, would we know? Would they have left traces of their presence etched into the record of our planet?

They called the idea the “Silurian hypothesis” — after a fictional advanced race of reptiles in the TV show “Doctor Who” — and published a paper on it in 2018.

It is a mind-bending idea that not only looks deep into the Earth’s past but also far into its future, as humans change the climate swiftly and dramatically by burning fossil fuels. “It is a thought experiment, but one that exposes our potential vulnerability globally,” said Katharine Hayhoe, an atmospheric scientist at Texas Tech University, who was not involved in the research.

The Silurean hypothesis opens big questions about the trajectory of advanced civilizations and whether all those that harness energy on a global scale are destined to burn out, or whether there’s a chance to live sustainably and survive for millions of years.

“These questions are all relevant to exactly the moment that we’re in,” said Frank. “We are pushing the planet into a new climate regime … we know this is true, right? And yet we’re not doing anything about it.”

The reason Schmidt had been mulling the possibility of previous advanced civilizations on Earth was an event deep in the past.

Around 55 million years ago, the planet’s climate signals went haywire. Global temperatures spiked up to 15 degrees Fahrenheit and ecosystems collapsed, as huge amounts of planet-heating carbon were injected into the atmosphere.

This period, known as the Paleocene-Eocene Thermal Maximum and first noted in the 1990s, may have been the result of an enormous volcanic eruption, or maybe a sudden release of methane. But exactly why it happened is still something of a mystery. What intrigued Schmidt were the similarities to what’s happening now as humans push up temperatures and drive extinctions.

And it’s not the only unexplained change deep in the planet’s past that echoes our present, other periods have seen oceans starved of oxygen and sharp hikes in temperature.

The likelihood is these are natural climate events, but what if they weren’t? How would we know? In their report, Frank and Schmidt go through the chain of possible evidence that might exist in support of a past advanced civilization.

Today, only around 1% of the Earth’s surface is urbanized and it wouldn’t take long for what we’ve built to be erased. The further back in time a hypothetical civilization existed, the harder it would be to find direct evidence, like objects.

Over millions of years, the planet is resculpted as tectonic plates shift, volcanoes erupt and land erodes. The Earth’s oldest large-scale stretch of land is the Negev Desert in southern Israel, at around 1.8 million years old.

Surface features may not provide much evidence, so the scientists looked at fossils. A vanishingly small fraction of animal species is fossilized, however, and even fewer are ever found. Dinosaurs roamed the Earth for about 180 million years, but only a few thousand near-complete fossils have been discovered.

It might be easy to miss a civilization that only existed for 100,000 years, a mere blip in the 400-million-year-old history of complex life on the planet.