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

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.

The researchers then considered traces left by chemical changes. Think of the Grand Canyon, Frank said, where you can see line after line representing different ages of the Earth. "Every one of those strata is ground up rock that keeps a memory of what the chemistry of the Earth was like then." If a previous civilization had altered the planet's chemistry — as humans have done by releasing vast amounts of carbon pollution — there might be a record.

The scientists also considered what other evidence humans might leave behind after our civilization has ended, like the vast amounts of plastics as well as artificial hormones for agriculture and persistent chemicals, which do not break down in the environment. These might all show up in future sediments, many millions of years from now. As might the heavy metals used in electronics.

The point of the paper wasn't to establish whether the Silurian hypothesis is correct, Frank said. Instead, it was "to say here's the things you'd want to look for in a much more high-resolution way in order to see whether or not there was ever a civilization before us."

Both Frank and Schmidt are in the strange position of having written a study about a theory neither believes in — they just say we cannot yet rule it out.

No one appears to have picked up the call for a deeper investigation in the years since, but some scientists have raised questions.

Jan Zalasiewicz, a geologist, paleontologist and emeritus professor at the University of Leicester in the UK, said the Silurian hypothesis is "wonderful and deeply thought-provoking idea." But Schmidt and Frank underestimated just how much modern humans have transformed the planet, as well as "the effectiveness of the fossilization process and how good it is at preserving a history of what has happened on our planet," he said.

We can't rule out intelligent pre-human societies, "dinosaurs that could sing complex arias, say, or Jurassic ammonites that could compose poetry," said Zalasiewicz, who co-wrote the 2025 book "Discarded: How Technofossils Will be Our Ultimate Legacy". But as soon cities are built, metals are smelted, plastics are made and discarded — these will leave a clear footprint, he said.

Zalasiewicz is comfortable ruling out the prior existence of the kind of advanced civilization we live in today — "energy-guzzling, manipulative, constructional, and producing waste in colossal quantities."

And that's the beauty of science, Frank said. "You publish a paper, it's like, 'Woo, this is great,' and then somebody comes along and says, you're wrong." Someone else may find something that points in a different direction, and eventually, as research accumulates, a consensus will be formed.

For now, perhaps the most important reason to explore this hypothesis is the important questions it poses about the longevity of advanced technological civilizations like ours.

We're smart enough to develop technology, but we might not be smart enough to figure out how to deal with its climate-shifting consequences, Frank said. "Are we an experiment that the planet has run, which is going to fail?"

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