In the grand sweep of Earth’s four-billion-year history, 300 years is barely a blink. And yet, that blink — our industrial era — has already reshaped the atmosphere, oceans, and sediments. If civilization collapsed tomorrow, would anything of us remain 100 million years from now? Could some alien beings visiting Earth in the future ever tell that this planet was inhabited by an advanced civilization? More startling still: if another civilization once existed on Earth long before us, could we even tell?
This question lies at the heart of the Silurian Hypothesis — a playful yet serious scientific proposition by NASA climate scientist Gavin Schmidt and astrophysicist Adam Frank, detailed in a 2018 paper in the International Journal of Astrobiology. The name nods to the Silurians, a fictional species of ancient intelligent reptiles from Doctor Who, but the premise is grounded in geology, astrobiology, and climate science.
“We are not however suggesting that intelligent reptiles actually existed in the Silurian age,” the authors clarify in their paper, just to be sure no one misreads them. “Nor that experimental nuclear physics is liable to wake them from hibernation.”
A Civilization Lost to Time?
At its core, the Silurian Hypothesis asks: if an industrial civilization arose millions of years ago — say, during the Devonian or the Paleocene — would we find any trace of it today?
According to Schmidt and Frank, the odds are slim.
For one, the geological record is woefully incomplete. Ocean crust, where much sediment settles, recycles every 170 million years or so. On land, surface preservation is even rarer. “The current area of urbanization is less than 1% of the Earth’s surface,” the researchers note, and ancient surfaces that remain intact are scarcer still.
Even human fossils — just a few hundred thousand years old — are hard to come by. So, for a civilization that existed 10 or 100 million years ago, the chance of finding fossilized bones or a lost city is vanishingly small.
Yet, there’s another way to detect such a presence: its planetary footprint.
Ghosts in the Sediment
If a civilization had industry, it likely burned energy, changed land use, and altered the atmosphere. These changes could leave subtle markers in the rock record — what the scientists call “geochemical fingerprints.”
Our own civilization, brief as it is, has already left such a mark. Carbon dioxide levels have surged, oceans have warmed and acidified, and plastic particles are raining into marine sediments. Persistent synthetic chemicals, like PCBs, and even radioactive isotopes from nuclear tests may linger for millions of years.
“The longer human civilization lasts, the larger the signal one would expect in the record,” the authors write. But that raises a paradox: “The more sustainable a society… the smaller the footprint.”
A long-lived, solar-powered civilization might leave hardly a trace. By contrast, a short-lived one — fueled by fossil carbon — would leave a distinctive isotopic spike, just as humans are doing now.
As they scoured Earth’s geological past, Schmidt and Frank identified events that eerily resemble today’s Anthropocene. Chief among them: the Paleocene-Eocene Thermal Maximum (PETM), a rapid global warming event 56 million years ago, triggered by a mysterious pulse of carbon.
During the PETM, global temperatures jumped by 5–7°C, ocean acidity rose, and mass extinctions rippled through the deep sea. Some metal levels spiked, and erosion intensified — features that mirror human-driven changes today.
There are others: Ocean Anoxic Events in the Cretaceous and Jurassic left behind black shale deposits and strange chemical signatures. In some cases, they also coincided with large carbon isotope anomalies — just like today.
Still, those ancient episodes are generally linked to volcanic activity or tectonic upheaval. To claim they came from a vanished civilization would require extraordinary evidence. Frank and Schmidt themselves don’t really believe any industrial civilization existed prior to that of humans.
“We are aware that raising the possibility of a prior industrial civilization… might lead to rather unconstrained speculation,” the authors caution. “Care must be taken not to postulate such a cause until actually positive evidence is available.”
Why It Matters for Today — and for the Stars
This thought experiment may have real implications. It highlights how little we know about the long-term survival of civilizations — ours included. If industrial civilizations tend to collapse quickly, their geological legacy might be thin. We might never find a trace of them, just like countless human societies — along with their language, customs, stories, and inventions — have been lost across history.
It also reframes how we search for intelligent life elsewhere. The famous Drake Equation, which estimates the number of communicating civilizations in our galaxy, includes a term for how long such civilizations last. If advanced societies frequently self-destruct — or shift to sustainable models that leave little trace in the geological record — then the odds of detecting them plummet.
Frank and Schmidt urge scientists to think creatively: “Are there other classes of compounds that will leave unique traces in the sediment geochemistry on multi-million year timescales?” they ask. Could deep drilling on Mars or Venus reveal similar fingerprints?
The Silurian Hypothesis is not about proving an ancient lost civilization existed. As we’ve seen, it would be almost impossible to tell from very subtle geological traces. Rather, it’s more about asking what signs civilizations leave behind — and what that tells us about our own.
“While we strongly doubt that any previous industrial civilization existed before our own,” the authors conclude, “asking the question in a formal way… raises its own useful questions related both to astrobiology and to Anthropocene studies.”
