The Silurians are fictional creatures that appeared in an episode of the cult science fiction TV show Dr. Who. In the story, these lizard-like creatures achieved industrial expertise about 450 million years ago, long before humans evolved on Earth.
In recent years, the idea of advanced prehistoric life has turned out to be intriguing far beyond its entertainment value, raising various exciting questions. Not least is this: if an industrial civilization had existed in the past, what traces would it have left? In other words, how do we know that ours is the first technological civilization on Earth?
Modern humans have been around for 200,000 years, but life has existed on this planet for 3.5 billion. That leaves more than enough time for the rise and fall of not one but several pre-human industrial civilizations. Yet, so far, little serious thought has been given to the possibility that we are not the first species to build a civilization in the Solar System’s history.
We’re used to imagining extinct civilizations in terms of underwater statues and subterranean ruins. These artifacts of previous societies are acceptable if we are only interested in timescales of a few thousand years. But things get trickier once we are interested in “deep time,” meaning tens or hundreds of millions of years ago. It is unlikely that any massive artificial structures would remain preserved through eons of geological activity.
Urban areas presently comprise less than 1% of the Earth’s surface. So any comparable settlements from an earlier civilization would be easy for modern-day paleontologists to miss. And no one should count on finding a Jurassic-era artifact, e.g., the Antikythera mechanism, used by the ancient Greeks and considered the world’s first computer. Complex items don’t last millions of years. The same is true for fossils of beings who might have lived in industrial civilizations. Moreover, the fraction of life that gets fossilized is tiny; of all the many dinosaurs that ever lived, only a few thousand fossil specimens have been discovered. Given that the oldest known fossils of Homo Sapiens are only about 300,000 years old, there is no certainty that our species might even appear in the fossil record in the long run.
For these reasons, Adam Frank, a physicist at the University of Rochester, and Gavin Schmidt, director of NASA’s Goddard Institute for Space Studies, have recently focused on what kind of signature a technological species might leave behind.
The best way to answer this question is to figure out what evidence we would leave behind if our civilization collapsed at the current stage of development.
Humanity’s collective activity is laying down a variety of traces that will be detectable by scientists 100 million years in the future. The extensive use of fertilizers, for example, means we’re redirecting the planet’s nitrogen flow into food production. Future researchers should see this in characteristics of nitrogen showing up in sediments from their era. And then there’s all the plastic.
Increasing amounts of plastic are deposited on the seafloor everywhere, even in the Arctic. Sooner or later, all this plastic turns into microscopic particles, creating a layer that could persist for geological timescales. Still, the most promising marker of humanity’s presence is the carbon released into the atmosphere as a by-product of the burning of fossil fuels. This ancient carbon derives from plant life, which preferentially absorbs more of the lighter isotope carbon-12 than the heavier isotope carbon-13. The more fossil fuels we burn, the more the balance of these carbon isotopes shifts. Temperature increases also leave isotopic signals.
So if these are traces our civilization is bound to leave for the future, might the same “signals” waiting to be uncovered right now in ancient geological strata? Fifty-five million years ago, global temperatures rose from 9 to 14 degrees Fahrenheit. This is called the Paleocene-Eocene Thermal Maximum (PETM). During the PETM, Earth’s surface temperature rose because of a sudden release of greenhouse gases from the Earth’s crust. These greenhouse gases caused the climate to warm and the ocean to become more acidic. Such environmental conditions may have led to the extinction of most of the dinosaurs.
There are also other similar events in Earth’s history. For example, an event that occurred only a few million years after the PETM is called the Eocene Layers of Mysterious Origin. More massive events in the Cretaceous left the ocean without oxygen for millennia.
These events are almost certainly not caused by previous non-human civilizations. The present era (known as Anthropocene) is remarkable because of the speed at which we dump fossil carbon into the atmosphere. There have been geological periods where Earth’s CO2 has been as high or higher than it is today, but never before in the planet’s long history has so much buried carbon been dumped back into the atmosphere so quickly. Still, the PETM carbon isotope spike mostly shows Earth’s timescales for responding to whatever caused it, not necessarily the timescale of the cause. Finding evidence of a short-lived event in ancient sediments might take reliable and novel detection methods.
