Introduction
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 skill about 450 million years ago, long before humans evolved on Earth.
In recent years, the concept of advanced prehistoric life has proven intriguing far beyond its entertainment value, raising various exciting questions. Not least is the so-called Silurian Hypothesis: If an industrial civilization had existed in the past, what traces would it have left behind? 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 for 3.5 billion years. 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 earlier societies are acceptable if we are only interested in timescales of a few thousand years. However, things become more complex when we are interested in “deep time,” referring to periods spanning tens or hundreds of millions of years ago. It is unlikely that any massive artificial structures would stay preserved through eons of geological activity.
Urban areas now occupy less than 1% of the Earth’s surface. So, modern-day paleontologists would easily overlook any comparable settlements from an earlier civilization. And no one should count on finding a Jurassic-era artifact, like the Antikythera mechanism, used by the ancient Greeks, which is 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 (see their paper here), have recently focused on what signature a technological species leaves behind.
The best way to answer this question is to determine what evidence we would leave behind if our civilization collapsed at its current stage of development.
Humanity’s collective activity is leaving a variety of traces that will be detectable by scientists 100 million years from now. The extensive use of fertilizers, for example, means we’re redirecting the planet’s nitrogen flow into food production. Future researchers should consider this in determining the characteristics of nitrogen in sediments from our era.
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 breaks into microscopic particles, creating a layer that can 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 the traces our civilization is bound to leave for the future, will the same “signals” await discovery 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 have contributed to the extinction of most dinosaurs.
Other similar events in Earth’s history also occur. 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 thousands of years.
These events are almost certainly not caused by earlier non-human civilizations. The current era (known as Anthropocene) is remarkable because of the speed at which we dump fossil carbon into the atmosphere. There have been geological periods when Earth’s CO2 levels were as high or higher than they are today, but never before in the planet’s long history has so much carbon been released back into the atmosphere so quickly.
Still, the PETM temperature spike primarily reflects Earth’s timescales for responding to whatever caused it, rather than the timescale of the cause itself. Finding evidence of a relatively short-lived event (like a civilization) in ancient sediments might take newer, more reliable detection techniques.
