The Silurian Hypothesis: How Do We Know That Humans Were the First Civilization on Earth?

Updated | What if another industrial civilization had existed on Earth tens of millions of years ago, long before humans, but all traces of it have now been lost?

While it may seem like an absurd idea, this thought experiment is the focus of a new scientific paper authored by Adam Frank, an astrophysicist from the University of Rochester, and Gavin Schmidt, director of the NASA Goddard Institute for Space Studies.

They have named the paper "The Silurian Hypothesis" after a fictional race of intelligent, bipedal reptiles from the British sci-fi series Doctor Who—known as the Silurians—that supposedly lived on Earth hundreds of millions of years ago.

In the study, which has been published in the International Journal of Astrobiology, Frank and Schmidt ask what traces human civilization would leave behind and how future scientists might find evidence of our existence.

The researchers looked at the likely geological fingerprint of the Anthropocene—a term used by many researchers to denote the current geological age in which human activity has been the primary influence on the climate and environment.

While the Anthropocene has not yet been officially classified as a distinct geological era, it is already clear that humans are having an impact on the geological record being laid down today, the authors wrote in the paper.

"We are already a geophysical force, and our presence is being recorded in carbon, oxygen and nitrogen isotopes, extinctions, extra sedimentation, spikes in heavy metals and synthetic chemicals (including plastics)," Schmidt told Newsweek.

The human burning of fossil fuels, for example, is already having an impact on the geological record, despite industrialization only beginning around 300 years ago. What's more, global warming, agriculture and the spread of synthetic pollutants are all making their mark.

So let's imagine that, perhaps, some other species on Earth rose briefly to civilization millions of years ago, would there be any traces of them today, for example, fossils or the remains of buildings?

How do we know that there weren’t industrial civilizations on Earth long before human beings appeared? That’s the question posed in a new scientific thought experiment. University of Rochester / Michael Osadciw

"Possibly," Schmidt said, "but it might also be that all such traces have been ground to dust and that the only remaining traces are in the more subtle perturbations in geochemistry." In addition, "fossilization is extremely rare and very partial, so evidence could easily have been missed," especially if a civilization had lasted just a few thousand or tens of thousands of years, much like our own.

The issues raised by Frank and Schmidt with respect to the planetary impact of civilizations could also have implications for the future exploration of other planets and our search for intelligent alien life.

"We know early Mars and, perhaps, early Venus were more habitable than they are now, and conceivably we will one day drill through the geological sediments there, too," Schmidt said in the statement. "This helps us think about what we should be looking for."

This article was updated to include comment from Gavin Schmidt.