We think we're the first advanced earthlings—but how do we really know?

Imagine if, many millions of years ago, dinosaurs drove cars through cities of mile-high buildings. A preposterous idea, right? Over the course of tens of millions of years, however, all of the direct evidence of a civilization—its artifacts and remains—gets ground to dust. How do we really know, then, that there weren't previous industrial civilizations on Earth that rose and fell long before human beings appeared?

It's a compelling thoughtexperiment, and one that Adam Frank, a professor of physics and astronomy at the University of Rochester, and Gavin Schmidt, the director of the NASA Goddard Institute for Space Studies, take up in a paper published in the International Journal of Astrobiology.

"Gavin and I have not seen any evidence of another industrial civilization," Frank explains. But by looking at the deep past in the right way, a new set of questions about civilizations and the planet appear: What geological footprints do civilizations leave? Is it possible to detect an industrial civilization in the geological record once it disappears from the face of its host planet? "These questions make us think about the future and the past in a much different way, including how any planetary-scale civilization might rise and fall."

In what they deem the "Silurian Hypothesis," Frank and Schmidt define a civilization by its energy use. Human beings are just entering a new geological era that many researchers refer to as the Anthropocene, the period in which human activity strongly influences the climate and environment. In the Anthropocene, fossil fuels have become central to the geological footprint humans will leave behind on Earth. By looking at the Anthropocene's imprint, Schmidt and Frank examine what kinds of clues future scientists might detect to determine that human beings existed. In doing so, they also lay out evidence of what might be left behind if industrial civilizations like ours existed millions of years in the past.