Would it be possible that an industrialized civilization existed on Earth before the current human civilization? Two scientists proposed a hypothesis that says that if an industrial civilization did exist, how would people be able to tell that a civilization had been there before.

These two professors called their theory the Silurian Hypothesis.

Silurian Hypothesis

In a study by Professor Adam Frank from the University of Rochester and Dr. Gavin Schmidt from NASA's Goddard Institute for Space Studies published in the International Journal of Astrobiology, they discuss the idea of whether an industrialized civilization existed before the current industrialized civilization. After not finding any evidence of a previous civilization, the two decided to examine what clues may be left behind by a civilization.

The study was sparked by a conversation that Frank and Schmidt had while discussing climate change. One of the problems with detecting another civilization is that the fossil record doesn't go back further than 2.6 million years ago. Everything before this time period has already been turned to dust over time. This would also make it impossible that the civilization would've been human since they didn't appear until much later.

The two called their idea the Silurian hypothesis after a Doctor Who episode. Silurians were a reptilian race on the show that existed prior to humans that went into deep hibernation beneath the Earth.

Humans' Geological Footprint

Humans' largest footprint is left by the burning of fossil fuels. Since the beginning of industrialization, humans have burned enough fuels to disrupt the carbon cycle. This leaves evidence in the carbon isotope records.

More evidence that humans lived on Earth is global warming, which changed the nitrogen cycle due to fertilizers. Farming leaves behind a footprint by increasing soil erosion. Materials left behind by humans will also be around for millions of years--this includes plastics, synthetic pollutants, and steroids.

The seafloor is littered with plastics. It's not large particles because they've been worn down but microscopic particles. This creates a layer that will be preserved in the layer of soil.

Fossils would exist but only very few. Adam Frank says that the fraction of life that gets fossilized is tiny and depends on factors such as the time and habitat of the organism. This would make it easy to miss an industrial civilization that lasted 100,000 years. To put that into perspective, that would be 500 times longer than the current industrial civilization.

Frank and Schmidt come to the conclusion that another civilization may not have lived on Earth before humans. Instead, they focus on the impact that climate change will have on the footprint left behind by humans. This could make it easier for people to understand the impact that humans are having on the planet and how they could focus on reducing the impact that is left behind.