For centuries, humanity has been intrigued by the possibility of encountering advanced civilizations beyond our planet. But what if such a society existed on Earth long before humans evolved?

In 2018, physicist Professor Adam Frank and climate modeler Dr. Gavin Schmidt published a paper exploring whether modern science might uncover traces of an extinct industrial civilization from millions of years ago. The paper is available here.

Dubbed “Silur’s Hypothesis,” after the advanced reptilian species from the long-running BBC science fiction series Doctor Who, the researchers concluded that, while unlikely, evidence of such a civilization may be elusive.

The study focuses on the timeframe between 400 million and 4 million years ago, investigating what remnants this hypothetical society might have left behind.

Over just a few centuries, our industries have significantly altered global climate and ecosystems. If humanity were to vanish over millions of years, however, any direct evidence of our society would likely fade away.

Our largest cities could vanish within a geological instant due to erosion and tectonic activity.

Consequently, scientists searching for an ancient civilization should look for geological signatures of their existence.

Advanced civilizations, much like modern humans, would demand substantial energy and food production. As a result, we might anticipate similar indicators in Earth’s geologic layers, such as evidence of extensive carbon emissions, climate change, and rising sea levels.

The challenge lies in distinguishing climate change caused by fossil fuel-dependent civilizations from that induced by natural processes in the geological record.

Interestingly, there is a striking resemblance between current climate change and historical events on Earth referred to as “hyperthermal” events. One such instance occurred around 55 million years ago, where global temperatures surged by up to 8°C (14.4°F) and were accompanied by intense geological upheavals.

Another consideration is that the longer a sophisticated civilization endures, the more evidence it generates. However, for a civilization to have longevity, it must be sustainable, leading to reduced geological traces.

For instance, a civilization relying on wind and solar energy would leave less physical evidence compared to one powered by fossil fuels. This paradox explains why the traces of such civilizations, if they indeed existed, would be infrequent.

Silur’s Hypothesis encourages us to reflect on the imprints humanity leaves behind. Addressing these inquiries may enhance our search for advanced civilizations on other planets.

This article answers the question posed by Exeter’s Joshua Stucky: “If advanced civilizations lived on Earth millions of years ago, could we recognize their existence?”

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