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

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.

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“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,” Frank said in a statement.

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 as a unique geological era is still pending, “it is already clear that our human efforts will impact the geologic record being laid down today,” the authors wrote in the paper.

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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.

But how would we find traces of previous civilizations when we start to look back further in time?

“We’re used to imagining extinct civilizations in terms of the sunken statues and subterranean ruins,” Frank wrote in an article for The Atlantic. “These kinds of artifacts of previous societies are fine if you’re only interested in timescales of a few thousands of years. But once you roll the clock back to tens of millions or hundreds of millions of years, things get more complicated.”

During these time periods, we would be talking about another species, in any case, seeing as homo sapiens only appeared around 300,000 years ago. So let's imagine that, perhaps, some early mammal rose briefly to civilization around 60 million years ago.

2660_DinoStreet_FeatureImage 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

“There are fossils, of course," Frank wrote in the article, "but the fraction of life that gets fossilized is always minuscule and varies a lot depending on time and habitat. It would be easy, therefore, to miss an industrial civilization that only lasted 100,000 years—which would be 500 times longer than our industrial civilization has made it so far.”

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.

“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.”

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