Silurian Hypothesis: What If Humans Aren’t the First Civilization on Earth?

Earth is completely overrun with humans. There are billions of us living and dying on this planet, leaving signs of our existence in the soil, atmosphere, and even in space. Are we the first, though? Researchers from the University of Rochester and NASA have produced a paper that details how we might be able to tell if humans are the first civilization to arise on Earth. They call the possibility that we’re not the first rulers of Earth the Silurian Hypothesis, named after the ancient reptilian civilization from Doctor Who.

Human history can seem extensive, but we’ve only existed for a geologic blink of the eye. Earth is around 4.5 billion years old, and complex life has existed for hundreds of millions of years. Modern humans, on the other hand, have only existed for around 300,000 years. That leaves a great deal of time during which other intelligent species could evolve and become extinct on Earth before us.

The study comes from University of Rochester astrophysicist Adam Frank and NASA’s Goddard Institute director Gavin Schmidt. They address a number of misconceptions about studying the past, and present several ways we might test for a pre-human Earth civilization. One of the first things they discuss is the fossil record. We’ve all seen the remains of ancient creatures on display, but these specimens represent only a tiny sliver of life on Earth. Organisms only fossilize under very specific conditions. An extinct species could have numbered in the millions, and maybe none of them fossilized.

If you’re searching for an ancient industrialized civilization, what about ruins? As the paper notes, human urban centers currently only account for about 1 percent of Earth’s surface area. Without constant maintenance, structures tend to be swallowed up by nature in a matter of decades. In a million years, any physical evidence would have long ago vanished. The best we could hope to find are minor, inconclusive anomalies in sedimentary layers.

There are, however, several signs of past civilizations that may survive long into the future. We need only look at the activity of humans to learn what they are. For example, humans have burned massive volumes of fossil fuels. We release more than 500 billion tons of carbon from coal, oil, and natural gas, and this alters the ratio of carbon-12 and carbon-13 in plants, soil, and water. Detecting similar spikes in sediment samples from the past could indicate an industrial civilization. Agriculture also causes long-term changes to the planet — nitrogen-based fertilizers could be detectable for eons in the fossil record.

While ancient buildings would not survive millions of years, specific building materials might. Humans have created long-lived synthetic molecules like plastic, so maybe a hypothetical ancient lizard civilization would have as well. If we were to find something similar in the fossil record, that would be substantial evidence for a pre-human civilization.

It’s an interesting thought experiment, but that’s all it is right now. There’s no evidence of Silurians or any other intelligent beings on Earth before us — though we can’t rule it out completely.