The Silurian hypothesis is a thought experiment which assesses modern science's ability to detect evidence of a prior advanced civilization, perhaps several millions years ago. In a 2018 paper, Adam Frank, an astrophysicist at the University of Rochester, and Gavin Schmidt, director of the NASA Goddard Institute, imagined an advanced civilization before humans and pondered whether it would "be possible to detect an industrial civilization in the geological record?". They wrote, "While we strongly doubt that any previous industrial civilization existed before our own, asking the question in a formal way that articulates explicitly what evidence for such a civilization might look like raises its own useful questions related both to astrobiology and to Anthropocene studies."  The term 'silurian hypothesis' was inspired by a 1970s Doctor Who episode with intelligent reptiles.
According to Frank and Schmidt, since fossilization is relatively rare and little of Earth's exposed surface is from before the quaternary time period, the chances of finding direct evidence of such a civilization, such as technological artifacts, is small. After a great time span the researchers concluded we would be more likely to find indirect evidence such as anomalies in the chemical composition or isotope ratios of sediments. Objects which could indicate possible evidence of past civilizations include plastics and nuclear wastes residues buried deep underground or on the ocean floor.
Prior civilizations could have gone to space and left artifacts on other celestial bodies, such as the Moon and Mars. Evidence for artifacts on these two worlds would be easier to find than on Earth, where erosion and tectonic activity would erase much of it.
Frank first approached Schmidt to discuss how to detect alien civilizations via their potential impact upon climate through the study of ice cores and tree rings. They both realized that the hypothesis could be expanded and applied to Earth and humanity due to the fact that humans have been in their current form for the past 300,000 years and have had sophisticated technology for only the last few centuries.
- Wright, Jason (30 Apr 2017). "Prior Indigenous Technological Species". International Journal of Astrobiology. 17: 96–100. arXiv:1704.07263. doi:10.1017/S1473550417000143.
- Whitwam, Ryan (April 19, 2018). "Silurian Hypothesis: What If Humans Aren't the First Civilization on Earth?". Extreme Tech.
- Schmidt, Gavin A.; Frank, Adam (Apr 10, 2018). "The Silurian Hypothesis: Would it be possible to detect an industrial civilization in the geological record?". arXiv:1804.03748 [astro-ph.EP].
- Choi, Charles Q. (April 21, 2018). "Could intelligent life have existed on Earth millions of years before humans?". The Washington Post.
- "If we weren't the first industrial civilization on Earth, would we ever know?". Technology Review. April 20, 2018.
- Frank, Adam (2018-04-13). "Was There a Civilization On Earth Before Humans?". The Atlantic.
A look at the available evidence