The Silurian hypothesis speculates on the possibility of a prior advanced civilization on Earth, extant perhaps on the order of millions of years ago, and the means by which its past existence may be detected. The hypothesis was framed by Adam Frank, an astrophysicist at the University of Rochester, and Gavin Schmidt, director of the NASA Goddard Institute in 2018. The term was named by Schmidt after a 1970s Doctor Who episode with intelligent reptiles.
Given the extreme rarity of fossilization and the fact that very little of pre-quaternary Earth surface is exposed, the hypothesis argues that potential civilizations older than about 4 Ma, the chances of finding direct evidence of their existence via objects or fossilized examples of their population is small, and evidence for such civilization must be looked for through indirect observations. Although evidence for extinct civilizations could include technological artifacts, but after a great time span is more likely to appear in such indirect forms as anomalies in the chemical composition or isotope ratios of sediments. Objects indicating the possible evidence of past civilizations include plastics and nuclear wastes 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 Gavin to hash out an idea of how to detect alien civilizations by their potential impact upon climate by the study of ice cores and tree rings, but they both came to realize that the hypothesis could be expanded, and applied to Earth and humanity, considering that humans have been in its current form for the past 300,000 years and has had sophisticated technology for only the past few centuries.
- 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: [astro-ph.EP], Bibcode:2018arXiv180403748S
- Whitwam, Ryan (April 19, 2018). "Silurian Hypothesis: What If Humans Aren't the First Civilization on Earth?". Extreme Tech.
- 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. "Was There a Civilization On Earth Before Humans?". The Atlantic.
A look at the available evidence