Silurian hypothesis

The Silurian hypothesis is a thought experiment which assesses modern science's ability to detect evidence of a prior advanced civilization, perhaps several million years ago. The most probable clues for such a civilization could be carbon, radioactive elements or temperature variation. The name "Silurian" derives from the eponymous sapient species from the BBC science fiction series Doctor Who, who in the series established an advanced civilization prior to humanity, not from the eponymous geological period.

Astrophysicists Adam Frank and Gavin Schmidt proposed the "Silurian Hypothesis" in a 2018 paper, exploring the possibility of detecting an advanced civilization before humans in the geological record. They argued that there has been sufficient fossil carbon to fuel an industrial civilization since the Carboniferous Period (~350 million years ago). However, finding direct evidence, such as technological artifacts, is unlikely due to the rarity of fossilization and Earth's exposed surface. Instead, researchers might find indirect evidence, such as climate changes, anomalies in sediment, or traces of nuclear waste. The hypothesis also speculates that artifacts from past civilizations could be found on the Moon and Mars, where erosion and tectonic activity are less likely to erase evidence. The concept of pre-human civilizations has been explored in popular culture, including novels, television shows, and short stories.

Explanation
The idea was presented in a 2018 paper by Adam Frank, an astrophysicist at the University of Rochester, and Gavin Schmidt, director of the Goddard Institute for Space Studies. Frank and Schmidt imagined an advanced civilization before humans and pondered whether it would "be possible to detect an industrial civilization in the geological record". They argue as early as the Carboniferous period (~350 million years ago) "there has been sufficient fossil carbon to fuel an industrial civilization comparable with our own". However, they also 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 the fictional species called the Silurians from the British television series Doctor Who.

According to Frank and Schmidt, since fossilization is relatively rare and little of Earth's exposed surface is from before the Quaternary time period (~2.5 million years ago), there is low probability of finding direct evidence of such a civilization, such as technological artifacts. After a great time span, the researchers concluded, contemporary humans would be more likely to find indirect evidence such as rapid changes in temperature or climate (as occurred during the Paleocene–Eocene Thermal Maximum ~55 million years ago); evidence of tapping geothermal power sources; or anomalies in sediment such as their chemical composition (e.g., evidence of artificial fertilizers) or isotope ratios (e.g., there is no naturally occurring plutonium-244 outside a supernova, so evidence of this isotope could indicate a technologically advanced civilization). Objects that could indicate possible evidence of past civilizations include plastics and nuclear wastes residues buried deep underground or on the ocean floor. The paper also mentions the Natural fission reactors at Oklo, Gabon which were active some two billion years BP — while none of the transuranic elements it produced are still present (they have since decayed to longer-lived or stable daughter nuclides), the depletion of 235U and the characteristic isotope ratios of fission products were used to confirm that fission had indeed occurred.

Frank and Schmidt speculate such a civilization 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.

In popular culture
The idea of pre-human civilizations is explored in H.P. Lovecraft's work, for example in his 1931 short novel At the Mountains of Madness and later novelette The Shadow out of Time.

In Isaac Asimov's Big Game (written in 1941 and published in 1974) and Day of the Hunters (1950), the extinction of the dinosaurs was caused by an intelligent race of humanoid dinosaurs. In Asimov's No Connection (1948), a non-human scientist of a future civilization tries to prove the existence of an ancient primate civilization on Earth.

Andre Norton's The Time Traders (1958) and later books in the series discussed the idea that most physical evidence of ancient advanced civilizations on Earth could be removed in mere millennia by glaciers, volcanic eruptions, and natural decay.

The eponymous Silurians on Doctor Who are a race of reptilian humanoids from Earth's past, making their first appearance in the show in 1970.

Frank and Schmidt cite Inherit the Stars, a 1977 novel by J. P. Hogan as containing a similar hypothesis, but also say they were surprised by how rarely the concept was explored in science fiction.

In Larry Niven's 1980 short story "The Green Marauder", an alien over 700 million years old (due to relativistic travel) tells a human about the last time it visited Earth, and the hopeless plea from Earth's anaerobic civilization for help against the growing environmental threat of chlorophyll.

The Star Trek Voyager 1997 episode "Distant Origin" has the crew encounter the Voth, a spacefaring race that appear to have evolved on Earth from dinosaurs. When discussing this theory with a Voth scientist, Chakotay speculates that their ancestors evolved on an isolated continent that was destroyed by cataclysm, with all traces buried under oceans or kilometers of rock.

In the video game Honkai Impact 3rd, there was a Previous Era with a technologically-advanced civilization that was wiped out by the Honkai. Evidence of this Previous Era has been mostly destroyed, but the game takes place in the Current Era and you play as a member of the new technologically-advanced civilization.