The Silurian hypothesis is an idea that checks whether modern science can find signs of a highly advanced civilization that may have existed millions of years ago. Possible clues could include carbon, radioactive materials, or changes in Earth's temperature. The name "Silurian" comes from a smart species in the BBC science fiction show Doctor Who, which in the series built a civilization before humans.

In 2018, astrophysicist Adam Frank and climate scientist Gavin Schmidt introduced the "Silurian Hypothesis" in a paper. They studied whether evidence of an advanced civilization could be found in Earth's geological history before humans. They noted that enough carbon has been left behind since the Carboniferous Period (about 350 million years ago) to support an industrial society. However, finding direct proof, like tools or machines, is unlikely because most things do not become fossils, and Earth's surface is mostly covered by layers of rock and soil. Instead, scientists might look for indirect signs, such as changes in climate, strange patterns in rock layers, or traces of nuclear waste. The hypothesis also suggests that evidence of ancient civilizations could be found on the Moon and Mars, where weathering and moving of Earth's surface are less common. The idea of civilizations existing before humans has also appeared in books, TV shows, stories, and video games.

Explanation

In 2018, a paper written by Adam Frank, a scientist who studies space, and Gavin Schmidt, who leads a research institute about space, proposed an idea. They imagined a civilization more advanced than humans and asked if it would be possible to find signs of such a civilization in Earth’s geological history. They suggested that as early as the Carboniferous period, which was about 350 million years ago, Earth had enough fossil fuel to support an industrial civilization similar to ours. However, they also noted that while they believe no such civilization existed before humans, exploring this question helps scientists better understand life in the universe and human history. The term "Silurian hypothesis" comes from a fictional group called the Silurians in a British television show called Doctor Who.

Frank and Schmidt explained that finding direct evidence, such as technology from a past civilization, is unlikely because fossilization is rare, and most of Earth’s surface that is visible today is from the Quaternary period, which began about 2.5 million years ago. Instead, they suggested looking for indirect signs, such as sudden changes in Earth’s temperature or climate, like those seen during the Paleocene–Eocene Thermal Maximum about 55 million years ago. Other clues might include signs of using geothermal energy, unusual chemical patterns in sediment, or artificial materials like fertilizers. For example, plutonium-244, a substance not found naturally on Earth except in space, could indicate advanced technology if discovered. Evidence might also include plastics or nuclear waste buried underground or on the ocean floor. The paper also mentions natural nuclear reactors in Oklo, Gabon, which operated about 2 billion years ago. Scientists studied the chemical changes in the area to confirm that nuclear reactions had occurred.

Frank and Schmidt suggested that a past civilization might have traveled to space and left objects on the Moon or Mars. These objects would be easier to find than on Earth, where natural processes like erosion and moving rock layers could destroy evidence. Frank first talked to Schmidt about finding signs of alien civilizations by studying ice cores and tree rings to understand climate changes. They realized the idea could also apply to Earth and humans, as humans have existed in their current form for about 300,000 years but have had advanced technology for only a few hundred years. The Silurian hypothesis has also been linked to the cryptoterrestrial hypothesis, which tries to explain the existence of unidentified flying objects.

History in science fiction

The Silurians from the TV show Doctor Who are a group of reptile-like human beings from Earth's ancient past. They first appeared on the show in 1970. In the story, the Silurians actually lived during the Eocene period, not the Silurian period as their name suggests. Frank and Schmidt mentioned that a 1977 novel called Inherit the Stars by J.P. Hogan had a similar idea, but they were surprised that this concept was not often used in science fiction stories.

An early example of this idea appears in H.P. Lovecraft's short story The Shadow out of Time, published in 1926. This story introduces the Great Race of Yith, a group of beings that existed long ago on Earth. In this story, the minds of the Great Race came from intelligent beings from space, which later used the bodies of Earth's life forms.

A common idea in fiction is that dinosaurs developed intelligence and used tools. This idea appears in the books The Dreaming Dragons (1980, later revised as The Dreaming) by Damien Broderick and Toolmaker Koan (1987) by John McLoughlin. The Dreaming Dragons won the Ditmar Award. Toolmaker Koan also explores the question of why intelligent life has not been found in space. This idea also appears in the Star Trek: Voyager episode Distant Origin (1997). In this episode, the Voyager encounters the Voth, a space-faring species that seems to have evolved from dinosaurs on Earth. A Voth scientist explains that their ancestors lived on a continent that was destroyed by a disaster, leaving no traces except on the ocean floor or buried deep under rock.

Another example is Larry Niven's 1980 short story The Green Marauder. In this story, an alien over 700 million years old (due to space travel that makes time pass differently) tells a human about a past visit to Earth. During this visit, the alien helped an Earth civilization that did not need oxygen fight against the growing threat of plants that produce oxygen.