Astrobiologist Adam Frank and NASA scientist Gavin Schmidt have proposed a thought experiment titled the "Silurian hypothesis," which examines whether an industrial civilization could have existed on Earth millions of years ago without leaving a trace. The researchers published their findings in a 2018 paper, investigating the geological signatures that an energy-harnessing society would leave behind. While both authors state they do not personally believe such a civilization existed, they argue the possibility cannot be scientifically ruled out based on current geological data.
The hypothesis was prompted by the Paleocene-Eocene Thermal Maximum, a period approximately 55 million years ago characterized by a rapid spike in global temperatures and a massive injection of carbon into the atmosphere. While scientists generally attribute this event to volcanic activity or methane releases, Schmidt and Frank noted the similarities between these ancient climate shifts and the changes currently driven by human fossil fuel consumption. They named the theory after the Silurians, a fictional race of advanced reptiles from the television series "Doctor Who."
In their research, Frank and Schmidt identified several markers that might indicate a past industrial society, including changes in carbon isotopes, the presence of synthetic chemicals, plastics, and heavy metals used in electronics. They noted that over millions of years, tectonic plate movement and erosion erase most surface evidence, with the oldest large-scale land surface, the Negev Desert, dating back only 1.8 million years. Furthermore, they pointed out that because only a small fraction of species are fossilized, a civilization lasting 100,000 years could easily be missed in the 400-million-year history of complex life.
The research suggests that the concrete change a future observer would notice in the geological record includes "technofossils" such as refined metals and synthetic hormones that do not break down naturally. Jan Zalasiewicz, an emeritus professor at the University of Leicester, noted that while a subtle civilization might be missed, one as "energy-guzzling" and "manipulative" as modern human society would leave a clear, colossal footprint. This framing forces a comparison between current industrial activity and the natural cycles of the planet that have occurred over hundreds of millions of years.
The knock-on effects of this study involve how future researchers might search for extraterrestrial life or evaluate the longevity of advanced civilizations. It raises questions about whether civilizations that harness energy on a global scale are inherently prone to climate-induced collapse or if sustainable long-term survival is possible. While no further large-scale investigations have been launched since the 2018 paper, the authors state that scientific consensus will depend on whether future researchers find high-resolution evidence in geological strata that contradicts or supports the theory. Currently, the hypothesis remains a theoretical framework rather than an established historical fact.