Astrobiologist Adam Frank and NASA scientist Gavin Schmidt have proposed a thought experiment examining whether an advanced industrial civilization could have existed on Earth millions of years before humans. The researchers, who published their findings in a 2018 paper, titled the concept the "Silurian hypothesis." The study explores whether such a civilization would leave detectable traces in the geological record after millions of years of tectonic activity and erosion.
The hypothesis originated from observations of the Paleocene-Eocene Thermal Maximum, a period approximately 55 million years ago when global temperatures rose by up to 15 degrees Fahrenheit. While scientists generally attribute this spike to natural causes like volcanic eruptions or methane releases, Schmidt and Frank used the event to consider what markers a short-lived, energy-harnessing society might leave behind. They noted that human activity is currently altering the planet's chemistry and climate in ways that may mirror past anomalies.
The researchers analyzed various forms of evidence that could persist over geological timescales, including chemical signatures in rock strata, synthetic compounds, and plastics. They noted that because urbanization currently covers only about 1% of the Earth's surface and the fossil record is highly incomplete, a civilization lasting 100,000 years could be difficult to detect after millions of years. However, other scientists, including paleontologist Jan Zalasiewicz, suggest that large-scale industrial activities would leave a permanent "technofossil" footprint that would remain visible to future geologists.
For humans today, the study highlights how current industrial practices might appear to future observers. The researchers point to the accumulation of artificial hormones, heavy metals from electronics, and non-biodegradable plastics as specific materials that may persist in sediment for millions of years. These substances represent a shift in the planet's chemistry that could eventually be buried and preserved in rock layers, similar to the chemical signatures scientists study today to understand Earth's history 55 million years ago.
The hypothesis serves as a model for evaluating the longevity of advanced civilizations and their potential for sustainability. While the authors state they do not necessarily believe a prior civilization existed, the framework provides a method for searching for high-resolution anomalies in the geological record. Future consensus on the matter will depend on whether new research identifies specific evidence that contradicts natural explanations for past climate events. As of now, no further large-scale investigations into the hypothesis have been reported since the 2018 paper.