University of Rochester professor Adam Frank and NASA scientist Gavin Schmidt developed the “Silurian Hypothesis” to examine whether an advanced civilization could have existed on Earth before humans and left detectable traces. Their discussion began in 2017, and they published a paper in 2018. The researchers say the project was not intended to prove such a civilization existed, but to identify what subtle evidence scientists should seek if one had existed and disappeared millions of years ago.

The inquiry links Earth’s unusual climate episodes with present-day human-driven warming. About 55 million years ago, during the Paleocene-Eocene Thermal Maximum, carbon emissions were associated with a global temperature increase of up to 15 degrees Fahrenheit, though the precise cause remains unknown. Texas Tech University climate researcher Katharine Hayhoe, who was not involved, called the idea a thought experiment that highlights humanity’s potential vulnerability.

The researchers argue that surface structures and fossils may be difficult to preserve because of tectonic shifts, volcanic eruptions and erosion. They therefore consider chemical changes in sedimentary layers as possible evidence. Plastic, synthetic agricultural hormones, toxic chemicals and heavy metals used in electronics could remain as future markers of human civilization. Geologist Jan Zalasiewicz said cities, metal smelting and plastic waste would leave a clear footprint, and rejected the prior existence of a similarly energy-intensive civilization.