What if humanity was not the first industrial civilization? What if, untold millions of years ago, amid the climate fluctuations and the struggles for dominance between the species, a society arose? Would its traces be detectable in the rocks and soil, the element isotopes themselves, after such a long period?

The hypothesis is an exercise published recently in the International Journal of Astrobiology. And although the scientists have their doubts about a proto-human population dominating the globe long before humanity’s family tree sprouted branches, they nonetheless hold that the investigation holds scientific value.

“Perhaps unusually, the authors of this paper are not convinced of the correctness of their proposed hypothesis,” write Gavin A. Schmidt, of NASA’s Goddard Institute of Space Studies, and Adam Frank, of the University of Rochester. “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 Anthropocene studies.”

The “Silurian Hypothesis” is their ultimate focus. It is a theory named after a plot in the 1970s incarnation of the Doctor Who sci-fi TV series in Britain, in which the “Silurians” were a buried race of intelligent reptiles, according to the paper.

The possible window of time for an unknown “Silurian” factor would be between 400 million years ago and approximately 4 million years ago, write the scientists.

Direct evidence like artifacts, even if they did exist, would not be plentiful, considering the sheer timescales, they add.

“Despite impressive recent gains in the ability to detect the wider impacts of civilization on landscapes and ecosystems, we conclude that for potential civilizations older than about four million years ago, the chance of finding direct evidence of their existence via objects or fossilized examples of their population is small.”

So the two scientists identified some of the hallmarks of what humanity has wrought in its approximately 300 years of industrial history—and how that would possibly be reflected by a separate race in the physical evidence available for humans now to observe and assess.

The evidence would include the byproducts of mass production, and the ability to harness energy sources at global scales.

Stable isotope anomalies, like those produced by fossil fuel burning, would be present.

The sedimentary layers, especially in the oceans, would show fluctuations, write the scientists.

The faunal record would show radiation and extinctions at varying rates for key layers, they add.

Most tellingly, unnatural traces would present themselves, the scientists argue. The compounds would include synthetic biomarkers and pollutants, plastics, and likely even radioactive substances.

The authors looked at these markers, and then compared them to the fluctuations represented in the sedimentary record.

But from their look at the abrupt events in the Paleoxoic, Mesozoic and Cenozoic eras, they found no combination of the hallmark tracers that would be explained by an intelligent race of Earth dwellers millions of years ago, according to the paper.

“Impacts to warming and/or carbon influx (such as increase runoff, erosion, etc.) appear to be qualitatively similar whenever in the geological period they occur,” they conclude. “These changes are thus not sufficient evidence for prior industrial civilizations.”

That doesn’t mean it’s not definitively impossible, the paper adds.

“Were it to be true it would have profound implications and not just for astrobiology,” write the scientists. “However most readers do not need to be told that it is always a bad idea to decide on the truth or falsity of an idea based on the consequences of it being true.”

The paper’s findings were first reported by The Atlantic.