If you’ve ever wished that a new study came packaged with some science fiction exploring the implications, this is your lucky day. Of course, not every research paper lends itself to a short story, but a manuscript by NASA’s Gavin Schmidt and the University of Rochester’s Adam Frank asks a fun question: are we sure that humans built the first industrial civilization in Earth’s history?

In recent years, scientists have debated defining a new geologic epoch—the “Anthropocene”—based on the idea that humans have done enough to leave a recognizable mark in Earth’s geologic archives. Theoretically, if another world harbored life that produced an industrial civilization, we could find the proof written in that world’s rocks, too.

To examine that idea, Schmidt and Frank pawed through the pages of Earth’s history—after all, it’s not impossible that some earlier species built a civilization that was subsequently wiped out, right? By looking for funky signals in the rock record, you can think about how clear the signs might be on another world.

Signs in the rocks

Here on Earth, the Anthropocene will appear as seemingly sudden and simultaneous chemical changes in layers of rock or sediment. Rapid global warming is reflected in the relative proportion of different isotopes of oxygen in certain materials. Isotopes of carbon have shifted due to our burning of fossil fuels. Isotopes of nitrogen record intensive farming practices and the production of fertilizer.

Many rivers are dumping more sediment into the ocean—carrying heavy metals—as a result of things like soil erosion; continuing ocean acidification will scrub calcium carbonate from that sediment, changing the character of the layer. And of course, the fossil record will record a host of sudden extinctions.

And then there are the distinctively “artificial” clues—long-lived pollutants like PCBs, particles of plastics that escape eventual decay, or isotopes of plutonium or curium produced in nuclear explosions.

So what could we expect a past civilization to leave behind? We’re much more likely to find chemical signals than to chance upon a preserved artifact, but different (hypothetical) civilizations would probably leave different chemical signals. On top of that, most geologic archives typically can’t resolve time much shorter than a thousand years—and burrowing critters that stir up sediments can smear that signal across thousands of additional years. So while humanity’s rapid changes to our world over a century or so would seem distinctive, we can’t bank on identifying such suspiciously rapid changes in the rock record.

Instead, you would want to look for sudden changes in many markers at once—and in a way that implies large-scale energy use.

A bit of history

To play with this, the researchers pulled out a set of rapid events in Earth’s history. There’s the Paleocene-Eocene Thermal Maximum, a time of rapid warming about 55 million years ago. That event came with changing carbon isotopes, sediment changes, species extinctions, and spikes in heavy metal concentrations. The Cretaceous and Jurassic periods saw a handful of low-oxygen events in the ocean that left distinctive sediments. One can find changes in carbon isotopes, nitrogen isotopes, and sulfur isotopes, as well as metal concentrations.

For these and the other time periods they looked at, researchers have found correlations with tectonic or volcanic events that could plausibly have played a role. They also appear to have played out more slowly. But there are also some parallels to the Anthropocene.

“We are aware that raising the possibility of a prior industrial civilization as a driver for events in the geological record might lead to rather unconstrained speculation,” Schmidt and Frank write. “One would be able to fit any observations to an imagined civilization in ways that would be basically unfalsifiable. Thus, care must be taken not to postulate such a cause until actually positive evidence is available.”

The two researchers don’t actually think that reptiles were digging up coal to fuel an industrial civilization in the Jurassic; it’s just that thinking about this question can lead to some worthwhile realizations. First, they say, we don’t actually understand enough about how long our industrial pollutants will last in the rock record—or if there are other possible industrial pollution markers you could look for besides the ones we’ve made.

And apart from that, the researchers point out that we haven’t bothered to look for “unexpected” chemical signals in these past events. Samples get analyzed for markers of climate change, for example, but looking for goofy signals that might not make sense to us isn’t a priority in a world with very limited project funding.

That’s actually the premise of a short story that Schmidt (the director of climate research at NASA’s Goddard Institute of Space Studies) wrote to go with the paper. The story, published by Motherboard, involves a young, overworked researcher who accidentally runs a 55-million-year-old paleoclimate sample for PCBs—and finds some.

So did Earth see flying cars millions of years ago? Probably not, but it’s surprisingly difficult to truly rule out the possibility of prior civilizations based on the rock record. So if we ever want to look for signs of civilizations on other worlds, we’ll have to think about how to do it right.

International Journal of Astrobiology, 2018. DOI: 10.1017/S1473550418000095 (About DOIs).