Are we the first advanced civilization to have emerged on Earth? Or is it possible that others have done so, millions of years in the past, and we simply don’t know it because there is no longer any surviving evidence for it? After all, most signs of human presence disappear within ‘just’ 10,000 years – the blink of an eye in terms of the age of the planet. Surprisingly, that is the question posed by a new peer-reviewed scientific paper published in the International Journal of Astrobiology, titled “The Silurian hypothesis: would it be possible to detect an industrial civilization in the geological record?“. The name of the paper is a tongue-in-cheek Doctor Who reference to the reptile-humanoids known as the Silurians, who in the cult science fiction show were once part of an advanced civilization on Earth during the Silurian period (some 400 million years ago). The paper was born when physicist Adam Frank visited NASA’s Goddard Institute for Space Studies (GISS) – an internationally renowned climate-science facility – with a proposal to study global warming from an “astrobiological perspective”: that is, asking whether any industrial civilization that arises on another planet will, through their own activity, trigger their own version of a climate shift. But he was stopped in his tracks by NASA scientist Gavin A. Schmidt, who asked a question that he hadn’t considered: “How do you know we’re the only time there’s been a civilization on our own planet?” It took me a few seconds to pick my jaw off the floor. I had certainly come into Gavin’s office prepared for eye rolls at the mention of “exo-civilizations.” But the civilizations he was asking about would have existed many millions of years ago. Sitting there, seeing Earth’s vast evolutionary past telescope before my mind’s eye, I felt a kind of temporal vertigo. “Yeah,” I stammered, “Could we tell if there’d been an industrial civilization that deep in time?” We never got back to aliens… Though neither of us could see it at that moment, Gavin’s penetrating question opened a window not just onto Earth’s past, but also onto our own future. However, Schmidt’s penetrating question is not exactly easy to answer. We are used to thinking about discovering a lost civilization by finding things like a city hidden in the jungle, or artifacts on the floor of the sea. But, as mentioned earlier, these sorts of things only last for periods measured in thousands of years, if we’re lucky. Once we start looking back on the scale of millions of years, we encounter a major problem when we’re looking for things that existed on the surface of the Earth: geological upheaval. The geologic record doesn’t go back past what’s called the Quaternary period 2.6 million years ago. For example, the oldest large-scale stretch of ancient surface lies in the Negev Desert. It’s “just” 1.8 million years old—older surfaces are mostly visible in cross section via something like a cliff face or rock cuts. Go back much farther than the Quaternary and everything has been turned over and crushed to dust. But, I hear you say, we have evidence for the dinosaurs that existed millions of years ago, via fossil remains! It is worth noting at this stage then, that the fraction of life that gets fossilized is actually extremely small and “varies widely as a function of time, habitat and degree of soft tissue versus hard shells or bones…for all the dinosaurs that ever lived, there are only a few thousand near complete specimens, or equivalently only a handful of individual animals across thousands of taxa per 100,000 years.” As such, given Homo sapiens have not been around very long at all (geologically speaking), if we became extinct overnight we might not even register in the fossil record millions of years from now. Frank and Schmidt note that our industrial civilization “has lasted (so far) roughly 300 years…a small fraction of the time we have existed as a species, and a tiny fraction of the time that complex life has existed on the Earth’s land surface (∼400 million years ago).” So, there has been a massive amount of time during which many earlier civilizations could have risen and fallen – but is there any evidence for that actually happening, and what form would that evidence take? Instead of looking for fossils and technological objects, Frank and Schmidt instead suggest looking for “physicochemical tracers for previous industrial civilizations” –that is, anomalous changes in the chemicals found in the geological record that point at something odd happening. For example, if we consider our own era, the Anthropocene, we have rare earth elements being used in computers, massive amounts of fertilizer being used for agriculture, radiation resulting from nuclear reactors and bomb tests, plastics flooding the ocean, and a massive increase in atmospheric carbon. All of these things might be visible to scientists when they look at the chemistry of past eras. And there are certainly already ‘mysteries’ in the geological record that could be candidates for the ‘Silurian hypothesis’. For example, during the Eocene (56-34 million years ago), there appear to have been a number of ‘hyperthermal events’ – characterized by “significant negative carbon isotope excursions, warming and relatively high sedimentation rates driven by increases in terrigenous input”, and Arctic conditions showing “evidence of warming, lower salinity and greater anoxia” – that have been collectively named ELMOs: “Eocene Layers of Mysterious Origin”. And before that, during the Paleocene/Eocene transition, there is another anomalous event known as the Paleocene–Eocene thermal maximum (PETM): There are undoubted similarities between previous abrupt events in the geological record and the likely Anthropocene signature in the geological record to come. Negative, abrupt δ13C excursions, warmings and disruptions of the nitrogen cycle are ubiquitous. More complex changes in biota, sedimentation and mineralogy are also common. Specifically, compared with the hypothesized Anthropocene signature, almost all changes found so far for the PETM are of the same sign and comparable magnitude. Some similarities would be expected if the main effect during any event was a significant global warming, however caused. Furthermore, there is evidence at many of these events that warming was driven by a massive input of exogeneous (biogenic) carbon, either as CO2 or CH4. One of the great difficulties, however, is in accounting for anomalies that might be caused by natural events, rather than industrial civilizations. For example, many cases of the massive input of carbon in ancient times have been linked with significant episodes of tectonic and/or volcanic activity – these changes therefore obviously can’t be considered as evidence for the ‘Silurian hypothesis’. In the paper, Frank and Schmidt run through a number of anomalies in the record of various geological eras, but in the end, their investigation is only a first, small, admittedly speculative step. And yet… …We nonetheless find the above analyses intriguing enough to motivate some additional research. Firstly, despite copious existing work on the likely Anthropocene signature, we recommend further synthesis and study on the persistence of uniquely industrial byproducts in ocean sediment environments. Are there other classes of compounds that will leave unique traces in the sediment geochemistry on multi-million year timescales? Secondly, and this is indeed more speculative, we propose that a deeper exploration of elemental and compositional anomalies in extant sediments spanning previous events be performed (although we expect that far more information has been obtained about these sections than has been referenced here). Oddities in these sections have been looked for previously as potential signals of impact events (successfully for the K–T boundary event, not so for any of the events mentioned above), ranging from iridium layers, shocked quartz, micro-tectites, magnetites, etc. But it may be that a new search and new analyses with the Silurian hypothesis in mind might reveal more. The researchers note that they are aware that their hypothesis “might lead to rather unconstrained speculation”, and there is a danger of some people fitting theories to what is observed in the geological record, rather than working forward from the observations. “The Silurian hypothesis cannot be regarded as likely”, they state, “merely because no other valid idea presents itself.” Still, returning to Frank’s original motivation – considering global warming from an astrobiology perspective – the remains of ancient civilizations might not just be here on Earth. Other planets, like Mars, once had lots of water and could have supported a civilization millions of years ago. So could we drill and take samples from those planets when we are capable of doing so, and study other planets’ geological records looking for evidence of advanced alien civilizations as well? The discovery of an ancient culture, here or elsewhere, would – in the view of the researchers – “have profound implications”. As such… 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 to Anthropocene studies. Thus, we hope that this paper will serve as motivation to improve the constraints on the hypothesis so that in future we may be better placed to answer our title question. You can read the entire paper online – “The Silurian hypothesis: would it be possible to detect an industrial civilization in the geological record?” – or for a more accessible account see Adam Frank’s article about his research at The Atlantic.