We will restrict the scope of this paper to geochemical constraints on the existence of pre-Quaternary industrial civilizations, that may have existed since the rise of complex life on land. This rules out societies that might have been highly organized and potentially sophisticated but that did not develop industry and probably any purely ocean-based lifeforms. The focus is thus on the period between the emergence of complex life on land in the Devonian (~400 Ma) in the Paleozoic era and the mid-Pliocene (~4 Ma).

The likelihood of objects surviving and being discovered is similarly unlikely. Zalasiewicz ( Reference Zalasiewicz2009 ) speculates about preservation of objects or their forms, but the current area of urbanization is <1% of the Earth's surface (Schneider et al., Reference Schneider, Friedl and Potere2009 ), and exposed sections and drilling sites for pre-Quaternary surfaces are orders of magnitude less as fractions of the original surface. Note that even for early human technology, complex objects are very rarely found. For instance, the Antikythera Mechanism (ca. 205 BCE) is a unique object until the Renaissance. Despite impressive recent gains in the ability to detect the wider impacts of civilization on landscapes and ecosystems (Kidwell, Reference Kidwell2015 ), we conclude that for potential civilizations older than about 4 Ma, the chances of finding direct evidence of their existence via objects or fossilized examples of their population is small. We note, however, that one might ask the indirect question related to antecedents in the fossil record indicating species that might lead downstream to the evolution of later civilization-building species. Such arguments, for or against, the Silurian hypothesis would rest on evidence concerning highly social behaviour or high intelligence based on brain size. The claim would then be that there are other species in the fossil record which could, or could not, have evolved into civilization-builders. In this paper, however, we focus on physicochemical tracers for previous industrial civilizations. In this way, there is an opportunity to widen the search to tracers that are more widespread, even though they may be subject to more varied interpretations.

The fraction of life that gets fossilized is always extremely small and varies widely as a function of time, habitat and degree of soft tissue versus hard shells or bones (Behrensmeyer et al., Reference Behrensmeyer, Kidwell and Gastaldo2000 ). Fossilization rates are very low in tropical, forested environments but are higher in arid environments and fluvial systems. As an example, 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. Given the rate of new discovery of taxa of this age, it is clear that species as short-lived as Homo sapiens (so far) might not be represented in the existing fossil record at all.

That this paper's title question is worth posing is a function of the incompleteness of the geological record. For the Quaternary (the last 2.5 million years), there is widespread extant physical evidence of, for instance, climate changes, soil horizons and archaeological evidence of non-Homo Sapiens cultures (Denisovians, Neanderthals, etc.) with occasional evidence of bipedal hominids dating back to at least 3.7 Ma (e.g. the Laetoli footprints) (Leakey & Hay, Reference Leakey and Hay1979 ). The oldest extant large-scale surface is in the Negev Desert and is ~1.8 Ma old (Matmon et al., Reference Matmon2009 ). However, pre-Quaternary land-evidence is far sparser, existing mainly in exposed sections, drilling and mining operations. In the ocean sediments, due to the recycling of ocean crust, there only exists sediment evidence for periods that post-date the Jurassic (~170 Ma) (ODP Leg 801 Team, 2000 ).

Consideration of previous civilizations on other solar system worlds has been taken on by Wright ( Reference Wright2017 ) and Haqq-Misra & Kopparapu ( Reference Haqq-Misra and Kopparapu2012 ). We note here that abundant evidence exists of surface water in ancient Martian climates (3.8 Ga) (e.g. Achille & Hynek, Reference Achille and Hynek2010 ; Arvidson et al., Reference Arvidson2014 ), and speculation that early Venus (2 Ga to 0.7 Ga) was habitable (due to a dimmer sun and lower CO 2 atmosphere) has been supported by recent modelling studies (Way et al., Reference Way2016 ). Conceivably, deep drilling operations could be carried out on either planet in future to assess their geological history. This would constrain consideration of what the fingerprint might be of life, and even organized civilization (Haqq-Misra & Kopparapu, Reference Haqq-Misra and Kopparapu2012 ). Assessments of prior Earth events and consideration of Anthropocene markers such as those we carry out below will likely provide a key context for those explorations.

Determination of the ‘pessimism line’ emphasizes the importance of three Drake equation terms f l , f i and f c . Earth's history often serves as a template for discussions of possible values for these probabilities. For example, there has been considerable discussion of how many times life began on Earth during the early Archean given the ease of abiogenisis (Patel et al., Reference Patel2015 ) including the possibility of a ‘shadow biosphere’ composed of descendants of a different origin event from the one which led to our Last Universal Common Ancestor (LUCA) (Cleland & Copley, Reference Cleland and Copley2006 ). In addition, there is a long-standing debate concerning the number of times intelligence has evolved in terms of dolphins and other species (Marino, Reference Marino, Vakoch and Dowd2015 ). Thus, only the term f c has been commonly accepted to have a value on Earth of strictly 1.

This is a particularly cogent issue in light of recent developments in astrobiology in which the first three terms, which all involve purely astronomical observations, have now been fully determined. It is now apparent that most stars harbour families of planets (Seager, Reference Seager2013 ). Indeed, many of those planets will be in the star's habitable zones (Dressing & Charbonneau, Reference Dressing and Charbonneau2013 ; Howard, Reference Howard2013 ). These results allow the next three terms to be bracketed in a way that uses the exoplanet data to establish a constraint on exo-civilization pessimism. In Frank & Sullivan ( Reference Frank and Sullivan2016 ) such a ‘pessimism line’ was defined as the maximum ‘biotechnological’ probability (per habitable zone planets) f bt for humans to be the only time a technological civilization has evolved in cosmic history. Frank & Sullivan ( Reference Frank and Sullivan2016 ) found f bt in the range ~10 −24 –10 −22 .

If over the course of a planet's existence, multiple industrial civilizations can arise over the span of time that life exists at all, the value of f c may in fact be >1.

The Drake equation is the well-known framework for estimating of the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy (Drake, Reference Drake1961 , Reference Drake, Mamikunian and Briggs1965 ). The number of such civilizations, N, is assumed to be equal to the product of; the average rate of star formation, R*, in our Galaxy; the fraction of formed stars, f p , that have planets; the average number of planets per star, n e , that can potentially support life; the fraction of those planets, f l , that actually develop life; the fraction of planets bearing life on which intelligent, civilized life, f i , has developed; the fraction of these civilizations that have developed communications, f c , i.e., technologies that release detectable signs into space, and the length of time, L, over which such civilizations release detectable signals.

While much idle speculation and late night chatter has been devoted to this question, we are unaware of previous serious treatments of the problem of detectability of prior terrestrial industrial civilizations in the geologic past. Given the vast increase in work surrounding exoplanets and questions related to detection of life, it is worth addressing the question more formally and in its astrobiological setting. We note also the recent work of Wright ( Reference Wright2017 ) which addressed aspects of the problem and previous attempts to assess the likelihood of solar system non-terrestrial civilization such as Haqq-Misra & Kopparapu ( Reference Haqq-Misra and Kopparapu2012 ). This paper is an attempt to remedy the gap in a way that also puts our current impact on the planet into a broader perspective. We first note the importance of this question to the well-known Drake equation. Then we address the likely geologic consequences of human industrial civilization and then compare that fingerprint to potentially similar events in the geologic record. Finally, we address some possible research directions that might improve the constraints on this question.

One of the key questions in assessing the likelihood of finding such a civilization is an understanding of how often, given that life has arisen and that some species are intelligent, does an industrial civilization develop? Humans are the only example we know of, and our industrial civilization has lasted (so far) roughly 300 years (since, for example, the beginning of mass production methods). This is 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, Ma). This short time period raises the obvious question as to whether this could have happened before. We term this the ‘Silurian hypothesis’ Footnote 1 .

The search for life elsewhere in the universe is a central occupation of astrobiology and scientists have often looked to Earth analogues for extremophile bacteria, life under varying climate states and the genesis of life itself. A subset of this search is the prospect for intelligent life, and then a further subset is the search for civilizations that have the potential to communicate with us. A common assumption is that any such civilization must have developed industry of some sort. In particular, the ability to harness those industrial processes to develop radio technologies capable of sending or receiving messages. In what follows, however, we will define industrial civilizations here as the ability to harness external energy sources at global scales.

The geological footprint of the Anthropocene

While an official declaration of the Anthropocene as a unique geological era is still pending (Crutzen, Reference Crutzen2002; Zalasiewicz et al., Reference Zalasiewicz2017), it is already clear that our human efforts will impact the geologic record being laid down today (Waters et al., Reference Waters2014). Some of the discussion of the specific boundary that will define this new period is not relevant for our purposes because the markers proposed (ice core gas concentrations, short-half-lived radioactivity, the Columbian exchange) (e.g. Lewis & Maslin, Reference Lewis and Maslin2015; Hamilton, Reference Hamilton2016) are not going to be geologically stable or distinguishable on multi-million year timescales. However, there are multiple changes that have already occurred that will persist. We discuss a number of these below.

There is an interesting paradox in considering the Anthropogenic footprint on a geological timescale. The longer human civilization lasts, the larger the signal one would expect in the record. However, the longer a civilization lasts, the more sustainable its practices would need to have become in order to survive. The more sustainable a society (e.g. in energy generation, manufacturing or agriculture) the smaller the footprint on the rest of the planet. But the smaller the footprint, the less of a signal will be embedded in the geological record. Thus, the footprint of civilization might be self-limiting on a relatively short timescale. To avoid speculating about the ultimate fate of humanity, we will consider impacts that are already clear, or that are foreseeable under plausible trajectories for the next century (e.g. Nazarenko et al., Reference Nazarenko2015; Köhler, Reference Köhler2016).

We note that effective sedimentation rates in ocean sediment for cores with multi-million-year-old sediment are of the order of a few cm/1000 years at best, and while the degree of bioturbation may smear a short-period signal, the Anthropocene will likely only appear as a section a few cm thick, and appear almost instantaneously in the record.

Faunal radiation and extinctions The last few centuries have seen significant changes in the abundance and spread of small animals, particularly rats, mice and cats, etc. that are associated with human exploration and biotic exchanges. Isolated populations almost everywhere have now been superseded in many respects by these invasive species. The fossil record will likely indicate a large faunal radiation of these indicator species at this point. Concurrently, many other species have already, or are likely to become, extinct, and their disappearance from the fossil record will be noticeable. Given the perspective from many million years ahead, large mammal extinctions that occurred at the end of the last ice age will also associated with the onset of the Anthropocene.

Non-naturally occurring synthetics There are many chemicals that have been (or were) manufactured industrially that for various reasons can spread and persist in the environment for a long time (Bernhardt et al., Reference Bernhardt, Rosi and Gessner2017). Most notably, persistent organic pollutants (organic molecules that are resistant to degradation by chemical, photo-chemical or biological processes), are known to have spread across the world (even to otherwise pristine environments) (Beyer et al., Reference Beyer2000). Their persistence is often tied to being halogenated organics since the bond strength of C–Cl (for instance) is much stronger than C–C. For instance, polychlorinated biphenyls are known to have lifetimes of many hundreds of years in river sediment (Bopp, Reference Bopp1979). How long a detectable signal would persist in ocean sediment is, however, unclear. Other chlorinated compounds may also have the potential for long-term preservation, specifically CFCs and related compounds. While there are natural sources for the most stable compound (CF 4 ), there are only anthropogenic sources for C 2 F 6 and SF 6 , the next most stable compounds. In the atmosphere, their sink via photolytic destruction in the stratosphere limits their lifetimes to a few thousand years (Ravishankara et al., Reference Ravishankara1993). The compounds do dissolve in the the ocean and can be used as tracers of ocean circulation, but we are unaware of studies indicating how long these chemicals might survive and/or be detectable in ocean sediment given some limited evidence for microbial degradation in anaerobic environments (Denovan & Strand, Reference Denovan and Strand1992). Other classes of synthetic biomarkers may also persist in sediments. For instance, steroids, leaf waxes, alkenones and lipids can be preserved in sediment for many millions of years (i.e. Pagani et al., Reference Pagani2006). What might distinguish naturally occurring biomarkers from synthetics might be the chirality of the molecules. Most total synthesis pathways do not discriminate between D- and L-chirality, while biological processes are almost exclusively monochiral (Meierhenrich, Reference Meierhenrich2008) (for instance, naturally occurring amino acids are all L-forms, and almost all sugars are D-forms). Synthetic steroids that do not have natural counterparts are also now ubiquitous in water bodies.

Plastics Since 1950, there has been a huge increase in plastics being delivered into the ocean (Moore, Reference Moore2008; Eriksen et al., Reference Eriksen2014). Although many common forms of plastic (such as polyethylene and polypropylene) are buoyant in sea water, and even those that are nominally heavier than water may be incorporated into flotsam that remains at the surface, it is already clear that mechanical erosional processes will lead to the production of large amounts of plastic micro- and nano-particles (Cozar et al., Reference Cozar2014; Andrady, Reference Andrady2015). Surveys have shown increasing amounts of plastic ‘marine litter’ on the seafloor from coastal areas to deep basins and the Arctic (Pham et al., Reference Pham2014; Tekman et al., Reference Tekman, Krumpen and Bergmann2017). On beaches, novel aggregates ‘plastiglomerates’ have been found where plastic-containing debris comes into contact with high temperatures (Corcoran et al., Reference Corcoran, Moore and Jazvac2014). The degradation of plastics is mostly by solar ultraviolet radiation and in the oceans occurs mostly in the photic zone (Andrady, Reference Andrady2015) and is notably temperature dependent (Andrady et al., Reference Andrady1998) (other mechanisms such as thermo-oxidation or hydrolysis do not readily occur in the ocean). The densification of small plastic particles by fouling organisms, ingestion and incorporation into organic ‘rains’ that sink to the sea floor is an effective delivery mechanism to the seafloor, leading to increasing accumulation in ocean sediment where degradation rates are much slower (Andrady, Reference Andrady2015). Once in the sediment, microbial activity is a possible degradation pathway (Shah et al., Reference Shah2008) but rates are sensitive to oxygen availability and suitable microbial communities. As above, the ultimate long-term fate of these plastics in sediment is unclear, but the potential for very long-term persistence and detectability is high.

Transuranic elements Many radioactive isotopes that are related to anthropogenic fission or nuclear arms, have half-lives that are long, but not long enough to be relevant here. However, there are two isotopes that are potentially long-lived enough. Specifically, Plutonium-244 (half-life 80.8 million years) and Curium-247 (half-life 15 million years) would be detectable for a large fraction of the relevant time period if they were deposited in sufficient quantities, say, as a result of a nuclear weapon exchange. There are no known natural sources of 244Pu outside of supernovae. Attempts have been made to detect primordial 244Pu on Earth with mixed success (Hoffman et al., Reference Hoffman1971; Lachner et al., Reference Lachner2012), indicating the rate of actinide meteorite accretion is small enough (Wallner et al., Reference Wallner2015) for this to be a valid marker in the event of a sufficiently large nuclear exchange. Similarly, 247Cm is present in nuclear fuel waste and as a consequence of a nuclear explosion. Anomalous isotopic ratios in elements with long-lived radioactive isotopes are also possible signatures, for instance, lower than usual 235U ratios, and the presence of expected daughter products, in uranium ores in the Franceville Basin in the Gabon have been traced to naturally occurring nuclear fission in oxygenated, hydrated rocks ~2 Ga (Gauthier-Lafaye et al., Reference Gauthier-Lafaye, Holliger and Blanc1996).