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The Silurian Hypothesis

Filed under: — gavin @ 17 April 2018

One of the benefits of working for NASA is that the enormous range of science the agency covers – from satellite records for the present day, to exoplanet climates, from early Mars and deep time on Earth to the far future – and the opportunity to think ‘big’. This week sees the publication of a paper I wrote with Adam Frank that we hope might provoke some ‘big’ thinking.

The Silurian Hypothesis (preprint) is the idea if industrial civilization had arisen on Earth prior to the existence of hominids, what traces would be left that could be detectable now? As a starting point, we explore what the traces of the Anthropocene will be in millions of years – carbon isotope changes, global warming, increased sedimentation, spikes in heavy metal concentrations, plastics and more – and then look at previous examples of similar events in the geological record. What is unique about our presence on Earth and what might be common to any industrial civilization? Can we rule out similar causes?


(Dino Street (University of Rochester illustration/Michael Osadciw)

Adam had a nice piece in the Atlantic and there is also a good write up on Motherboard.

The naming of this idea comes from a 1970 Dr. Who episode where an ancient race of reptilians (“Silurians”) who had put themselves in hibernation to avoid a global catastrophe were awakened by experimental nuclear physics experiments. (I tried to find ‘prior art’ on pre-human terrestrial civilization that wasn’t based on notions of panspermia or ancient astronauts, but I haven’t yet been successful – anyone?). Needless(?) to say, we aren’t proposing any such occurrence (not least because the Silurian period is too early for the development of complex life on land).

The ideas in the paper lead naturally to many lines of speculation, some of which are relevant to us today, and some of which are just interesting (to us at least). For instance, given that the more sustainable a civilization is, the smaller its geophysical footprint might be, what does that imply for the detectability of long-term civilizations? Does the onset of ocean anoxia at the end of many of these events suggest a possibility of cycle where the collapse of one civilization provides the seeds (fossil fuels) for the next?

The whole idea is so intriguing that I wanted to do more with it than is possible in a journal article. Other scientists have occasionally dabbled in science-fiction (notably Carl Sagan and Fred Hoyle) and so, following their lead, I wrote a short story “Under the Sun” about the consequences for finding such a signal.

Literary as well as scientific criticism welcomed!

References

  1. G.A. Schmidt, and A. Frank, "The Silurian hypothesis: would it be possible to detect an industrial civilization in the geological record?", International Journal of Astrobiology, pp. 1-9, 2018. http://dx.doi.org/10.1017/S1473550418000095

112 Responses to “The Silurian Hypothesis”

  1. 1
    gavin says:

    Some additional comments and links on twitter:

    https://twitter.com/ClimateOfGavin/status/985926956771430401

  2. 2

    The pseudoscience folks refer to these as “Legacy Civilizations,” usually from the idea that the Pyramids are much, much older than they really are and various other objects are really “legacy artifacts” from vanished civilizations–Atlantis, Mu, etc. I think Asimov (1980) speculated that intelligence might repeatedly arise and be destroyed on a habitable planet, so that another term would be needed for the Drake equation–fraction of time an intelligent civilization actually exists during the habitable period.

    I wonder what might succeed us if we wipe ourselves out. Rats and raccoons are pretty close to having hands.

  3. 3
  4. 4
    Auz says:

    Prior art… I can think of Harry Harrison’s “West of Eden”.

    https://en.wikipedia.org/wiki/West_of_Eden

  5. 5
    Mitch says:

    Interesting speculation. Since the larger the geophysical disruption, the easier it is to detect a previous civilization, there are a couple of interesting followups–what would be the change in our detectability following the different RCP’s over the next century or so, and what would minimize detectability? Is it possible to detect a “sustainable” civilization after a few million years?

  6. 6
    RickFromTexas says:

    There was an episode of Star Trek Voyager where they had an encounter with sentient dinosaurs from Earth who long ago migrated to a different quadrant in the galaxy. Very good episode, too.

  7. 7
    DAVID ALAN JONES RIDGE says:

    As a Christian I would go so far as to subscribe to an Atlantis and/or a Lemuria. There could possibly be clues that the society that Cain in the Bible built was a very advanced society, intellectually and technologically. And this was 1,500-2,000 years before the Biblical flood.

  8. 8
    Ric Merritt says:

    Nice story (Under the Sun). A not entirely distant cousin to Asimov’s classic Nightfall.

  9. 9
    Matthias says:

    One word: “Fantastic”!

  10. 10
    sidd says:

    Bravo !

  11. 11
    Ron R. says:

    “As a starting point, we explore what the traces of the Anthropocene will be in millions of years – carbon isotope changes, global warming, increased sedimentation, spikes in heavy metal concentrations, plastics and more – and then look at previous examples of similar events in the geological record. What is unique about our presence on Earth and what might be common to any industrial civilization?”

    See the International Geosphere-Biosphere Programme’s Great Acceleration”. More here.

    https://midmiocene.wordpress.com/2015/01/23/1950/

    And now to read your story!

  12. 12
    Russell says:

    DAVID ALAN JONES RIDGE says:
    17 Apr 2018 at 11:41 AM
    As a Christian I would go so far as to subscribe to an Atlantis and/or a Lemuria.

    Could David be one one of the Mid Atlantic Ridges?

    While deep time has pretty much caught up with the Old Testament and the New,
    the present epoch could use some more eras

  13. 13

    #8, Ric–

    Mmm, yeah. I see what you mean.

  14. 14
    Peter Ellis says:

    Isn’t there a “negative space” aspect to consider? Any civilisation that comes after us will have to contend with a lack of easily-accessible deposits of iron, copper, tin, coal, oil etc.

    You argue that anoxic exents leading to mass carbon deposition might form the future fossil fuel beds… how long will it take for anything that’s buried on the ocean floor during the Anthropocene to become near-surface accessible on land? A putative future civilisation won’t start off with access to deep-sea drilling rigs, fracking capability, etc., it needs to work with what’s accessible via open pit shallow mining.

    Similarly, the fact that our own Industrial Revolution was build on coal beds dating to the Carboniferous rather seems to suggest that no other intervening civilisation got there first!

  15. 15
    Ron R. says:

    “Right. You need to generate really large numbers of free and energetic neutrons. Like in a bomb. Oh…, oh, I see…”

    Interesting and well written. I like it and hope you do more fiction. Of course the real evidence would have been if they found an oopart dated 55ma with something stamped on the bottom reading the equivalent of “Made in Japan.

    I’ve teased with a story like that.

    https://midmiocene.wordpress.com/2017/11/22/the-last-option/

  16. 16
  17. 17
    Radge Havers says:

    So, thinking out loud here, wouldn’t there be fossil evidence no matter how squashed in sedimentary rocks: nuclear waste hazards, landfill linings, velociraptors with titanium hip replacements, that sort of thing?

    I was sort of intrigued by this in terms of some ‘concurrent art’, Bears Discover Fire, by Terry Bisson. (“The premise is that bears have discovered fire, and are having campfires on highway medians.”* But then I discovered that Michael Bishop was inspired by this to write a story entitled Bears Discover Smut and got distracted.)

    Anyway, I know it’s not exactly the point of the exercise, but you have to wonder what would need to happen to lizard brains (and why) in order to make the leap to industrialization. How would that shape the kinds signals they’d leave behind?

    Just wondering…

    ——
    *Wikipedia.

  18. 18
    Greg Simpson says:

    I seem to recall a story about finding a gold (?) watch (?) in ancient sediments from a dinosaur civilization, but the details are lost to me.

    Minor point:

    “It turned out that Audrey III was as well suited to detecting pollution levels in local river mud as she was past climate anomalies.”

    Should probably say “…as she was in detecting past climate anomalies.”

    Major point:

    You apparently are trying to split the story up into two time lines. That is difficult for the best authors, but in your case it’s just confusing. It looks like something you did at the last minute to make the story more “interesting” (it isn’t).

  19. 19
    Night-Gaunt49 says:

    In my own fiction and past-future history it is peopled with other intelligences. Some truly fanciful (like “fairies” are Carboniferous super intelligences) to a Permian civilization that split 3 ways- one left, one stayed and the last one stayed but went into long term stasis. Based off the mammal-like reptiles as they were called. I chose the Therocephalians which are an extinct suborder of carnivorous eutheriodont therapsids that lived from the middle and late Permian into the Triassic. Therocephalians (“beast-heads”) are named after their large skulls, which, along with the structure of their teeth, suggest that they were successful carnivores. Like other non-mammalian synapsids, therocephalians are described as mammal-like reptiles, although in fact, Therocephalia is the group most closely related to the cynodonts, which gave rise to the mammals. The earlier therocephalians were in many respects as primitive as the gorgonopsids, but they did show certain advanced features. The discovery of maxilloturbinal ridges in some specimens suggests that at least some therocephalians may have been warm-blooded. (And at one point they gift us an easy to build FTL drive globally.)

    Not counting any we create there could be more to appear some based off of elephant offshoots and even further in the future to arboreal terrestrial cephelopods onward even greater time to creatures that wouldn’t look out of place in the Silurian arthropods with god-like powers.

    As with the answer is there intelligent life out there? When and where? Say if only 2 such appear there in our galaxy say every 15 million years are so many pitfalls involved as we have found in our own species experience. It is a matter of timing. Wouldn’t have to be millions of years it could be by thousands of years and distances too great for us to ever travel at our present knowledge.

    We are a lowly Type 0 civilization yet our accidental wrecking of our whole climate is Type 1 level. But it is a mess and ultimately deleterious to us and all life on our planet. My hypothesis is that that a civilization can cause disaster in the next level of development. Which means the trials and tribulations of intelligence, should be mature and fix our coming Hot House Earth sooner than Nature can do there will be others for other generations to contend with.

    Gene Roddenberry understood the dilemma of intelligence and showed other species who also fell though they were thousands of years further in technology and intelligence. Thought experiments done as tv drama.

  20. 20

    Well, it won’t last millions of years into the future, but, frankly, I think our longest-lived Legacies will be (a) the Carbon Dioxide we emit, and (b) our plastics.

    After all, in terms of our collective influence on the planet, we are Carbon Dioxide.

  21. 21
    jack o. lantern says:

    As a sxity year old geologist I’ve spent a good part of my life looking at sedimentary, metamorphic and igneous rocks in all parts of N. America. The “past civilizations” questions is a tough one.

  22. 22
    Russell says:

    I7:”wouldn’t there be fossil evidence no matter how squashed in sedimentary rocks: nuclear waste hazards, landfill linings, velociraptors with titanium hip replacements, that sort of thing?”

    Here you go, Roger

    https://vvattsupwiththat.blogspot.com/2014/11/petrology-for-poets.html

  23. 23
    SCM says:

    I enjoyed the story very much, although I concur with Greg that a double timeline in a short story is a little confusing.

  24. 24
    Bill Bedford says:

    >I think Asimov (1980) speculated that intelligence might repeatedly arise and be destroyed on a habitable planet, so that another term would be needed for the Drake equation–fraction of time an intelligent civilization actually exists during the habitable period.

    Intelligence may well have arisen repeatedly, but it has to be linked with the ability to manipulate the environment in order to produce technology. Dolphins may be as intelligent, and have similar communication skills, as humans but the chances of them producing a metalworking technology seem to be vanishingly slim.

  25. 25
    thefordprefect says:

    Some of the more environmentally aware dinosaurs were worried about the
    consequences of an accident with the new Iridium enriched fusion reactor.
    “If it goes off only the cockroaches and mammals will survive…” they said.

  26. 26
    Carrie says:

    The Silurian Hypothesis? Given the state of the real climate of the world this ‘big thinking work’ by Gavin is very sad. This article and the paper says a lot but not a word about climate science or how to improve the world and focus on sustaining life.

    I find it a disgrace to everyone at Nasa/Giss and to anyone who has ever lifted a finger to support climate science.

    Because it is all Ego all the way down.

  27. 27
    James Cross says:

    I’ve had a variety of speculations around the related topic of the Fermi Paradox. My most recent one is here.

    https://broadspeculations.com/2017/09/24/im-still-here-where-are-the-aliens/

    I think life is probably abundant in the universe but complex life not so much. It takes a lot of planetary good fortune over a long period of time combined with fortuitous evolutionary sequences to get a big brained organism with ability to significantly manipulate the environment. I suppose some dinosaurs might have evolved this capability but then they had the planetary misfortune of the asteroid strike. The octopus, if it evolved to be out of the water and to live longer, might develop a civilization but that would need quite a bit of fortuitous evolutionary sequences.

    So, all in all, I would say technological civilizations are probably rare in the universe and, even if they do not wipe themselves out, probably suffer some sort of cosmic misfortune. That only leaves a few fortunate (and wise?) civilizations to survive and it is unlikely ours will be in that group.

  28. 28
    Carrie says:

    The Silurian Hypothesis by Dumb and Dumber?

    This is what happens when non-experts who have no clue what they are doing think they know it all when they do not.

    Quote

    Ancient civilisation? Was prehistoric global warming caused by pre-human species

    AN ANCIENT civilisation may have rose up and died out quickly scientists have said after discovering a short-lived but massive peak in global warming in pre-historic times.

    A new study from a climatologist and a professor of astrophysics has revealed a massive spike in global warming 56 million years ago.

    This spike was discovered in a dramatic change in the geological composition buried deep beneath Earth’s surface in an era known as the Paleocene-Eocene Thermal Maximum (PETM).

    To investigate the idea of a possible pre-historic intelligent civilisation, the duo, Professor Adam Frank, of the University of Rochester and Gavin Schmidt, director of Nasa’s Goddard Institute for Space Studies (GISS), examined what evidence we, as humans, would leave behind if we were to become extinct.

    Writing in The Atlantic, Professor Frank said: “There is a conundrum here. If an earlier species’s industrial activity is short-lived, we might not be able to easily see it.

    https://www.express.co.uk/news/science/947879/ancient-civilisation-global-warming-life-climate-change

    Some people have way too much spare time on their hands when they should be doing something more constructive and useful with the power of their Scientific position.

    I expect calls for Gavin’s resignation as Lab Director for GISS to be coming thick and fast.

    [Response: I wouldn’t use the Express as a primary source for information on new scientific papers. Or indeed anything beyond the day of the week. – gavin]

  29. 29
    Frank P. Johnston says:

    The Henson TV series Dinosaurs “..is initially set in 60,000,003 BC in Pangaea. The show centers on the Sinclair family: Earl Sinclair (the father), Fran Sinclair (née Phillips – the mother and Earl’s wife), their three children (son Robbie, daughter Charlene, and Baby Sinclair) and Fran’s mother, Ethyl.

    Earl’s job is to push over trees for the Wesayso Corporation with his friend and coworker Roy Hess where they work under the supervision of their boss Bradley P. Richfield.”

    https://en.wikipedia.org/wiki/Dinosaurs_(TV_series)

  30. 30
    Radge Havers says:

    Russell @ ~ 22

    I’m digging it!
    (So to speak. And glad that you kept the p-chem to a minimum…)
    :-)

    Now who is Roger?

  31. 31
    Radge Havers says:

    My stupid iPad hangs whenever I try to access the story at Motherboard. Is it mirrored?

    I’ve scanned the paper, and it looks convincing so far–to me at least.

    I wonder how far ideas have to spread before they reach critical mass and catch on, spreading even farther–which might increase the strength of the signal and incidence of fossils.

    BPL @ ~ 2

    Raccoons do have hands, and if I understand correctly they are even more innervated than humans since they rely on them so heavily for exploring their environment. Life span and amount of time spent learning before adulthood may be limiting factors in developing a material culture… or maybe not. I have great hope for raccoons as a replacement species for humans.

    Or maybe lemurs will get it right.

  32. 32
    Tom Fuller says:

    It’s a well-plotted narrative. It reads more like a treatment than a conventional short story.

    That’s a hint. It wouldn’t be too tough to get this in front of Netflix, Amazon or Hulu. Disney has a website where you can submit this type of thing online–I think one of the other major studios does too.

    Nice job.

  33. 33

    Time to call attention to the book “Out of Antarctica” – which uses cultural clues to suggest that humans once lived on that continent and left as the original climate migrants. No proof, only conjecture supported by plausibility. https://www.amazon.com/Out-Antarctica-Reflections-origins-peoples/dp/1902699459

    One paper at https://www.sciencedirect.com/science/article/pii/S1055790309004965 mentions the, “Divergence of the Pleurobranchinae into the Antarctic Tomthompsonia and the remaining species in Early Oligocene coincides with two major geological events; namely the onset of glaciation in Antarctica and the opening of the Drake Passage with following formation of the Antarctic Circumpolar Current (ACC). These sudden and dramatic changes in climate probably led to subsequent migration of the last common ancestor of the remaining Pleurobranchinae into warmer regions, while the ACC may have accounted for larval dispersal to the Eastern Atlantic.”

  34. 34
    Mark says:

    Suppose intelligence had arisen in much earlier life forms, and they determined that their continued emissions of oxygen would have significant consequences on the planet, so they decided to stop their emissions.

  35. 35
    Jon Herim says:

    https://www.atmos-chem-phys.net/18/5147/2018/acp-18-5147-2018.pdf

    global average sfc T anomalies [as] indicative of anomalies in outgoing energy…is not well supported over the historical temperature record in the model ensemble or more recent satellite observations

  36. 36
    nigelj says:

    Excellent little essay. There is almost certainly intelligent life out there in the universe, but what are the chances of such life going through the long series of inventions and discoveries that eventually lead humans to harness electricity? And having all the suitable materials? Not high one would think.

  37. 37
    Paul D. says:

    #21

    Oh surely you have entertained the notion of finding a little IC or microprocessor chip or the like lying amongst the brachiopods or trilobites every time you visit a sedimentary rock outcrop…

    I certainly entertain such thoughts whenever I pick around an outcrop… just for fun of course…

  38. 38
    Jim Eager says:

    Peter Ellis wrote @14: “Isn’t there a “negative space” aspect to consider? Any civilisation that comes after us will have to contend with a lack of easily-accessible deposits of iron, copper, tin, coal, oil etc.

    Coal and oil yes, as they are combustible, but homo sapiens have quite nicely concentrated iron, copper, tin and other metals into multiple deposits of far higher purity than those found in nature. We know them as garbage dumps, scrap yards, factories and cities.

  39. 39
    S.B. Ripman says:

    Great story Gavin. Well-written, readable, thought-provoking, meaningful but not preachy. The publication of fiction has can stimulate public awareness nearly as much as the publication of scientific articles. Here’s a poem recently written by a chum, touching on your field. Thanks for your continuing work.

    Out on a Record-Breaking
    Warm Saturday in January
    by Tim Gillespie

    The shocking day: the sun blinding, gloves
    crammed in my pocket, jacket over my arm,
    steam rising from the drying-out, early
    flowering viburnum and daphne in the air—
    a bounty so unexpected, a slap of joy.

    “Beautiful day,” I say to a woman digging
    in her yard. We’ve nodded other times when
    I’ve walked by her house a half-mile from mine.
    “Yes,” she says, “unseasonably warm,” brushing back
    a strand of hair with her muddy glove. “I wonder
    what it means, though,” she says, her trowel
    plunging into the dirt, “and if we should worry.”

    The what-it-means: glacial melting, coral
    reefs bleached of life, tsunami and storm surge,
    rising seas swamping small villages, drought
    looming and fire and mudslide to follow—-
    a small scent of wintersweet punches my nose.

    -In Windfall: A Journal of Poetry of Place, spring 2018 issue

  40. 40
    sidd says:

    “For instance, on the planet Earth, man had always assumed that he was more intelligent than dolphins because he had achieved so much—the wheel, New York, wars and so on—whilst all the dolphins had ever done was muck about in the water having a good time. But conversely, the dolphins had always believed that they were far more intelligent than man—for precisely the same reasons.”

    Douglas Adams

  41. 41
    Jon Kirwan says:

    I just skimmed this pre=print in about 60 seconds. Interesting enough that I’ll read it, later. This is what stood out to me:

    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 time-scale

    Nice.

    I’ll need to read it more carefully. But that stood out like a sore thumb in seconds of skimming. Interesting point to spend more time thinking about.

  42. 42
    Thomas P says:

    I think Lovecraft’s ‘At the Mountains of Madness” counts.

  43. 43
    Simon C says:

    I really enjoyed reading the story – apart from the drama of the dawning insight into what happened, it’s a great way of talking about the arc of publishing a scientific advance – the trepidation, the cautious publication, the media frenzy and the reactions of colleagues. And then the questions that arise from those new insights …
    One missing component (next episode, extinction permitting?) would be the story of identification of the technological species. With human geological debris (in the event of a sudden extinction) this would be fairly straightforward – the skulls with the largest brains, the plastic models showing miniature humans using technology, the scale and structural adaptions of the technology (eg cars) … with a much older species, who knows? there might have been many large-brained Eocene bipeds in this alternative world, but depending on the way they disposed of their dead, it might not be easy to find fossils of them. They might not have been as fond of plastic (or similar) models. But the associations between a species and its technology would probably still be reasonably clear.
    Humans will leave a massive geologial “footprint” – roads, rubbish dumps, graveyards, buildings, shipwrecks – that will still be reasonably clear in a hundred million years’ time – unless we clear up after ourselves …
    Thanks for the great story!

  44. 44

    Any evolving intelligence throughout the Universe will at one point discover the climate trigger. A primary requirement is likely a stable period priori. A species which is unaware of the climate trigger, will likely also not be able to hibernate.

    Given these circumstances we could make a metaphor for this climate situation, comparing it with an organisms puberty phase. The question is to what extent we will play with the climate trigger game, and the question is what it entails on longer time scales. For instance does it means dwarfing, wide spread species retardations, chain extinctions …

  45. 45
    Alastair B. McDonald says:

    Sometimes the truth can be stranger than fiction. Judging by the d13C spike, the PETM was caused by the oxidation of organic carbon, just as we are doing now. However, there seems to have been an additional source of CO2, so the oxidation of organic carbon was just the trigger for the release of even more carbon dioxide.

    We have already released more CO2 than that which triggered the PETM extinction. Will our emissions trigger further emissions as happened in the PETM? I have evidence to suggest the answer is yes.

    Will the scramble for scarce resources when temperatures soar trigger a nuclear war? I hope not.

  46. 46
  47. 47
    Nick O. says:

    I agree with Radge Havers, #17: where are all the fossils? The fact of a civilisation being sustainable does not exclude recyclable materials leaving a trace. For example, just down the road from where I live we have fossil tree (not really a tree, more like an enormous horsetail) dating from over 300 million years ago, found well preserved in the Durham coal measures, and showing lovely detail of its bark and roots. Well a tree is pretty recyclable, it’s just a load of wood and cells after all, and it has evolved to decompose, but this one still got preserved. Regarding the past civilisations, during their development phase they must have had artefacts, surely? Or tools? Factories? Vehicles? Dwellings? And the creatures themselves of course, maybe wearing some sort of clothing or body protection? Natural processes of weathering, transport and sedimentation can still allow the feather imprints of archaeopteryx to be preserved, or the delicate structures of certain leaves or invertebrates, so I think it is reasonable to expect the ‘people’ of these civilisations would have left something behind, including occasionally fossils of themselves and what they wore, or the things they carried. If they had spread out across the whole planet, one might even expect to find assemblages of them sometimes, rather like the fossil community clusters seen in the Burgess Shale or the Rhynie Chert.

  48. 48
    Omega Centauri says:

    Unless this civilization only occupied a small area, I would think there would be ample fossil evidence. We are calling the present the Anthropocene, we have come to have a huge influence of the planets environment, not just by a CO2 spike, but in many other ways, including enhanced erosion. The current civilization will have left important traces on this planet until such time as the sun becomes a red giant and melts all the rocks.

  49. 49
    Simon C says:

    The astrobiology article on the Silurian hypothesis makes a very good accompaniment to the story – or perhaps the aricle is the main course, and the story is the dessert! Anyway, plenty of food for thought. The article emphasises the difficulties of identifying an unambiguous geochemical marker for industrial activity.While this is probably true of a marine core, any extensive terrestrial exposure of sediments from the period would have lots of evidence of human activity, especially onshore or nearshore uplifted sediments. Objects do not have to survive millions of years to be fossilised, they only need to last long enough for their shape to leave a distinctive geological trace. Otherwise dinosaur footprints and soft-bodied fossils would be non-existent. The remains of human artifacts and structures would be incredibly abundant relative to eg the terrestrial vertebrate record. Interpreting them might not be easy though – and what would they make of the sort of assemblage they would encounter in the debris of a natural history museum? Dinosaurs, trilobites, humans and mammoths all together in a single structure … lets hope the future geologists don’t have a creationist problem!
    It would be hard to rule out the hypothesis of an alien civilisation arriving (and then departing post-industrially) unless the geological history of the evolution of the industrial species could be reconstructed at least to some extent.

  50. 50
    Roger Albin says:

    See The Toolmaker Koan by John C. McLaughlin

    and his superior novel, The Helix and The Sword