Remnants of forgotten civilizations

As silly as it can get sometimes, I am a dedicated Doctor Who fanatic.  I'm late to the game -- I only watched my first-ever episode of the long-running series four years ago -- but after that, I went at it with the enthusiasm you only see in the born-again.

The best of the series tackles some pretty deep stuff.  The ugly side of tribalism ("Midnight"), the acknowledgement that some tragedies are unavoidable ("The Fires of Pompeii"), the Butterfly Effect ("Turn Left"), the fact that you can't both "play God" and avoid responsibility ("The Waters of Mars"), and the terrible necessity of personal self-sacrifice ("Silence in the Library").  Plus, the series invented what would be my choice for the single most terrifying, wet-your-pants-inducing alien species ever dreamed up, the Weeping Angels (several episodes, most notably "Blink").

So it shouldn't have been a surprise when Doctor Who got a mention in this month's Scientific American, but it still kinda was.  It came up in a wonderful article by Caleb Scharf called "The Galactic Archipelago," which was about the possibility of intelligent life in the universe (probably very high) and the odd question of why, if that's true, we haven't been visited (Fermi's paradox).  Here at Skeptophilia we've looked at one rather depressing answer to Fermi -- the "Great Filter," the idea that intelligent life is uncommon in the universe either because there are barriers to the formation of life on other worlds, or that once formed, it's likely to get wiped out completely at some point.

It's even more puzzling when you consider the fact that it would be unnecessary for the aliens themselves to visit.  Extraterrestrial life paying a house call to Earth is unlikely considering the vastness of space and the difficulties of fast travel, whatever the amazingly-coiffed Giorgio Tsoukalos (of Ancient Aliens fame) would have you believe.  But Scharf points out that it's much more likely that intelligent aliens would have instead sent out self-replicating robot drones, which not only had some level of intelligence themselves (in terms of avoiding dangers and seeking out raw materials to build new drones), but could take their time hopping from planet to planet and star system to star system.  And because they reproduce, all it would take is one or two civilizations to develop these drones, and given a few million years, you'd expect they'd spread pretty much everywhere in the galaxy.

But, of course, it doesn't seem like that has happened either.

Scharf tells us that there's another possibility than the dismal Great Filter concept, and that's something that's been nicknamed the "Silurian Hypothesis."  Here's where Doctor Who comes in, because as any good Whovian will tell you, the Silurians are a race of intelligent reptilians who were the dominant species on Earth for millions of years, but who long before humans appeared went (mostly) extinct except for a few scattered remnant populations in deep caverns.

Last year, astronomers Gavin Schmidt and Adam Frank, of NASA and the University of Rochester (respectively), considered whether it was possible that an intelligent technological species like the Silurians had existed millions of years ago, and if so, what traces of it we might expect to find in the modern world.  And what Schmidt and Frank found was that if there had been a highly complex, city-building, technology-using species running the Earth, (say) fifty million years ago, what we'd find today as evidence of its existence is very likely to be...

... nothing.

Scharf writes:

[Astrophysicist Michael] Hart's original fact [was] that there is no evidence here on Earth today of extraterrestrial explorers...  Perhaps long, long ago aliens came and went.  A number of scientists have, over the years, discussed the possibility of looking for artifacts that might have been left behind after such visitations of our solar system.  The necessary scope of a complete search is hard to predict, but the situation on Earth alone turns out to be a bit more manageable.  In 2018 another of my colleagues, Gavin Schmidt of NASA's Goddard Institute for Space Studies, together with Adam Frank, produced a critical assessment of whether we could even tell if there had been an earlier industrial civilization on our planet. 

As fantastic as it may seem, Schmidt and Frank argue -- as do most planetary scientists -- that it is actually very easy for time to erase essentially all signs of technological life on Earth.  The only real evidence after a million or more years would boil down to isotopic or chemical stratigraphic anomalies -- odd features such as synthetic molecules, plastics, or radioactive fallout.  Fossil remains and other paleontological markers are so rare and so contingent on special conditions of formation that they might not tell us anything in this case. 

Indeed, modern human urbanization covers only on order of about one percent of the planetary surface, providing a very small target area for any paleontologists in the distant future.  Schmidt and Frank also conclude that nobody has yet performed the necessary experiments to look exhaustively for such nonnatural signatures on Earth.  The bottom line is, if an industrial civilization on the scale of our own had existed a few million years ago, we might not know about it.  That absolutely does not mean one existed; it indicates only that the possibility cannot be rigorously eliminated.

(If you'd like to read Schmidt and Frank's paper, it appeared in the International Journal of Astrobiology and is available here.)

It's a little humbling, isn't it?  All of the massive edifices we've created, the far-more-than Seven Wonders of the World, will very likely be gone without a trace in only a few million years.  A little more cheering is that the same will be true of all the damage we're currently doing to the global ecosystem.  It's not so surprising if you know a little geology; the current arrangement of the continents is only the most recent, and won't be the last the Earth will see.  Because of erosion and natural disasters, not to mention the rather violent clashes that occur when the continents do shift position, it stands to reason that our puny little efforts to change things won't last very long.

Entropy always wins in the end.

The whole thing puts me in mind of one of the first poems I ever read that made a significant impact on me -- Percy Bysshe Shelley's devastating "Ozymandias," which I came across when I was a freshman in high school.  It seems a fitting way to conclude this post.

I met a traveller from an antique land,
Who said—Two vast and trunkless legs of stone
Stand in the desert. . . . Near them, on the sand,
Half sunk a shattered visage lies, whose frown,
And wrinkled lip, and sneer of cold command,
Tell that its sculptor well those passions read
Which yet survive, stamped on these lifeless things,
The hand that mocked them, and the heart that fed;
And on the pedestal, these words appear:
"My name is Ozymandias, King of Kings;
Look on my Works, ye Mighty, and despair!"
Nothing beside remains. Round the decay
Of that colossal Wreck, boundless and bare
The lone and level sands stretch far away.

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Relics of a lost world

There are times that I get impatient with people doing what I call hypothesizing in a vacuum -- coming up with pointless "what ifs" that are unverifiable.  A lot of it seems to me to be useless mental messing-about that doesn't tell us anything new about how the universe actually works.

So it was a little surprising that I reacted as positively as I did to the paper that appeared last week in the International Journal of Astrobiology called, "The Silurian Hypothesis: Would It Be Possible to Detect an Industrial Civilization in the Geological Record?" by Gavin Schmidt and Adam Frank.  The authors write:

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."

You're reading this correctly; the authors are trying to parse whether we could detect the presence of an industrial civilization on Earth -- if it last existed, say, 250 million years ago.

They're not the first ones to think about this.  It's showed up in fiction, most notably in the short story "At the Mountains of Madness" by H. P. Lovecraft, wherein some explorers in Antarctica discover a colossal ruin that is not centuries, or even millennia, old, but tens of millions of years old -- when Antarctica was far north of its present location and had a tropical climate, and when it was inhabited by an intelligent civilization that was decidedly not human.  Being Lovecraft, of course this discovery presages several of the main characters losing important body parts, but I still remember that when I first read this story, when I was maybe fifteen years old, it wasn't the horror element that struck me most; it was the idea that maybe 90 million years ago, the world hosted a non-human intelligence of which most of the traces had been wiped out.

Schmidt and Frank first look at the likelihood of we ourselves becoming fossils, and they conclude that the answer is -- it's not very high:

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.  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.

So the mind-blowing outcome of this reasoning is that the vast majority of species that have ever lived left no fossil record at all -- and that our knowledge of prehistoric life is so scanty that using the word "incomplete" to describe it is a woeful understatement.

[Image courtesy of Wellcome Images and the Wikimedia Commons]

As far as our artifacts, they're not much more hopeful:

The likelihood of objects surviving and being discovered is similarly unlikely. Zalasiewicz (2009) speculates about preservation of objects or their forms, but the current area of urbanization is less than 1% of the Earth’s surface, 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, 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.

Of course, the thing is, all it would take is one single artifact; you don't need an entire city to be preserved (as it was in Lovecraft).  We've made a number of very durable things -- of which, surprisingly, glass is one of the most resistant.  Most metal objects corrode on the scale of a human lifetime, much less millions of years; and plastics, long thought of as indestructible, are likely to break down to microscopic dust within a few centuries.  Any Jurassic-era plastics would long since be undetectable.  But a single glass marble or fragment of a drinking glass, encased in sediment -- that'd definitely do the trick.

Schmidt and Frank aren't the only scientists to consider the question.  Alan Weisman, in his fascinating book The World Without Us, considers the sequence of events that would occur if humanity disappeared, and concludes that not only would life march on just fine, most of our impacts would be gone in short order.  Within three days, he says, all the lights would have gone out; the loss of electricity would have results like the entire New York City subway system, and most of New Orleans, flooding.  Structures in tropical climates -- like the Panama Canal and most cities within twenty degrees either side of the Equator -- would be overgrown and swallowed by jungle within a few decades.  Within five hundred years, Weisman says, just about all that would be left is aluminum cookware, plastic residue, some of the more durable glass objects, remnants of buildings, especially in dry climates, and monuments like Mount Rushmore.

And that's after five hundred years.  Which is 0.001% of fifty million years -- and even that time span doesn't bring us back to the extinction of the dinosaurs.

What's fascinating about this study is that Schmidt and Frank aren't trying to tell us that there have been ancient civilizations in previous eras; they're simply considering the question of whether it'd be detectable if there had been.  And the answer is: probably not.  Even such factors as an industrial civilization's impact on the climate and the chemistry of the atmosphere might not leave traces that would still be discernible a hundred million years later.  The authors end their paper thusly:

Perhaps unusually, the authors of this paper are not convinced of the correctness of their proposed hypothesis.  Were it to be true it would have profound implications and not just for astrobiology.  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.  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.

Which is it exactly.  Science isn't just the description of what we know, it also asks questions like "What would the universe look like if X were true?"  The ability to create a model of some version of reality, and then see if the predictions of that model line up with the evidence, is a powerful tool for understanding.

So as bizarre as it seems, apparently it is likely that even if there were a highly advanced industrial civilization that got wiped out by the Permian-Triassic Extinction 252 million years ago (along with damn near everything else; paleontologists Jack Sepkoski and David Raup, who specialize in studying the cheerful topic of mass extinctions, have estimated that the Permian-Triassic event obliterated 95% of the species on Earth), we would probably not have a single detectable trace of it left.

But maybe, just maybe, there's an Eldritch Cyclopean City still awaiting discovery somewhere.  Like Antarctica.  If so, though, I'm not going to be the one who explores it.  When that happened in Lovecraft's story, it resulted in a number of people having their brains sucked out by Shoggoths.

Which could seriously ruin one's day.

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