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We think we’re the first advanced earthlings—but how do we really know?

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Imagine if, many millions of years ago, dinosaurs drove cars through cities of mile-high buildings. A preposterous idea, right? Over the course of tens of millions of years, however, all of the direct evidence of a civilization—its artifacts and remains—gets ground to dust. How do we really know, then, that there weren’t previous industrial civilizations on Earth that rose and fell long before human beings appeared?

It’s a compelling thought experiment, and one that Adam Frank, a professor of physics and astronomy at the University of Rochester, and Gavin Schmidt, the director of the NASA Goddard Institute for Space Studies, take up in a paper published in the International Journal of Astrobiology .

“Gavin and I have not seen any evidence of another industrial civilization,” Frank explains. But by looking at the deep past in the right way, a new set of questions about civilizations and the planet appear: What geological footprints do civilizations leave? Is it possible to detect an industrial civilization in the geological record once it disappears from the face of its host planet? “These questions make us think about the future and the past in a much different way, including how any planetary-scale civilization might rise and fall.”

In what they deem the “Silurian Hypothesis,” Frank and Schmidt define a civilization by its energy use. Human beings are just entering a new geological era that many researchers refer to as the Anthropocene, the period in which human activity strongly influences the climate and environment. In the Anthropocene, fossil fuels have become central to the geological footprint humans will leave behind on Earth. By looking at the Anthropocene’s imprint, Schmidt and Frank examine what kinds of clues future scientists might detect to determine that human beings existed. In doing so, they also lay out evidence of what might be left behind if industrial civilizations like ours existed millions of years in the past.

Human beings began burning fossil fuels more than 300 years ago, marking the beginnings of industrialization. The researchers note that the emission of fossil fuels into the atmosphere has already changed the carbon cycle in a way that is recorded in carbon isotope records. Other ways human beings might leave behind a geological footprint include:

• Global warming, from the release of carbon dioxide and perturbations to the nitrogen cycle from fertilizers
• Agriculture, through greatly increased erosion and sedimentation rates
• Plastics, synthetic pollutants, and even things such as steroids, which will be geochemically detectable for millions, and perhaps even billions, of years
• Nuclear war, if it happened, which would leave behind unusual radioactive isotopes

Light of the Stars Read more about Professor Frank’s latest book and other work

“As an industrial civilization, we’re driving changes in the isotopic abundances because we’re burning carbon,” Frank says. “But burning fossil fuels may actually shut us down as a civilization. What imprints would this or other kinds of industrial activity from a long dead civilization leave over tens of millions of years?”

The questions raised by Frank and Schmidt are part of a broader effort to address climate change from an astrobiological perspective, and a new way of thinking about life and civilizations across the universe. Looking at the rise and fall of civilizations in terms of their planetary impacts can also affect how researchers approach future explorations of other planets.

“We know early Mars and, perhaps, early Venus were more habitable than they are now, and conceivably we will one day drill through the geological sediments there, too,” Schmidt says. “This helps us think about what we should be looking for.”

Schmidt points to an irony, however: if a civilization is able to find a more sustainable way to produce energy without harming its host planet, it will leave behind less evidence that it was there.

“You want to have a nice, large-scale civilization that does wonderful things but that doesn’t push the planet into domains that are dangerous for itself, the civilization,” Frank says. “We need to figure out a way of producing and using energy that doesn’t put us at risk.”

That said, the earth will be just fine, Frank says. It’s more a question of whether humans will be.

Can we create a version of civilization that doesn’t push the earth into a domain that’s dangerous for us as a species?

“The point is not to ‘save the earth,’” says Frank. “No matter what we do to the planet, we’re just creating niches for the next cycle of evolution. But, if we continue on this trajectory of using fossil fuels and ignoring the climate change it drives, we human beings may not be part of Earth’s ongoing evolution.”

The Silurian Hypothesis: A nod to Doctor Who

Adam Frank and Gavin Schmidt call their study the Silurian Hypothesis after a race of intelligent, bipedal reptiles—known as the Silurians—introduced in a 1970 episode of the British science fiction series Doctor Who . The Silurians supposedly evolved on Earth during the eponymous era, a geological time period lasting from 443 million to 416 million years ago. To avoid any kind of catastrophe, the reptiles went into hibernation for millions of years before being awakened by secret nuclear experiments in a Welsh mine.

“When we were writing this paper,” Schmidt says, “I tried to find examples of terrestrial, non-human civilizations in the science-fiction literature, but I wasn’t able to find anything earlier than the 1970s. Despite it being exceedingly unlikely that there were any civilizations in the Silurian period—this was before the land plants and animals had really established themselves—it seemed fitting to name our idea after the first example that people thought about, even if this is fiction.”

Astrophysicist Adam Frank

A self-described “evangelist of science,” Frank regularly writes   and speaks about subjects like intelligent life forms in the universe, high-energy-density physics, space exploration and missions, climate change, and more.

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The Silurian Hypothesis

*We may not be the first Anthropocene.

And if so, how would we even know

Today we get an answer thanks to Gavin Schmidt at the NASA Goddard Institute for Space Studies in New York city and Adam Frank at the University of Rochester. These guys have a name for the idea that an industrial civilization may have predated humanity: the Silurian hypothesis. They study the signature that our own civilization is likely to leave behind and ask whether it will be detectable millions of years from now. Their conclusion is that our probable impact on the planet will be palpable but in some ways hard to distinguish from various other events in the geological record.

Their work has some interesting implications for how we should study Earth and the impact we have on it. The research should also help astrobiologists decide what to look for elsewhere in the universe.

Schmidt and Frank begin by setting out just how little we know about ancient Earth. The oldest part of Earth’s surface is the Negev Desert in southern Israel, which is 1.8 million years old. Older surfaces exist only in exposed areas or as a result of mining and drilling operations. Given these constraints, the evidence of activity by Homo sapiens stretches back some 2.5 million years — not really that far in geological terms. The ocean floor is relatively young too, because ocean crust is constantly recycled. As a result, all ocean sediment post-dates the Jurassic Period and is therefore less than 170 million years old.

In any case, say Schmidt and Frank, the fraction of life that gets fossilized is tiny. Dinosaurs roamed Earth for some 180 million years, and yet only a few thousand near-complete specimens exist. Modern humans have existed for just a few tens of thousands of years. “Species as short-lived as homo sapiens (so far) might not be represented in the existing fossil record at all,” say Schmidt and Frank.

What of human artifacts — roads, buildings, baked-bean tins, and silicon chips? These, too, are unlikely to survive long, or to be found even if they do. “The current area of urbanization is less than 1% of the Earth’s surface,” point out the researchers.

“We conclude that for potential civilizations older than about 4 million years, the chances of finding direct evidence of their existence via objects or fossilized examples of their population is small,” they say. But there is another type of evidence: our civilization also leaves a chemical footprint....

Was There a Civilization on Earth Before Humans?

A look at the available evidence

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It only took five minutes for Gavin Schmidt to out-speculate me.

Schmidt is the director of NASA ’s Goddard Institute for Space Studies (a.k.a. GISS), a world-class climate-science facility. One day last year, I came to GISS with a far-out proposal. In my work as an astrophysicist, I’d begun researching global warming from an “astrobiological perspective.” That meant asking whether any industrial civilization that rises on any planet will, through its own activity, trigger its own version of a climate shift. I was visiting GISS that day hoping to gain some climate-science insights and, perhaps, collaborators. That’s how I ended up in Gavin’s office.

Just as I was revving up my pitch, Gavin stopped me in my tracks.

“Wait a second,” he said. “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 up 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. Instead, that first conversation launched a new study we’ve recently published in the International Journal of Astrobiology . 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.

We’re used to imagining extinct civilizations in terms of sunken statues and subterranean ruins. These kinds of artifacts of previous societies are fine if you’re only interested in timescales of a few thousands of years. But once you roll the clock back to tens of millions or hundreds of millions of years, things get more complicated.

When it comes to direct evidence of an industrial civilization—things like cities, factories, and roads—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 further than the Quaternary, and everything has been turned over and crushed to dust.

And, if we’re going back this far, we’re not talking about human civilizations anymore. Homo sapiens didn’t make their appearance on the planet until just 300,000 years or so ago . That means the question shifts to other species, which is why Gavin called the idea the Silurian hypothesis, after an old Doctor Who episode with intelligent reptiles.

So could researchers find clear evidence that an ancient species built a relatively short-lived industrial civilization long before our own? Perhaps, for example, some early mammal rose briefly to civilization building during the Paleocene epoch, about 60 million years ago. There are fossils, of course. But the fraction of life that gets fossilized is always minuscule and varies a lot depending on time and habitat. It would be easy, therefore, to miss an industrial civilization that lasted only 100,000 years—which would be 500 times longer than our industrial civilization has made it so far.

Given that all direct evidence would be long gone after many millions of years, what kinds of evidence might then still exist? The best way to answer this question is to figure out what evidence we’d leave behind if human civilization collapsed at its current stage of development.

Now that our industrial civilization has truly gone global, humanity’s collective activity is laying down a variety of traces that will be detectable by scientists 100 million years in the future. The extensive use of fertilizer , for example, keeps 7 billion people fed, but it also means we’re redirecting the planet’s flows of nitrogen into food production. Future researchers should see this in characteristics of nitrogen showing up in sediments from our era. Likewise our relentless hunger for the rare-Earth elements used in electronic gizmos. Far more of these atoms are now wandering around the planet’s surface because of us than would otherwise be the case. They might also show up in future sediments, too. Even our creation, and use, of synthetic steroids has now become so pervasive that it too may be detectable in geologic strata 10 million years from now.

And then there’s all that plastic. Studies have shown that increasing amounts of plastic “ marine litter ” are being deposited on the seafloor everywhere from coastal areas to deep basins, and even in the Arctic. Wind, sun, and waves grind down large-scale plastic artifacts, leaving the seas full of microscopic plastic particles that will eventually rain down on the ocean floor, creating a layer that could persist for geological timescales.

The big question is how long any of these traces of our civilization will last. In our study, we found that each had the possibility of making it into future sediments. Ironically, however, the most promising marker of humanity’s presence as an advanced civilization is a by-product of one activity that may threaten it most.

When we burn fossil fuels , we’re releasing carbon back into the atmosphere that was once part of living tissues. This ancient carbon is depleted in one of that element’s three naturally occurring varieties, or isotopes. The more fossil fuels we burn, the more the balance of these carbon isotopes shifts. Atmospheric scientists call this shift the Suess effect, and the change in isotopic ratios of carbon due to fossil-fuel use is easy to see over the past century. Increases in temperature also leave isotopic signals. These shifts should be apparent to any future scientist who chemically analyzes exposed layers of rock from our era. Along with these spikes, this Anthropocene layer might also hold brief peaks in nitrogen, plastic nanoparticles, and even synthetic steroids. So if these are traces our civilization is bound to leave for the future, might the same “signals” exist right now in rocks just waiting to tell us of civilizations long gone?

Fifty-six million years ago, Earth passed through the Paleocene-Eocene Thermal Maximum (PETM). During the PETM, the planet’s average temperature climbed as high as 15 degrees Fahrenheit above what we experience today. It was a world almost without ice, as typical summer temperatures at the poles reached close to a balmy 70 degrees Fahrenheit. Looking at the isotopic record from the PETM, scientists see both carbon and oxygen isotope ratios spiking in exactly the way we expect to see in the Anthropocene record. There are also other events like the PETM in Earth’s history that show traces like our hypothetical Anthropocene signal. These include an event a few million years after the PETM dubbed the Eocene Layers of Mysterious Origin, and massive events in the Cretaceous that left the ocean without oxygen for many millennia (or even longer).

Are these events indications of previous nonhuman industrial civilizations? Almost certainly not. While there is evidence that the PETM may have been driven by a massive release of buried fossil carbon into the air, it’s the timescale of these changes that matter. The PETM’s isotope spikes rise and fall over a few hundred thousand years. But what makes the Anthropocene so remarkable in terms of Earth’s history is the speed at which we’re dumping fossil carbon into the atmosphere. There have been geological periods where Earth’s CO 2 has been as high or higher than it is today, but never before in the planet’s multibillion-year history has so much buried carbon been dumped back into the atmosphere so quickly. So the isotopic spikes we do see in the geologic record may not be spiky enough to fit the Silurian hypothesis’s bill.

But there is a conundrum here. If an earlier species’s industrial activity is short-lived, we might not be able to easily see it. The PETM’s spikes mostly show us Earth’s timescales for responding to whatever caused it, not necessarily the timescale of the cause. So it might take both dedicated and novel detection methods to find evidence of a truly short-lived event in ancient sediments. In other words, if you’re not explicitly looking for it, you might not see it. That recognition was, perhaps, the most concrete conclusion of our study.

It’s not often that you write a paper proposing a hypothesis that you don’t support. Gavin and I don’t believe the Earth once hosted a 50-million-year-old Paleocene civilization. But by asking if we could “see” truly ancient industrial civilizations, we were forced to ask about the generic kinds of impacts any civilization might have on a planet. That’s exactly what the astrobiological perspective on climate change is all about. Civilization building means harvesting energy from the planet to do work (i.e., the work of civilization building). Once the civilization reaches truly planetary scales, there has to be some feedback on the coupled planetary systems that gave it life (air, water, rock). This will be particularly true for young civilizations like ours still climbing up the ladder of technological capacity. There is, in other words, no free lunch. While some energy sources will have lower impact—say solar versus fossil fuels—you can’t power a global civilization without some degree of impact on the planet.

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Once you realize, through climate change, the need to find lower-impact energy sources, the less impact you will leave. So the more sustainable your civilization becomes, the smaller the signal you’ll leave for future generations.

In addition, our work also opened up the speculative possibility that some planets might have fossil-fuel-driven cycles of civilization building and collapse. If a civilization uses fossil fuels, the climate change they trigger can lead to a large decrease in ocean oxygen levels. These low oxygen levels (called ocean anoxia) help trigger the conditions needed for making fossil fuels like oil and coal in the first place. In this way, a civilization and its demise might sow the seed for new civilizations in the future.

By asking about civilizations lost in deep time, we’re also asking about the possibility for universal rules guiding the evolution of all biospheres in all their creative potential, including the emergence of civilizations. Even without pickup-driving Paleocenians, we’re only now learning to see how rich that potential might be.

April 23, 2018

Could an Industrial Prehuman Civilization Have Existed on Earth before Ours?

A provocative new paper suggests some ways to find out

By Steven Ashley

How could we really know if industrial civilizations existed on Earth long before human beings appeared? That is the question posed in a scientific thought experiment by climate scientist Gavin Schmidt and astrophysicist Adam Frank.

Michael Osadciw University of Rochester

One of the creepier conclusions drawn by scientists studying the Anthropocene—the proposed epoch of Earth’s geologic history in which humankind’s activities dominate the globe—is how closely today’s industrially induced climate change resembles conditions seen in past periods of rapid temperature rise.

“These ‘hyperthermals,’ the thermal-maximum events of prehistory, are the genesis of this research,” says Gavin Schmidt, climate modeler and director of the NASA Goddard Institute for Space Studies. “Whether the warming was caused by humans or by natural forces, the fingerprints—the chemical signals and tracers that give evidence of what happened then—look very similar.”

The canonical example of a hyperthermal is the Paleocene-Eocene Thermal Maximum (PETM), a 200,000-year period that occurred some 55.5 million years ago when global average temperatures rose by five to eight degrees Celsius (nine to 14 degrees Fahrenheit). Schmidt has pondered the PETM for his entire career, and it was on his mind one day in 2017 when University of Rochester astrophysicist Adam Frank paid him a visit.

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Frank came to his office to discuss the idea of studying global warming from an “astrobiological perspective”—that is, investigating whether the rise of an alien industrial civilization on an exoplanet might necessarily trigger climate changes similar to those we see during Earth’s own Anthropocene. But almost before Frank could describe how one might search for the climatic effects of industrial “exocivilizations” on newly discovered planets, Schmidt caught him up short with a surprising question: “How do you know we’re the only time there’s been a civilization on our own planet?“

Frank considered a moment before responding with a question of his own: “Could we even tell if there had been an industrial civilization [long before this one]?“

Their subsequent attempt to address both questions yielded a provocative paper on the possibility that Earth might have spawned more than one technological society during its 4.5-billion-year history. And if indeed some such culture arose on Earth in the murky depths of geologic time, how might scientists today discern signs of that incredible development? Or, as they put it in the paper: “If an industrial civilization had existed on Earth many millions of years prior to our own era, what traces would it have left and would they be detectable today?“

Schmidt and Frank began by forecasting the geologic fingerprints the Anthropocene is likely to leave behind—such as hints of soaring temperatures and rising seas laid down in beds of sedimentary rock. These features, they noted, are very similar to the geologic leftovers of the PETM and other hyperthermal events. They then considered what tests could plausibly distinguish an industrial cause from otherwise naturally occurring climate changes. “These issues have never really been addressed to any great extent,” Schmidt notes. And that goes not only for scientists but evidently for science-fiction writers as well, he adds: “I looked back into the science-fiction literature to try to find the earliest example of a story featuring a nonhuman industrial civilization on Earth. The earliest I could find was in a Doctor Who episode.“

That 1970 episode of the classic TV series involves the present-day discovery of “Silurians”—an ancient race of technologically advanced, reptilian humanoids who predated the arrival of humans by hundreds of millions of years. According to the plot, these highly civilized saurians flourished for centuries until Earth’s atmosphere entered a period of cataclysmic upheaval that forced Homo reptilia to go into hibernation underground to wait out the danger. Schmidt and Frank paid tribute to the episode in the title of their paper: “The Silurian Hypothesis.”

Lost in Strata

Any plausibility of the Silurian hypothesis stems chiefly from the vast incompleteness of the geologic record, which only gets sparser the further back in time you go.

Today less than 3 percent of Earth’s surface is urbanized, and the chance that any of our great cities would remain over tens of millions of years is vanishingly low, says geologist Jan Zalasiewicz of the University of Leicester in England. A metropolis’s ultimate fate, he notes, mostly depends on whether the surrounding surface is subsiding (to be locked in rock) or rising (to be eroded away by rain and wind). “New Orleans is sinking; San Francisco is rising,” he says. The French Quarter, it seems, has much better chances of entering the geologic record than Haight-Ashbury.

“To estimate the odds of finding artifacts,” Schmidt says, “the back-of-the-envelope calculation for dinosaur fossils says that one fossil emerges every 10,000 years.” Dinosaur footprints are rarer still.

“After a couple of million years,” Frank says, “the chances are that any physical reminder of your civilization has vanished, so you have to search for things like sedimentary anomalies or isotopic ratios that look off.” The shadows of many prehuman civilizations could, in principle, lurk hidden in such subtleties.

But exactly what we would look for depends to some degree on how an Earthly but alien technological culture would choose to behave. Schmidt and Frank decided the safest assumption to make would be that any industrial civilization now or hundreds of millions of years ago should be hungry for energy. That means any ancient industrial society would have developed the capacity to widely exploit fossil fuels and other power sources, just as we did. “We’d be looking for globalized effects that would leave a worldwide trace”—planetary-scale physical-chemical tracers of energy-intensive industrial processes and their wastes, Schmidt says.

Next comes the issue of longevity—the longer a civilization’s energy-intensive period persists and the more its demands increase, the more obvious its presence should become in the geologic record. Consider our own industrial age, which has existed for only about 300 years out of a multimillion-year history of humanity. Now compare that minuscule slice of time with the half a billion years or so that creatures have lived on land. Humanity’s present rapacious phase of fossil-fuel use and environmental degradation, Frank says, is unsustainable for long periods. In time, it will diminish either by human choice or by the force of nature, making the Anthropocene less of an enduring era and more of a blip in the geologic record. “Maybe [civilizations like ours] have happened multiple times, but if they each only last 300 years, no one would ever see them,” Frank says.

Taking all this into consideration, what remains is a menu of diffuse long-lived tracers including fossil-fuel combustion residues (carbon, primarily), evidence of mass extinctions, plastic pollutants, synthetic chemical compounds not found in nature and even transuranic isotopes from nuclear fission. In other words, what we would need to look for in the geologic record are the same distinctive signals that humans are laying down right now.

Signs of Civilization

Finding signs of an altered carbon cycle would be one big clue to previous industrial periods, Schmidt says. “Since the mid-18th century, humans have released half a trillion tons of fossil carbon at high rates. Such changes are detectable in changes in the carbon isotope ratio between biological and inorganic carbon—that is, between the carbon incorporated into things like seashells and that which goes instead into lifeless volcanic rock.”

Another tracer would be distinctive patterns of sediment deposition. Large coastal deltas would hint at boosted levels of erosion and rivers (or engineered canals) swollen from increased rainfall. Telltale traces of nitrogen in the sediments could suggest the widespread use of fertilizer, fingering industrial-scale agriculture as a possible culprit; spikes in metal levels in the sediments might instead point to runoff from manufacturing and other heavy industry.

More unique, specific tracers would be nonnaturally occurring, stable synthetic molecules such as steroids and many plastics, along with well-known pollutants, including polychlorinated biphenyls (toxic chemical compounds from electrical devices) and chlorofluorocarbons (ozone-eating molecules from refrigerators and aerosol sprays).

The key strategy in distinguishing the presence of industry from nature, Schmidt notes, is developing a multifactor signature. Absent artifacts or convincingly clear markers, the uniqueness of an event may well be seen in many relatively independent fingerprints as opposed to the coherent set of changes that are seen to be associated with a single geophysical cause.

“I find it amazing that no one had worked all this out before, and I’m really glad that somebody has taken a closer look at it,” says Pennsylvania State University astronomer Jason Wright, who in 2017 published “a fluffy little paper” exploring the counterintuitive notion that the best place to find evidence of any of Earth’s putative prehuman civilizations may well be off-world. If, for instance, dinosaurs built interplanetary rockets, presumably some remnants of that activity might remain preserved in stable orbits or on the surfaces of more geologically inert celestial bodies such as the moon.

“Look, 200 years ago the question of whether there might be a civilization on Mars was a legitimate one,” Wright says. “But once the pictures came out from interplanetary probes, that was settled for good. And that view became ingrained, so now it’s not a valid topic for scientific inquiry; it’s considered ridiculous. But no one’s ever put the actual scientific limits on it—on what may have happened a long time ago.”

Wright also acknowledges the potential for this work to be misinterpreted. “Of course, no matter what, this is going to be interpreted as ‘Astronomers Say Silurians Might Have Existed,’ even though the premise of this work is that there is no such evidence,” he says. “Then again, absence of evidence is not evidence of absence.”

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What if we’re not Earth’s first big civilization?

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How do we really know that there weren’t civilizations on Earth before ours? A new paper addresses this question.

Imagine if, many millions of years ago, dinosaurs drove cars through cities of mile-high buildings. A preposterous idea, right? Over the course of tens of millions of years, however, all of the direct evidence of a civilization—its artifacts and remains—gets ground to dust. How do we know there weren’t previous industrial civilizations on Earth that rose and fell long before human beings appeared?

It’s a compelling thought experiment, and one that Adam Frank, a professor of physics and astronomy at the University of Rochester, and Gavin Schmidt, the director of the NASA Goddard Institute for Space Studies, take up in the International Journal of Astrobiology . Frank also considers the evidence in The Atlantic .

The history of the world

“Gavin and I have not seen any evidence of another industrial civilization,” Frank explains. But by looking at the deep past in the right way, a new set of questions about civilizations and the planet appears: What geological footprints do civilizations leave? Is it possible to detect an industrial civilization in the geological record once it disappears from the face of its host planet?

“These questions make us think about the future and the past in a much different way, including how any planetary-scale civilization might rise and fall.”

In what they deem the “Silurian Hypothesis,” Frank and Schmidt define a civilization by its energy use.

Human beings are just entering a new geological era that many researchers refer to as the Anthropocene, the period in which human activity strongly influences the climate and environment. In the Anthropocene, fossil fuels have become central to the geological footprint humans will leave behind on Earth.

By looking at the Anthropocene’s imprint, Schmidt and Frank examine what kinds of clues future scientists might detect to determine that human beings existed. In doing so, they also lay out evidence of what might be left behind if industrial civilizations like ours existed millions of years in the past.

Human beings began burning fossil fuels more than 300 years ago, marking the beginnings of industrialization. The researchers note that the emission of fossil fuels into the atmosphere has already changed the carbon cycle in a way that is recorded in carbon isotope records.

Other ways human beings might leave behind a geological footprint include:

• Global warming, from the release of carbon dioxide and perturbations to the nitrogen cycle from fertilizers
• Agriculture, through greatly increased erosion and sedimentation rates
• Plastics, synthetic pollutants, and even things such as steroids, which will be geochemically detectable for millions, and perhaps even billions, of years
• Nuclear war, if it happened, which would leave behind unusual radioactive isotopes

“As an industrial civilization, we’re driving changes in the isotopic abundances because we’re burning carbon,” Frank says. “But burning fossil fuels may actually shut us down as a civilization. What imprints would this or other kinds of industrial activity from a long dead civilization leave over tens of millions of years?”

A new perspective on climate change

The questions that Frank and Schmidt raise are part of a broader effort to address climate change from an astrobiological perspective, and a new way of thinking about life and civilizations across the universe. Looking at the rise and fall of civilizations in terms of their planetary impacts can also affect how researchers approach future explorations of other planets.

“We know early Mars and, perhaps, early Venus were more habitable than they are now, and conceivably we will one day drill through the geological sediments there, too,” Schmidt says. “This helps us think about what we should be looking for.”

Schmidt points to an irony, however: if a civilization is able to find a more sustainable way to produce energy without harming its host planet, it will leave behind less evidence that it was there.

“You want to have a nice, large-scale civilization that does wonderful things but that doesn’t push the planet into domains that are dangerous for itself, the civilization,” Frank says. “We need to figure out a way of producing and using energy that doesn’t put us at risk.”

That said, the earth will be just fine, Frank says. It’s more a question of whether humans will be.

Can we create a version of civilization that doesn’t push the earth into a domain that’s dangerous for us as a species?

“The point is not to ‘save the earth,'” says Frank. “No matter what we do to the planet, we’re just creating niches for the next cycle of evolution. But, if we continue on this trajectory of using fossil fuels and ignoring the climate change it drives, we human beings may not be part of Earth’s ongoing evolution.”

Inspiration from  Doctor Who

Frank and Schmidt call their study the Silurian Hypothesis after a race of intelligent, bipedal reptiles—known as the Silurians—introduced in a 1970 episode of the British science fiction series Doctor Who .

The Silurians supposedly evolved on Earth during the eponymous era, a geological time period lasting from 443 million to 416 million years ago. To avoid any kind of catastrophe, the reptiles went into hibernation for millions of years before secret nuclear experiments in a Welsh mine awakened them.

“When we were writing this paper,” Schmidt says, “I tried to find examples of terrestrial, non-human civilizations in the science-fiction literature, but I wasn’t able to find anything earlier than the 1970s. Despite it being exceedingly unlikely that there were any civilizations in the Silurian period—this was before the land plants and animals had really established themselves—it seemed fitting to name our idea after the first example that people thought about, even if this is fiction.”

Source: University of Rochester

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The ‘Silurian Hypothesis’ Debate: Could Earth Have Once Harbored a Pre-Human Industrial Civilization?

It might seem like something from Ancient Aliens , but it’s an important thought experiment.

• The astrobiologists who developed the thought experiment concluded that there is not strong evidence in Earth’s geologic record to support such a claim.
• However, we still lack the scientific methods that would allow us to dive deep into Earth’s behaviors over such a long time span, so we may want to keep an open mind.

Complex life on our planet has existed for at least 400 million years . Yet as a species, we only managed to create an industrial civilization around 300 years ago. But, what if an earlier industrial civilization existed on Earth millions of years ago? If that were so, how would we be able to prove it—or disprove it? This is the crux of the Silurian hypothesis, a fascinating thought experiment that appeared in a study published in 2018 in the International Journal of Astrobiology .

The possibility of life elsewhere in the universe and its parallels with the Anthropocene — the current geological epoch, during which humans have impacted Earth to the point of no return—has long puzzled Adam Frank , Ph.D., a professor of astrophysics at the University of Rochester, New York, and one of the two authors of the study.

“Is it common for any civilization that reaches our level of energy use to trigger their own version of climate change? I was wondering. If there are alien civilizations , would they also trigger climate change?” Frank ponders. With this whirlwind of thoughts in mind, Frank visited the NASA Goddard Institute of Space Sciences (GISS), an elite climate-science facility at the University of Columbia, New York. He wanted to share his thoughts with climate researchers, and surely expected lots of raised eyebrows and skeptical stares in the process.

“I went into the meeting with Gavin A. Schmidt [a climatologist and the director of NASA GISS with a Ph.D. in applied mathematics], and started talking about aliens. And then Gavin stopped me and said, ‘Wait a second. How do you know we’re the only time there’s been a civilization on our own planet?’’’ Frank tells Popular Mechanics . The question was an “Aha!” moment for Frank, mostly because it allowed him to consider revisiting facts he had taken for granted.

Evidence in Earth’s Geologic Record

Homo sapiens first appeared on Earth about 300,000 years ago . In the unlikely case that such an old industrial civilization had existed, it would predate the species to which we all belong. It was then that Schmidt called the idea the Silurian hypothesis, paying homage to the sophisticated reptilian humanoids awoken by nuclear testing after 400 million years of hibernation in a 1970s episode of the British science fiction television series Doctor Who. The study authors decided to laser in on the time period from 4 million years ago to 400 million years ago.

But going back hundreds of millions of years to find traces of a potential pre- Homo Sapiens civilization is not a piece of cake.

“After a few million years, Earth is pretty much resurfaced. You’re not going to have any statues , buildings , or anything left,” Frank says. Fossil records will be virtually nonexistent as everything will have crumbled to dust. The only evidence would come in the form of chemical imprints. “You’d have to look at each layer of rock, and then try and detect trends—look for changes in things like the carbon or oxygen isotopes, which are tracers of things like carbon dioxide. An industrial civilization would dump lots of carbon dioxide into the atmosphere, just like we do,” Frank says. Plastic or nanoparticles would also be good indicators of an industrial civilization that occurred in time immemorial.

Schmidt and Frank were intrigued by the period of geological history known as the Paleocene-Eocene Thermal Maximum (PETM), because something peculiar happened on our planet during it, 56 million years ago: Earth’s average temperature soared to 15 degrees Fahrenheit above what we have today, and the world became a temperate and iceless place. They investigated the carbon and oxygen isotope ratios from the PETM and indeed saw spikes, but they also saw declines, and all of these over a few hundred thousand years, which is nowhere near the speed at which carbon is currently suffocating the atmosphere. Frank says the PETM’s chemical differences pointed to a long-term climate change .

They also reviewed other “abrupt events” throughout time that are visible in the geologic record, including ocean anoxic events—when an ocean becomes depleted of oxygen—and extinction events . Unsurprisingly, and perhaps slightly disappointingly, they were not indicative of an industrial civilization, either.

Applying Occam’s Razor

“The hypothesis that Earth may have harbored long-extinct industrial civilizations and that existence may be recorded in the geologic record associated with climate change signatures is fascinating, however, even the authors are not sold on the likelihood of it being true,” Stephen Holler, Ph.D., an associate professor of physics at Fordham University in New York City, tells Popular Mechanics .

.css-2l0eat{font-family:UnitedSans,UnitedSans-roboto,UnitedSans-local,Helvetica,Arial,Sans-serif;font-size:1.625rem;line-height:1.2;margin:0rem;padding:0.9rem 1rem 1rem;}@media(max-width: 48rem){.css-2l0eat{font-size:1.75rem;line-height:1;}}@media(min-width: 48rem){.css-2l0eat{font-size:1.875rem;line-height:1;}}@media(min-width: 64rem){.css-2l0eat{font-size:2.25rem;line-height:1;}}.css-2l0eat b,.css-2l0eat strong{font-family:inherit;font-weight:bold;}.css-2l0eat em,.css-2l0eat i{font-style:italic;font-family:inherit;} If an earlier industrial civilization did exist, and its extinction was the result of catastrophic climate change due to industrial activities, then we should heed the warnings.

A great rule of thumb exists in science, and it goes by the name Occam’s razor . In the 14th century, English Franciscan philosopher and theologian William of Ockham proposed that the likeliest solution to a problem is the simplest one. “That is very likely the case here,” says Holler. “We can largely explain the geologic record in terms of natural phenomena, so there is no need to invoke lost civilizations .”

However, if an earlier industrial civilization did exist, and its extinction was the result of catastrophic climate change due to industrial activities, then we should heed the warnings because, as a civilization, we stand at the precipice. “It will be a hard landing when we go over the edge, and we very well might not survive,” says Holler.

Greater Sustainability Leads to Fewer Signs

There is an oxymoron to the Silurian hypothesis: the more sustainable a society is in the way it generates energy and manufactures resources—arguably, the more advanced a society is—the smaller the footprint it will leave on the planet. Yet, this smaller footprint would translate to few markers on the geologic record for that period.

For example, the more plastic or persistent synthetic molecules we produce, the higher the chances future civilizations will find traces of us. (Our society produces 300 million tons of plastic each year worldwide—almost the equivalent of the weight of the entire human population!). Even if we wipe ourselves off the face of Earth with a nuclear catastrophe, long-lived radioactive particles will endure in the soil eons later, signaling we had existed.

“With the Silurian hypothesis, we articulated the kinds of signals that our civilization would leave if we disappear and somebody looks for our civilization 10 or 20 millions of years from now,” Frank says.

But most of all, the experiment demonstrated certain shortcomings in our current scientific apparatus. “In case an earlier species’s industrial activity was particularly short-lived, we would not be able to detect it in ancient sediments with the tools and methods we have now,” Frank explains. “If you want to look for evidence of a previous civilization, you’d have to do studies that nobody’s done and develop novel methods—for example, you’d have to figure out ways to look at the rock record on a much finer timescale.”

Remember, we are talking about millions of years of evolution of complex life and an unsparing Earth that grinds down everything in its wake. And though both Frank and Schmidt don’t really believe an industrial civilization existed before our own, the main takeaway of the Silurian hypothesis, Frank says, is that if you’re not explicitly looking for something, you might not even see it.

Stav Dimitropoulos’s science writing has appeared online or in print for the BBC, Discover, Scientific American, Nature, Science, Runner’s World, The Daily Beast and others. Stav disrupted an athletic and academic career to become a journalist and get to know the world.

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Did another advanced species exist on Earth before humans?

Our Milky Way galaxy contains tens of billions of potentially habitable planets, but we have no idea whether we’re alone. For now Earth is the only world known to harbor life, and among all the living things on our planet we assume Homo sapiens is the only species ever to have developed advanced technology.

But maybe that’s assuming too much.

In a mind-bending new paper entitled “The Silurian Hypothesis” — a reference to an ancient race of brainy reptiles featured in the British science fiction show "Doctor Who" — scientists at NASA’s Goddard Institute for Space Studies and the University of Rochester take a critical look at the scientific evidence that ours is the only advanced civilization ever to have existed on our planet.

“Do we really know we were the first technological species on Earth?” asks Adam Frank, a professor of physics and astronomy at Rochester and a co-author of the paper. “We’ve had an industrial society for only about 300 years, but there’s been complex life on land for nearly 400 million years.”

If humans went extinct today, Frank says, any future civilization that might arise on Earth millions of years hence might find it hard to recognize traces of human civilization. By the same token, if some earlier civilization existed on Earth millions of years ago, we might have trouble finding evidence of it.

In search of lizard people

The discovery of physical artifacts would certainly be the most dramatic evidence of a Silurian-style civilization on Earth, but Frank doubts we’ll ever find anything of the sort.

“Our cities cover less than one percent of the surface,” he says. Any comparable cities from an earlier civilization would be easy for modern-day paleontologists to miss. And no one should count on finding a Jurassic iPhone; it wouldn't last millions of years, Gorilla Glass or no.

Finding fossilized bones is a slightly better bet, but if another advanced species walked the Earth millions of years ago — if they walked — it would be easy to overlook their fossilized skeletons — if they had skeletons. Modern humans have been around for just 100,000 years, a thin sliver of time within the vast and spotty fossil record.

For these reasons, Frank and Gavin Schmidt, a climatologist at Goddard and the paper's co-author, focus on the possibility of finding chemical relics of an ancient terrestrial civilization.

Using human technology as their guide, Schmidt and Frank suggest zeroing in on plastics and other long-lived synthetic molecules as well as radioactive fallout (in case factions of ancient lizard people waged atomic warfare). In our case, technological development has been accompanied by widespread extinctions and rapid environmental changes, so those are red flags as well.

After reviewing several suspiciously abrupt geologic events of the past 380 million years, the researchers conclude that none of them clearly fit a technological profile. Frank calls for more research, such as studying how modern industrial chemicals persist in ocean sediments and then seeing if we can find traces of similar chemicals in the geologic record.

He argues that a deeper understanding of the human environmental footprint will also have practical consequences, helping us recognize better ways to achieve a long-term balance with the planet so we don't end up as tomorrow's forgotten species.

Then again, he’s also a curious guy who's interested in exploring more far-out ideas for finding Silurian-style signatures: “You could try looking on the moon,” he says.

Lunar archaeology

The moon is a favored target of Penn State University astronomer Jason Wright, one of a handful of other researchers now applying serious scientific thinking to the possibility of pre-human technological civilizations.

“Habitable planets like Earth are pretty good at destroying unmaintained things on their surfaces,” Wright says. So he’s been looking at the exotic possibility that such a civilization might have been a spacefaring one. If so, artifacts of their technology, or technosignatures, might be found elsewhere in the solar system.

mach Space aliens could have died out long ago, scientist says

Wright suggests looking for such artifacts not just on the lunar surface, but also on asteroids or buried on Mars — places where such objects could theoretically survive for hundreds of millions or even billions of years.

SpaceX’s recent launch of a Tesla Roadster into space offers an insight into how such a search might go. Several astronomers pointed their telescopes at the car and showed that, even if you had no idea what you were looking at, you’d still quickly pick it out as one weird-looking asteroid.

Finding technosignatures in space is an extreme long shot, but Wright argues that the effort is worthwhile. “There are lots of other reasons to find peculiar structures on Mars and the moon, and to look for weird asteroids,” he says. Such studies might reveal new details about the history and evolution of the solar system, for instance, or about resources that might be useful to future spacefarers.

If the efforts turn up a big black obelisk somewhere, so much the better.

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If a technologically advanced species existed on Earth before us, they may have left a trace.

James Felton

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James is a published author with four pop-history and science books to his name. He specializes in history, strange science, and anything out of the ordinary.

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If an advanced species lived on Earth before us, would we know?

Image Credit: Liu zishan/Shutterstock.com

In Doctor Who,  an alien species called the Silurians exists – technologically-advanced humanoid reptiles who lived long before humans, going into hiding and being basically undiscovered again until everyone's favorite time-traveling alien came along in his phone box. So far, so not science. However, in 2018 two University of Cambridge scientists named their paper – The Silurian hypothesis: would it be possible to detect an industrial civilization in the geological record? – after the fictional species.

Published in the Journal of Astrobiology, the paper does not argue that there was a technologically-advanced species long before humanity, but proposed the interesting hypothetical question of whether it would be possible to find "geological fingerprints" of a bygone civilization that expired millions of years ago.

"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?" they  write in the paper . 

"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."

"This short time period raises the obvious question as to whether this could have happened before."

As well as being an interesting hypothesis to ponder, seeking to answer the question could also help us search for signs of advanced civilizations on exoplanets. As the paper points out, humans have left notable marks on the planet that will certainly last for many years in our (relatively short) time altering the planet's climate and ecosystems. However, that doesn't mean that these changes will be detectable millions of years from now. In fact, the record we leave – for instance, in the sediment – may only be a few centimeters thick. This may be true even if we survive for much longer than our current age.

"The longer human civilization lasts, the larger the signal one would expect in the record," the team write. "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."

The team discuss other markers we could leave for a species millions of years from now (or that may have been left for us). Some will be indistinguishable from naturally occurring phenomena such as the Cretaceous and Jurassic ocean anoxic events – but others would be clear signs that we were here, and we absolutely trashed the place.

"We speculate that some specific tracers that would be unique, specifically persistent synthetic molecules, plastics and (potentially) very long-lived radioactive fallout in the event of nuclear catastrophe," the team write. 

"Absent those markers, the uniqueness of the event may well be seen in the multitude of relatively independent fingerprints as opposed to a coherent set of changes associated with a single geophysical cause."

The team does not give a definitive answer in the paper, but suggest that if there were other ancient advanced species to be found, they would be discovered through exploration of elemental and compositional anomalies in the sediment record.

"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," they conclude. 

"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."

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The silurian hypothesis.

The art and life of Mark di Suvero

In his monthly column,  Conspiracy ,  Rich   Cohen  gets to the bottom of it all. 

When I was eleven, we lived in an English Tudor on Bluff Road in Glencoe, Illinois. One day, three strange men (two young, one old) knocked on the door. Their last name was Frank. They said they’d lived in this house before us, not for weeks but decades. For twenty years, this had been their house. They’d grown up here. Though I knew the house was old, it never occurred to me until then that someone else had lived in these rooms, that even my own room was not entirely my own. The youngest of the men, whose room would become mine, showed me the place on a brick wall hidden by ivy where he’d carved his name. “Bobby Frank, 1972.” It had been there all along. And I never even knew it.

That is the condition of the human race: we have woken to life with no idea how we got here, where that is or what happened before. Nor do we think much about it. Not because we are incurious, but because we do not know how much we don’t know.

What is a conspiracy?

It’s a truth that’s been kept from us. It can be a secret but it can also be the answer to a question we’ve not yet asked.

Modern humans have been around for about 200,000 years, but life has existed on this planet for 3.5 billion. That leaves 3,495,888,000 pre-human years unaccounted for—more than enough time for the rise and fall of not one but several pre-human industrial civilizations. Same screen, different show. Same field, different team. An alien race with alien technology, alien vehicles, alien folklore, and alien fears, beneath the familiar sky. There’d be no evidence of such bygone civilizations, built objects and industry lasting no more than a few hundred thousand years. After a few million, with plate tectonics at work, what is on the surface, including the earth itself, will be at the bottom of the sea and the bottom will have become the mountain peaks. The oldest place on the earth’s surface—a stretch of Israel’s Negev Desert—is just over a million years old, nothing on a geological clock.

The result of this is one of my favorite conspiracy theories, though it’s not a conspiracy in the conventional sense, a conspiracy usually being a secret kept by a nefarious elite. In this case, the secret, which belongs to the earth itself, has been kept from all of humanity, which believes it has done the only real thinking and the only real building on this planet, as it once believed the earth was at the center of the universe.

Called the Silurian Hypothesis, the theory was written in 2018 by Gavin Schmidt, a climate modeler at NASA ’s Goddard Institute, and Adam Frank, an astrophysicist at the University of Rochester. Schmidt had been studying distant planets for hints of climate change, “hyperthermals,” the sort of quick temperature rises that might indicate the moment a civilization industrialized. It would suggest the presence of a species advanced enough to turn on the lights. Such a jump, perhaps resulting from a release of carbon, might be the only evidence that any race, including our own, will leave behind. Not the pyramids, not the skyscrapers, not Styrofoam, not Shakespeare—in the end, we will be known only by a change in the rock that marked the start of the Anthropocene.

It was logical for Schmidt and Frank to turn their attention from the upper to the under, from the cosmos to our own earth. Why look for alien life there when we might find it here, removed not by miles but years. There was indeed a mysterious jump in surface heat; 55 million years ago, global temperatures rose from 9 to 14 degrees Fahrenheit. Called the Paleocene–Eocene Thermal Maximum, it left the same sort of geological evidence that will be left by our current carbon binge. There may have been other jumps, but we wouldn’t know it, as the geologic record only goes back so far. (We live in a compactor, where all things are crushed, recycled, and returned as new.) A meteor could’ve caused the Thermal Maximum, or it could’ve been the eruption of a monster volcano, the sort that presently smolders beneath the Atlantic. Or it could have been caused by the awakening of an ancient civilization, which rose like we rose, then fell as we will. That could be the fate of all advanced species, a rise and fall that flows as naturally as the change of seasons. Such a universe is ironic, continually creating characters whose technology brings on the very end they’re trying to avoid.

When Schmidt and Frank searched, they found a single forerunner to their idea of deep time. It came not from science, but from science fiction. At this level of conjecture, there’s little difference. It was an episode of Dr. Who, in which the time traveler visits an ancient species of advanced, long-extinct lizard people who’d achieved technological mastery 450 million years before modern man. The lizards were called Silurians, hence the Silurian Hypotheses.

It’s not really a new idea. Ancient mystical texts hint at earlier creation, the life that preceded the Garden, prequels to Genesis. These incarnations are not reported in the Bible because they are none of your goddamn business, but the evidence is everywhere. Some students of conspiracy believe there was a time when lizard people shared the earth with modern men, the older race dying as the younger emerged from the forest. The last of the lizards were worshipped as gods; these were the deities of ancient India and Greece. The technology—weapons and machines—created miracles. You can see the lizard kings in carvings from Mesopotamia, the oldest historical records, where humans bow before reptile men. You find them again in the Torah, where they appear as Nephilim, the so-called watchers—“The Nephilim were on the earth in those days and also afterward when the sons of God went to the daughters of humans and had children by them,” according to Genesis, “they were the heroes of old, men of renown”—which no priest, minister, or rabbi can properly explain. Just ask a clergyman and see for yourself. (I asked my rabbi.) There is some weird shit in the Bible.

According to a kabbalah-besotted friend, this world is God’s seventh creation, which explains dinosaur bones and other fossils. “The evidence is everywhere,” he told me one night. “They can say a meteor wiped out the past, but what is a meteor? God.” Some believe there are still Silurians walking the earth, holdovers who share their technology with a hidden elite—possibly Freemasons, possibly Jews. Some pseudoscientists speak of an atomic blast that took place in India 10,000 years ago. It might’ve been a natural phenomenon, or might’ve been the war that wiped out the Silurians or drove them off the earth. A website called Vedic Knowledge reported evidence “of an atomic blast dating back thousands of years. It destroyed most of the buildings and probably a half-million people in Rajasthan, India. One researcher estimates that the bomb used was about the size of the ones dropped on Japan in 1945.”

This ancient catastrophe, which some take as evidence of an ancient nuclear war, shows up in the Mahabharata, a Sanskrit epic, with the appearance of “a single projectile charged with all the power in the Universe … An incandescent column of smoke and flame as bright as 10,000 suns, rose in all its splendor … it was an unknown weapon, an iron thunderbolt, a gigantic messenger of death which reduced to ashes an entire race.”

It’s heartbreaking—the fact that, as we face the nightmare of climate change, some of us have read our own perilous present back into the geological past and have come to see even our apocalypse as unremarkable, something that’s been experienced before and was inevitable from the start. It’s thrilling, too, the idea of a pre-human industrial civilization. It means we don’t know anything: who we are, or where, or even the history of our own home.

Read more of  Rich   Cohen’s Conspiracy column here.

Rich Cohen is the author of  The Last Pirate of New York: A Ghost Ship, a Killer, and the Birth of a Gangster Nation .