“If humans where to become extinct, almost all traces of us ever existing would disappear within a few thousand years…. tens of thousands?
If it was a meteor or cataclysmic event of such magnitude that almost all life on Earth was rendered extinct… almost all trace would be wiped out over night.
How do we possibly know that intelligent, technologically advanced, ‘industrialised’ civilisations haven’t arose on Earth prior to homo sapiens?
Our fossil records of Earth are a tiny fraction of the life that has existed here. We know major extinctions events have happened in the past (and will happen again), meteor impacts, super volcanoes… the Silurian hypothesis suggests previous industrialised civilisations may have even wiped themselves out through environmental damage… much like we’re in the process of doing now! 😀
The Silurian hypothesis! I came up with pretty much the same hypothesis in 2012, when someone told me about the controversial ‘Dashka stone’.
(some claim to be possibly 120 million years old)
“How would we possibly know if another species had given rise to high intelligence or technological advancement MILLIONS or even TENS OF MILLIONS years ago?”… “What evidence would there be?”
I explained this theory to an American Ayahuasca ‘buddy’ in 2015, and all he kept saying was “Not the mama!… Not the mama!”… yeah I get it now! The TV show ‘Dinosaurs’ (we had it broadcast over in the UK, but it was shit so no-one watched it.. no -one I knew anyway). Designated to Sunday afternoon Channel 4… a TV show about ‘intelligent’ dinosaurs (American intelligence), called ‘Dinosaurs’… in which the baby was called ‘baby’.
The Silurian hypothesis, name taken from the intelligent British TV show Doctor Who! ;D
The Silurian hypothesis is a thought experiment which assesses modern science’s ability to detect evidence of a prior advanced civilization, perhaps several millions years ago. In a 2018 paper, Adam Frank, an astrophysicist at the University of Rochester, and Gavin Schmidt, director of the NASA Goddard Institute, imagined an advanced civilization before humans and pondered whether it would “be possible to detect an industrial civilization in the geological record?”. They wrote, “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.”  The term ‘silurian hypothesis’ was inspired by a 1970s Doctor Who episode with intelligent reptiles.
According to Frank and Schmidt, since fossilization is relatively rare and little of Earth’s exposed surface is from before the quaternary time period, the chances of finding direct evidence of such a civilization, such as technological artifacts, is small. After a great time span the researchers concluded we would be more likely to find indirect evidence such as anomalies in the chemical composition or isotope ratios of sediments. Objects which could indicate possible evidence of past civilizations include plastics and nuclear wastes residues buried deep underground or on the ocean floor.
Prior civilizations could have gone to space and left artifacts on other celestial bodies, such as the Moon and Mars. Evidence for artifacts on these two worlds would be easier to find than on Earth, where erosion and tectonic activity would erase much of it.
Frank first approached Schmidt to discuss how to detect alien civilizations via their potential impact upon climate through the study of ice cores and tree rings. They both realized that the hypothesis could be expanded and applied to Earth and humanity due to the fact that humans have been in their current form for the past 300,000 years and have had sophisticated technology for only the last few centuries.
Reptilian menaces called Silurians evolved on Earth before humankind — at least in the “Doctor Who” rendition of the universe. But science fiction aside, how would we know if some advanced civilization existed on our home planet millions of years before brainy humans showed up?
This is a serious question, and serious scientists are speculating about what traces these potential predecessors might have left behind. And they’re calling this possibility the Silurian hypothesis.
When it comes to the hunt for advanced extraterrestrial civilizations that might exist across the cosmos, one must reckon with the knowledge that the universe is about 13.8 billion years old. In contrast, complex life has existed on Earth’s surface for only about 400 million years, and humans have only developed industrial civilizations in the last 300 years. This raises the possibility that industrial civilizations might have been around long before human ones ever existed — not just around other stars, but even on Earth itself. [Greetings, Earthlings! 8 Ways Aliens Could Contact Us]
“Now, I don’t believe an industrial civilization existed on Earth before our own — I don’t think there was a dinosaur civilization or a giant tree sloth civilization,” said study co-author Adam Frank, an astrophysicist at the University of Rochester in New York. “But the question of what one would look like if it did [exist] is important. How do you know there hasn’t been one? The whole point of science is to ask a question and see where it leads. That’s the essence of what makes science so exciting.”
Artifacts of human or other industrial civilizations are unlikely to be found on a planet’s surface after about 4 million years, said Frank and study co-author Gavin Schmidt, director of NASA’s Goddard Institute for Space Studies in New York. For instance, they noted that urban areas currently take up less than 1 percent of Earth’s surface, and that complex items, even from early human technology, are very rarely found. A machine as complex as the Antikythera mechanism — which is considered to be the world’s first computer from ancient Greece — remained unknown until the development of elaborate clocks in RenaissanceEurope.
One may also find it difficult to unearth fossils of any beings who might have lived in industrial civilizations, the scientists added. The fraction of life that gets fossilized is always extremely small: Of all the many dinosaurs that ever lived, for example, only a few thousand nearly complete fossil specimens of the “terrible lizards” have been discovered. Given that the oldest known fossils of Homo sapiens are only about 300,000 years old, there is no certainty that our species might even appear in the fossil record in the long run, they added. [In Images: The Oldest Fossils on Earth]
Instead, the researchers suggested looking for more subtle evidence of industrial civilizations in the geological records of Earth or other planets. The scientists focused on looking at the signs of civilization that humans might create during the Anthropocene, the geological age characterized by humans’ influence on the planet.
“After a few million years, any physical reminder of your civilization may be gone, so you have to look for sedimentary anomalies, things like different chemical balances that just look wacky,” Frank said.
One sign of industrial civilization may have to do with isotopes of elements such as carbon. (Isotopes of an element vary in how many neutrons they possess in their atomic nuclei — for example, carbon-12 has six neutrons, while carbon-13 has seven.)
For instance, humans living in industrial civilizations have burned an extraordinary amount of fossil fuels, releasing more than 500 billion tons of carbon from coal, oil and natural gas into the atmosphere. Fossil fuels ultimately derive from plant life, which preferentially absorb more of the lighter isotope carbon-12 than the heavier isotope carbon-13. When fossil fuels get burned, they alter the ratio of carbon-12 to carbon-13 normally found in the atmosphere, ocean and soils — an effect that could later be detected in sediments as hints of an industrial civilization.
In addition, human industrial civilizations have also discovered ways to artificially “fix nitrogen” — that is, to break the powerful chemical bonds that hold nitrogen atoms together in pairs in the atmosphere, using the resulting single nitrogen atoms to create biologically useful molecules. The large-scale application of nitrogenous fertilizers generated via nitrogen fixing is already detectable in sediments remote from civilization, the scientists noted.
The Anthropocene is also triggering a mass extinction of a wide variety of species that’s likely visible in the fossil record. Human industrial activity may also prove to be visible in the geological record in the form of long-lived synthetic molecules from plastics and other products, or radioactive fallout from nuclear weapons.
One wild idea the Silurian hypothesis raises is that the end of one civilization could sow the seeds for another. Industrial civilizations may trigger dead zones in oceans, causing organic material (from the corpses of organisms in the zones) to get buried that could, down the line, become fossil fuels that could support a new industrial civilization. “You could end up seeing these cycles in the geological record,” Frank said.
All in all, thinking about the impact that a previous civilization has on Earth “could help us think about what effects one might see on other planets, or about what is happening now on Earth,” Frank said.
Schmidt and Frank detailed their findings online April 10 in a study to be published in a forthcoming issue of the journal International Journal of Astrobiology.
A provocative new paper suggests some ways to find out
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 5 to 8 degrees Celsius (about 9 to 14 degrees Fahrenheit). Schmidt has pondered the PETM for his entire career, and it was on his mind one day in his office last year when the University of Rochester astrophysicist Adam Frank paid him a visit.
Frank was there 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 has yielded a provocative paper on the possibility 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 the paper put it: “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 will likely 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 for the Silurian hypothesis stems chiefly from the vast incompleteness of the geologic record, which only gets sparser the farther back in time you go.
Today, less than 1 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 Jan Zalasiewicz, a geologist at 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 a
go should be hungry for energy. Which means any ancient industrial society would develop the capacity to widely exploit fossil fuels as well as other power sources, just as we have today. “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 grows, the more obvious its presence should become in the geologic record. Consider our own industrial age, which has only existed for about 300 years out of a multimillion-year history of humanity. Now compare that minuscule slice of time with the half-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 [civilization like ours] has happened multiple times, but if they each only last 300 years, no one would ever see it,” 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 a half-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 non-naturally occurring, stable synthetic molecules such as steroids and many plastics, along with well-known pollutants including PCBs—toxic polychlorinated biphenyls from electrical devices—and CFCs—ozone-eating chlorofluorocarbons 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 last year 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.”