{"id":124934,"date":"2018-12-31T12:00:39","date_gmt":"2018-12-31T12:00:39","guid":{"rendered":"https:\/\/www.transcend.org\/tms\/?p=124934"},"modified":"2019-01-07T12:04:54","modified_gmt":"2019-01-07T12:04:54","slug":"was-there-a-civilization-on-earth-before-humans-a-look-at-the-available-evidence","status":"publish","type":"post","link":"https:\/\/www.transcend.org\/tms\/2018\/12\/was-there-a-civilization-on-earth-before-humans-a-look-at-the-available-evidence\/","title":{"rendered":"Was There a Civilization on Earth Before Humans? A Look at the Available Evidence"},"content":{"rendered":"
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A view shows partial skeleton remains, with skull and bones, from an ancient burial area that was excavated at a building site in Bordeaux, France, December 6, 2016.
REUTERS\/Regis Duvignau – RC1D852EDCB0<\/p><\/div>\n

It only took five minutes for Gavin Schmidt to out-speculate me.<\/p>\n

Schmidt is the director of NASA\u2019s 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\u2019d begun researching global warming from an \u201castrobiological perspective.\u201d That meant asking whether any industrial civilization that rises on any planet will, through their own activity, trigger their own version of a climate shift. I was visiting GISS that day hoping to gain some climate science insights and, perhaps, collaborators. That\u2019s how I ended up in Gavin\u2019s office.<\/p>\n

Just as I was revving up my pitch, Gavin stopped me in my tracks.<\/p>\n

\u201cWait a second,\u201d he said. \u201cHow do you know we\u2019re the only time there\u2019s been a civilization on our own planet?\u201d<\/p>\n

It took me a few seconds to pick my jaw off the floor. I had certainly come into Gavin\u2019s office prepared for eye rolls at the mention of \u201cexo-civilizations.\u201d But the civilizations he was asking about would have existed many millions of years ago. Sitting there, seeing Earth\u2019s vast evolutionary past telescope before my mind\u2019s eye, I felt a kind of temporal vertigo. \u201cYeah,\u201d I stammered, \u201cCould we tell if there\u2019d been an industrial civilization that deep in time?\u201d<\/p>\n

We never got back to aliens. Instead, that first conversation launched a new study we\u2019ve recently published<\/a> in the International Journal of Astrobiology<\/em>. Though neither of us could see it at that moment, Gavin\u2019s penetrating question opened a window not just onto Earth\u2019s past, but also onto our own future.<\/p>\n

We\u2019re used to imagining extinct civilizations in terms of the sunken statues and subterranean ruins. These kinds of artifacts of previous societies are fine if you\u2019re 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.<\/p>\n

When it comes to direct evidence of an industrial civilization\u2014things like cities, factories, and roads\u2014the geologic record doesn\u2019t go back past what\u2019s 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\u2019s \u201cjust\u201d 1.8 million years old\u2014older surfaces are mostly visible in cross section via something like a cliff face or rock cuts. Go back much farther than the Quaternary and everything has been turned over and crushed to dust.<\/p>\n

And, if we\u2019re going back this far, we\u2019re not talking about human civilizations anymore. Homo sapiens<\/em> didn\u2019t 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 Dr. Who<\/em> episode with intelligent reptiles.<\/p>\n

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 only lasted 100,000 years\u2014which would be 500 times longer than our industrial civilization has made it so far.<\/p>\n

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\u2019d leave behind if human civilization collapsed at its current stage of development.<\/p>\n

Now that our industrial civilization has truly gone global, humanity\u2019s 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\u2019re redirecting the planet\u2019s 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\u2019s 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.<\/p>\n

And then there\u2019s all that plastic. Studies have shown increasing amounts of plastic \u201cmarine litter\u201d 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.<\/p>\n

The big question is how long any of these traces of our civilization will last. In our study, we found each had the possibility of making it into future sediments. Ironically, however, the most promising marker of humanity\u2019s presence as an advanced civilization is a by-product of one activity that may threaten it most.<\/p>\n

When we burn fossil fuels, we\u2019re releasing carbon back into the atmosphere that was once part of living tissues. This ancient carbon is depleted in one of that element\u2019s 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 last 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 to the future, might the same \u201csignals\u201d exist right now in rocks just waiting to tell us of civilizations long gone?<\/p>\n

Fifty-six million years ago, Earth passed through the Paleocene-Eocene Thermal Maximum (PETM). During the PETM, the planet\u2019s 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 the Earth\u2019s 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<\/a> for many millennia (or even longer).<\/p>\n

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\u2019s the timescale of these changes that matter. The PETM\u2019s isotope spikes rise and fall over a few hundred thousand years. But what makes the Anthropocene so remarkable in terms of Earth\u2019s history is the speed at which we\u2019re dumping fossil carbon into the atmosphere. There have been geological periods where Earth\u2019s CO2<\/sub> has been as high or higher than today, but never before in the planet\u2019s 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\u2019s bill.<\/p>\n

But there is a conundrum here. If an earlier species\u2019s industrial activity is short-lived, we might not be able to easily see it. The PETM\u2019s spikes mostly show us the Earth\u2019s 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\u2019re not explicitly looking for it, you might not see it. That recognition was, perhaps, the most concrete conclusion of our study.<\/p>\n

It\u2019s not often that you write a paper proposing a hypothesis that you don\u2019t support. Gavin and I don\u2019t believe the Earth once hosted a 50-million-year-old Paleocene civilization. But by asking if we could \u201csee\u201d truly ancient industrial civilizations, we were forced to ask about the generic kinds of impacts any civilization might have on a planet. That\u2019s 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 birth (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\u2014say solar vs. fossil fuels\u2014you can\u2019t power a global civilization without some degree of impact on the planet.<\/p>\n

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\u2019ll leave for future generations.<\/p>\n

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.<\/p>\n

By asking about civilizations lost in deep time, we\u2019re 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\u2019re only now learning to see how rich that potential might be.<\/p>\n

___________________________________________________<\/p>\n

Adam Frank<\/a> is a professor of astrophysics at the University of Rochester. His work has appeared in Scientific American, The New York Times, and NPR. He is the author of Light of the Stars: Alien Worlds and the Fate of the Earth. <\/em><\/p>\n

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