Was there a civilization on Earth before humans?

A look at the available tests

Gavin Schmidt only needed five minutes to overcome my speculation.

Schmidt is the director of NASA’s Goddard Institute for Space Studies (also known as GISS), a world-class climate science center. One day last year, I came to GISS with a crazy proposal. In my work as an astrophysicist, I had begun to investigate global warming from an “astrobiological perspective.” That meant wondering if any industrial civilization that arose on any planet would trigger, through its own activity, its own version of climate change. That day I visited the GISS hoping to get some ideas on climate science and maybe collaborators. That’s how I ended up in Gavin’s office.

Just as I was speeding up my speech, Gavin stopped me in my tracks.

“Wait a second,” he said. “How do you know that we are the only time there has been a civilization on our own planet?”

It took me a few seconds to lift my jaw off the ground. Certainly she had come to Gavin’s office prepared to roll her eyes at the mention of “exocivilizations.” But the civilizations she was asking about would have existed many millions of years ago. Sitting there, viewing Earth’s vast evolutionary past through the telescope before my mind’s eye, I felt a kind of temporal vertigo. Yes, I stuttered. “Could we know if there was an industrial civilization that deep in time?”

We never talked about aliens again. Instead, that first conversation launched a new study that we recently published in the International Journal of Astrobiology . Though neither of us could see it at the time, Gavin’s penetrating question opened a window not only into Earth’s past, but into our own future as well.

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

When it comes to direct evidence of an industrial civilization – things like cities, factories and highways – the geological record goes back no further than what is called the Quaternary period, 2.6 million years ago. For example, the oldest large-scale surface stretch is found in the Negev desert. It is “only” 1.8 million years old; the oldest surfaces are mostly visible in cross section through something like a cliff or rock cuts. If we go back beyond the Quaternary, everything has turned to dust.

And, if we go back that far, we are no longer talking about human civilizations. Homo sapiens did not make their appearance on the planet until about 300,000 years 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 sentient 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 primitive mammal briefly rose to civilization building during the Paleocene epoch, about 60 million years ago. There are fossils, of course. But the fraction of life that fossilizes is always minuscule and varies greatly with time and habitat. Therefore, it would be easy to miss an industrial civilization lasting only 100,000 years, which would be 500 times longer than our industrial civilization has lasted until now.

Given that all direct evidence would have disappeared after many millions of years, what kind of evidence could exist then? The best way to answer this question is to find out what evidence we would leave behind if human civilization collapsed at its current stage of development.

Now that our industrial civilization has become truly global, the collective activity of humanity is leaving a series of traces that may be detected by scientists in 100 million years. Extensive use of fertilizers, for example, keeps 7 billion people fed, but it also means we are redirecting the planet’s nitrogen flows towards food production. Researchers of the future should see this in the characteristics of nitrogen that appear in the sediments of our era. The same goes for our relentless demand for rare earth elements used in electronics. Because of us, there are many more such atoms roaming the surface of the planet than there would be. They could also show up in future sediments.

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

The big question is how long these traces of our civilization will last. In our study, we found that each of them had a chance of reaching future sediments. Ironically, however, the most promising marker of humanity’s presence as an advanced civilization is a byproduct of one of its most threatening activities.

When we burn fossil fuels, we put carbon back into the atmosphere that was once part of living tissue. This ancient carbon is depleted into one of three natural varieties of that element, or isotopes. The more fossil fuels we burn, the more the balance of these carbon isotopes changes. Atmospheric scientists call this change the Suess effect, and the change in carbon isotope ratios due to fossil fuel use is easy to see over the last century. Temperature increases also leave isotopic signals. These changes should be apparent to any future scientist chemically analyzing exposed rock layers from our era. Along with these spikes, this Anthropocene layer could also contain brief spikes of nitrogen, plastic nanoparticles and even synthetic steroids. So if these are the footprints our civilization is destined to leave for the future, could the same “signs” exist right now in the rocks waiting to tell us about civilizations long gone?

56 million years ago, the Earth went through the Paleocene-Eocene Thermal Maximum (PETM). During the PETM, the planet’s average temperature rose up to 15 degrees Fahrenheit above what we experience today. It was a nearly ice-free world, as typical summer temperatures at the poles reached nearly 70 degrees Fahrenheit. By looking at the PETM isotope record, scientists see the ratios of carbon and oxygen isotopes spike 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. Among them is an event that took place a few million years after the PETM, called the Eocene Layers of Mysterious Origin,

Are these events indications of earlier non-human industrial civilizations? Almost certainly not. Although there is evidence that the PETM may have been driven by a massive release of buried fossil carbon into the air, it is the timescale of these changes that matters. The isotopic peaks of the PETM rise and fall over a few hundred thousand years. But what makes the Anthropocene so remarkable in terms of Earth’s history is the rate at which we’re dumping fossil carbon into the atmosphere. There have been geological periods when the Earth’s CO2 has been as high or higher than it is today, but never before in the billions of years of the planet’s history has so much buried carbon been dumped into the atmosphere so quickly.

But here is a puzzle. If the industrial activity of a former species is short-lived, we may not be able to see it easily. The PETM spikes mostly show us the Earth’s time scales for responding to what caused it, not necessarily the time scale of the cause. So novel, dedicated detection methods might be needed to find evidence of a truly short-lived event in ancient sediments. In other words, if you don’t explicitly look for it, you may not see it. This recognition was, perhaps, the most concrete conclusion of our study.

It is not often that one writes an article proposing a hypothesis that is not supported. Gavin and I do not believe that the Earth was home to a 50 million year old Paleocene civilization. But in asking whether we could “see” really ancient industrial civilizations, we were forced to ask about the generic types of impacts any civilization could have on a planet. That is exactly what the astrobiological perspective on climate change is all about. Building a civilization means harvesting energy from the planet to do work (ie, the work of building the civilization). Once civilization reaches truly planetary scales, there has to be some feedback in the coupled planetary systems that gave it life (air, water, rock). This will be especially true for young civilizations like ours that are still climbing the ladder of technological capability. In other words, there is no free lunch. Although some energy sources will have less of an impact – for example, solar power versus fossil fuels – you can’t power a global civilization without some degree of impact on the planet.

Once you realize, through climate change, the need to find lower impact energy sources, the less impact you will leave behind. So the more sustainable your civilization is, the less of a mark you will leave for future generations.

In addition, our work also opened up the speculative possibility that some planets may have cycles of fossil fuel-driven build-up and collapse of civilization. If a civilization uses fossil fuels, the climate change they trigger can lead to a huge drop in oxygen levels in the ocean. These low oxygen levels (called ocean anoxia) help trigger the conditions necessary to make fossil fuels like oil and coal in the first place. In this way, a civilization and its disappearance could sow the seed of new civilizations in the future.

In asking about civilizations lost in deep time, we are also asking about the possibility of universal rules guiding the evolution of all biospheres to their full creative potential, including the emergence of civilizations. Even without the leading paleocenes, we are only now learning to see how rich that potential can be.

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