The Silurian Hypothesis:

We are always there talking about extra worlds, aliens, and how civilizations might be sitting safely on extrasolar planets, protected by the enormous distances involved.

And it is as if we take it for granted that nearly five billion years of geological and biological evolution of the Earth could have resulted in only one dominant form of intelligence: that of our species.

We, Homo sapiens, as the apex of evolution, the final, unique, and unrepeatable product of a time course all aimed at bringing us to the dominance of this planet…

But… what if this is not really the case? What if before us … long before … there was a way and time for evolutionary roulette to have birthed another terrestrial civilization?

Would it really be far-fetched to think so?

After all, complex life has existed for more than 500 million years. How sure are we that humans are the first intelligent species to have evolved on this planet?

Might there not have been – I don’t know – a Cambrian civilization of intelligent trilobites, building underwater cities powered by hydrothermal vents? Or big-brained Cretaceous theropods capable of using tools and language?

This is not as absurd as it sounds. On the geological time scale, we inhabited this planet for only an instant, and industrial civilization existed for only a fraction of that instant. And if humans became extinct tomorrow, the relics of our society would not last long.

But why these questions? And who are the Silurians?

Earth is the only planet known to us on which we know with certainty that life has existed and exists. And it is also the only planet known to have given rise to at least one industrial civilization, ours, meaning by this expression a civilization formed by beings capable of using external sources of energy -such as fossil fuels or solar energy- on a global scale.

A civilization with such characteristics has existed on our planet for no more than three centuries, that is, since the Industrial Revolution provided the means to transform local economies into a highly integrated system capable of ensuring the mass production of an ever-increasing number of goods and the ability to alter the environment globally.

As members of the only known industrial civilization, we face alone, at least for now, two boundless immensities: that of space and that of time.

As far as space is concerned, the exploration of the solar system by countless probes has produced so much data that we can conclude with reasonable certainty that, at least from here to Pluto, there is no other industrial civilization (and, for that matter, no sign of life either) outside of Earth.

Whether other civilizations exist beyond the solar system, then, we can only imagine, but our ignorance on the matter is total. Of course, we know that there are a great many planets: in nearly three decades, thousands have been discovered, thanks to the Kepler Space Telescope and other sophisticated methods of investigation. Starting from the ratio of the number of extrasolar planets discovered so far to the total number of stars observed, we can deduce by statistical methods that the potentially habitable planets in the Milky Way alone are probably more than 10 billion: too many even to count.

But there is one big problem: they are all beyond our reach, and by a lot.

Exploring the depths of space in search of another industrial civilization is therefore a daunting task.

But equally difficult is carrying out the same search across the chasms of time, without moving from Earth. We know from the fossil record that complex life forms evolved on our planet starting more than 400 million years ago. We also know that evolution does not proceed linearly, but follows a bumpy path that passes through bursts of life and mass extinctions; we also know that many ‘technical’ solutions to explore and manipulate the environment-such as eyes and wings, for example-have been ‘invented’ several times over millions of years by entirely different species in response to similar evolutionary pressures.

There is then a question to ask, seemingly bizarre, but entirely reasonable: is it possible that some other intelligent species existed on Earth in the distant past that created an industrial civilization somewhat analogous to ours? And from this question another question immediately arises: if another industrial civilization ever existed, what geological traces should we look for to prove its existence?

The second question has guided the work of two scientists, Gavin A. Schmidt, climatologist and director of the Goddard Institute for Space Studies, and Adam Frank, professor of astrophysics at the University of Rochester.

The title of the paper with which the two scholars attempted to provide an answer – The Silurian hypothesis – refers to an episode of the famous British television series “Doctor Who.”  That episode, dating back to 1970, concerned the discovery of the Silurians, an ancient race of technologically advanced reptilian humanoids that allegedly preceded the advent of humans by hundreds of millions of years. According to the plot, these highly civilized saurians thrived for centuries until Earth’s atmosphere entered a period of catastrophic change that forced Homo reptilia to hibernate underground to escape danger.

It should be said at the outset that the two scholars were dealing with entirely theoretical arguments, without even remotely thinking of dressing up as field paleontologists…

Schmidt and Frank have found no geological evidence that could be considered proof of the existence of a past industrial civilization; nor do they believe that such a civilization ever existed. However, it is interesting to follow their reasoning and, above all, to reflect with them on the immensity of past geological time and the rapidity with which the traces of civilization such as ours would be eroded and eventually hidden by the uninterrupted reshaping of the earth’s surface and ocean floor.

The first footprints of Homo sapiens are still fresh on the earth. Our species dates back about 300,000 years and our industrial capabilities to less than 300. We arrived late on a planet that has orbited the Sun for 4.5 billion years. Enough time, some speculate, for earlier civilizations to have developed and then disappeared without a trace.

The bottom line is this: the rocks beneath our feet probably constitute an incomplete encyclopedia of Earth’s history.

Over the past 2.5 million years, the geological record shows evidence of climate change, different soils, and fragments of older hominid cultures, such as Neanderthals and Denisovans. But the further we go back in time, the more sparse the physical remains become. All objects belonging to civilizations more than 4 million years old, the evidence suggests, are probably lost forever due to continuous soil remodeling and abrasion by the elements.

The built structures we have constructed-highways, railroads, bridges, cities, skyscrapers, dams, etc. – tower mightily before our eyes today; the very surface of the planet appears extensively altered by human activity with effects visible from space, as evidenced by satellite photos of cities illuminated at night by electric lights or those of the millions of km² of land, covered by the geometric grid of cultivated fields. But the unraveling of all these works on the scale of geological time will occur in roughly the blink of an eye.

If the human species were to disappear tomorrow, in less than two million years erosion and sedimentation, acting relentlessly on the currently inhabited territories, would erase any trace of the cities and artifacts produced by industrial civilization. Perhaps only Mount Rushmore, with the faces of American presidents carved in rock, and only the great pyramids of Giza would endure for a fraction of a million years.

It follows that the chances of coming across the remnants of an industrial civilization that existed millions of years earlier but lasted only a few thousand years are truly minimal.

Should we then resign ourselves to leaving no traces of our existence and being unable to find more in the past? Not exactly. While it is true that artifacts are deteriorating extremely rapidly and fossils are extremely rare, according to Schmidt and Frank, however, the overall impact of industrial civilization on the planet’s climate and mineralogy remains to be examined.

Humans are indeed changing the environment through man-made substances, such as plastics. Atomic bombs leave radioactive footprints. Burning fossil fuels is also an obvious chemical sign of industrial activity: it alters the ratio of carbon-13 to carbon-12 in the atmosphere.

Not only that… We know that there was a massive release of carbon into the atmosphere about 56 million years ago, which coincided with a dramatic increase in global temperature and the mass extinction of marine life.

Could this be a sign of climate change triggered by an advanced civilization that preceded our own?

Probably not… volcanic activity can also produce a similar signature by warming and unlocking carbon-rich ocean sediments; as can the fall of an asteroid… But how to be certain?

The two authors think it would be worth trying to search marine sediments for evidence of short-lived events, but they also admit that this would be extraordinarily difficult at present.

In conclusion, the study of sedimentary stratifications provides numerous examples of past epochs in Earth’s geological history in which conditions similar to those created by the impact of human industrial civilization on the environment occurred, namely: global increase in temperatures, decrease in the ratio of carbon isotopes, alterations in the nitrogen cycle, anoxia events, deforestation, extinctions, and peaks in the abundance of certain metals.

However, none of these geological traces can be considered conclusive evidence for the existence of an ancient and vanishing industrial civilization. Indeed, there is no way to prove through the study of sediments that past climatic changes, of which there is also clear evidence, were as rapid and sudden as those caused by human industrial activities.

Moreover, there is also to be considered that any culture sophisticated enough to become industrialized may eventually have harnessed entirely natural energies, such as the power of wind, waves, or the sun. And herein lies a paradox: today we know that our species is altering the environment, but we also know that we are working to mitigate the effects of our presence on the planet. This amounts to saying: the more civilization progresses on the path of environmental awareness, the less garbage it will leave around until it becomes completely invisible to the eyes and tools of later civilizations.

Frank and Schmidt-we have already said this-do not believe that such a civilization ever existed, but their article asks a broader question that is also relevant to exoplanet studies. Indeed, only by asking these kinds of questions will we be able to gain the knowledge that in the future (or already in the present, with the James Webb Telescope) will allow us to pick up on the fly signs of the existence, present or past, of some kind of alien civilization.

There is, however, one aspect we have not yet considered in the Silurian hypothesis. What if a remote terrestrial civilization – reptilian or otherwise – had acquired a level of technology that would allow it to fly in space?

Should that be the case, it is more likely that we could find traces of its very distant past, not on Earth, but rather on other nearby celestial bodies: the Moon, and Mars–all worlds on which tectonic and erosion phenomena are much less present.

Remember the black monolith discovered on the Moon inside the Tycho crater?

That’s it-something like that.

Again, that was literature, but let’s remember that Arthur Clarke, the author of 2001: A Space Odyssey, was someone who never missed a prediction…