The Silurian Hypothesis: How Could We Detect Extremely Ancient Civilizations

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Podcast Transcript

The universe is billions of years old. 

If, in the future, humanity were to explore the galaxy and visit other planets around other stars, we might be visiting places where at one time, an advanced civilization once existed. 

However, if such a civilization existed, it might have been millions of years in the past. If that was the case, how would we even know that it existed? 

Also, what if we ask that same question of Earth rather than of alien worlds?

Learn more about the Silurian Hypothesis on this episode of Everything Everywhere Daily.

This episode is going to be a bit different in that I’m not going to be covering a historical event or explaining the facts about something. 

This episode is going to be about questions. Questions that don’t necessarily have any definitive answers, but questions that can get you thinking, and questions that can help you understand how it is we know what we know about a great many things. 

The topic of this episode began with questions about intelligent life elsewhere in the universe. 

If we were to find intelligent life, how would we even know that we’ve found them? 

If there is some civilization on another distant planet, is there something in the atmosphere of that planet that we could use to determine that there was a civilization there?

Adam Frank, an astrophysicist at the University of Rochester, began wondering if every advanced civilization would do something to change their climate, similar to humans. 

In 2017 he took these questions to Gavin Schmidt, the director of NASA’s Goddard Institute for Space Studies. Schmidt is a climate scientist and climate modeler. 

Schmidt, instead of answering the question directly, turned the question around and asked, “How do we know there hasn’t been some other advanced civilization on Earth?”

At first, that question seems ridiculous. Humans are the only species on Earth that have ever created an advanced civilization. 

However, once you get past the initial incredulity, you are faced with a serious question. How do we know that we aren’t the only advanced civilization to have existed on Earth? 

That turns out to be a surprisingly difficult question to answer. 

Frank and Schmidt posed a thought experiment: If there was an industrial civilization on Earth tens or hundreds of millions of years ago, what evidence would let us know of their existence? 

They dubbed this the Silurian Hypothesis. They got the name from the race of lizard people from the Doctor Who television show. In the Doctor Who universe, the Silurians existed millions of years before humans, and their name was taken from the Silurian geologic period, which was 443 to 419 million years ago. 

Let me make it perfectly clear. The Silurian Hypothesis is just a thought experiment. No one is actually claiming there was an advanced industrial lizard civilization on Earth over 400 million years ago. There is zero evidence of this. 

So don’t go telling your friends about the podcast you listened to that said there were ancient lizard people on Earth. I’m sure such podcasts exist, but this is not one of them. 

The thought experiment simply asks, if such a civilization did exist, how would we know it? 

First of all, could such a civilization have even existed in the first place? It took about 500 million years from the rise of multicellular life to human beings. 

We like to think of evolution as one continuous path getting better and better, ultimately leading to the development of humans. However, that is not at all how it worked. While humans happened, it wasn’t destined to happen. We are not the end result of evolution. Every other species on the planet fits an ecological niche just fine. 

In fact, modern homo sapiens have only existed for about 300,000 years and the earliest things we can point to as human ancestors date back about 2,000,000 years.  Our industrial civilization has only existed for about 300 years. 

When you are looking at the time span that complex life has existed on Earth, there has been plenty of time for other intelligent species to have arisen. 

So, again, while no one is saying lizard people existed, it also isn’t impossible to say they could have existed. 

It is also believed that as early as 350 million years ago, there were already enough fossil fuels on Earth to have supported an industrial civilization. Coal, oil, and gas as energy sources would have been possible. 

Going back to the original question, if such an industrial lizard civilization existed, how would we know? 

Your first reaction is probably to say that we would just find their ancient artifacts as we do with other ancient human civilizations. We would find their fossilized remains as we do with other ancient creatures. 

There is a problem with that. 

Almost nothing gets fossilized. The odds of any living thing becoming a fossil is extremely remote. Various species of dinosaurs roamed the Earth for tens of millions of years. During that time, there would have been not billions but trillions of individual dinosaur creatures that existed.  

From that enormous pool, we only have a few thousand complete dinosaur skeletons. There is a good chance almost no living thing you have ever encountered will remain as a fossil in 100 million years. It is entirely possible, perhaps probable, that entire species are missing from the fossil record because no member of that species was ever fossilized. 

Moreover, even if the lizard people were fossilized, the odds of us finding those fossils are slim. Almost all fossil finds are discovered by accident. Someone digging something when they encounter a bone. 

Maybe you think that even if we don’t find fossils, maybe we’ll discover their artifacts. We find stone structures all the time from ancient people. Gobekli Tepe in Turkey, on which I’ve done a previous episode, is almost 11,000 years old. Even if we can’t find the remains of the actual people who built it, we can still find evidence of what they built.

Maybe there could be some sort of Planet of the Apes type moment when we discover the lizard civilization equivalent of the lizard State of Liberty. 

The problem is there is a huge difference between something surviving for thousands of years and something surviving for tens or hundreds of millions of years. 

The time scales we’d be talking about are geologic. Not only wouldn’t buildings and roads survive, entire mountain chains and continents wouldn’t even survive. 

It is easy to see with your own eyes how quickly concrete structures can decay after just a few decades. Skyscrapers made of steel will eventually corrode and collapse if not maintained, even if it took thousands of years for that to happen. Everything made of wood will eventually rot or burn if given enough time. 

Natural disasters, even if they only happen once a century, would be frequent enough to destroy most structures. Earthquakes, floods, volcanic eruptions, fires, tornadoes, and severe storms would take their toll over long periods of time. 

Structures that are buried would eventually become part of the sedimentary rock itself, given enough time.

So, once again, if an advanced industrial civilization did exist on Earth millions of years ago, how would we possibly know it? 

If there were evidence, it wouldn’t be direct evidence, it would have to be found somewhere in the geologic record. 

Frank and Schmidt attempted to address the problem in a 2019 paper published in the International Journal of Astrobiology titled “The Silurian Hypothesis: Would it be possible to detect an industrial civilization in the geological record?”

The key to finding evidence of a past civilization, or the key to a future civilization learning about us, would have to be found in the rocks themselves.

One of the first things that could possibly be evidence of industrial civilization would be radioactive isotopes. 

Certain isotopes and elements do not appear naturally. A good example is plutonium-244, which has a half-life of 80.8 million years. If there were nuclear weapons used, then a concentration of unnatural isotopes could be evidence of industrial civilization. 

Another more subtle clue could be stable isotopes. In particular, if you are burning fossil fuels, it would have had a biological origin, and the ratio of Carbon-13 to Carbon-12 would be lower than normal. 

Carbon-based particulate matter, as well as CO2, which was bound in rocks, would create an anomalous layer where the Carbon-13 to Carbon-12 changed. This could be a sign that fossil fuels were used that could be found in the geologic record. 

Another stable isotope that could signify the presence of a past industrial civilization would be Nitrogen-15.  The early 20th century saw the development of the Haber-Bosch process, which allowed for the artificial creation of nitrogen fertilizers. 

These fertilizers have skewed ratios of Nitrogen-14 to Nitrogen-15 that you would find in natural biological processes. As nitrogen is spread on farm fields that cover a large amount of area, this is a good candidate for discovery. 

Another more direct form of evidence would be discovering a layer of chemicals in the rock which is not produced naturally. Synthetic chemicals like chloro-fluoro-carbons or Per- and poly-fluoroalkyl substances, also known as forever chemicals, also would give a strong indication of industrial civilization. 

Another possibility would be the discovery of microplastics. There is now developing on Earth a layer of microplastics in ocean sediment. These are the extremely small particles of plastic that are left as plastics break down. They have actually been found all over the planet, so it is possible that a layer of plastics would be evidence of civilization.

However, we have no clue if plastics could, in fact, survive that long. Pressures and temperatures involved with geologic processes could turn them into different molecules, and they could be digested by microbes before that ever gets a chance to happen. 

One problem is that if signs like these were found in the geologic record, they wouldn’t necessarily be evidence of an advanced civilization. There are alternative theories for how many of these signs could appear without civilization.

Volcanism and meteor strikes could change stable isotope levels. If we searched for evidence of ancient civilizations on another planet, there is no guarantee that the planet would have the same amount of radioactive elements that we have on Earth. 

Moreover, things like plastics and synthetic chemicals reflect what humans developed, and they might not represent what some other civilization would have developed. An industrial civilization could have existed but never made any of those things or may only have done so in very limited amounts. 

One of the best places to look for evidence of a past civilization might not be on a planet at all. It would be up in space. If a civilization became space-faring, they would be able to put satellites in high orbit or on neighboring moons. 

These would be able to sit, mostly unmolested, for millions of years. Cosmic rays and solar winds might bombard them, but they could basically stay there indefinitely without being subject to a planet’s geologic processes.  

The entire Silurian Hypothesis thought experiment shows just how tricky of a problem this is. Given enough time, almost all evidence of our civilization will disappear.  100 million years from now, any intelligent civilization on Earth would have a hard time knowing that we existed. 

So, while we have no evidence of an ancient lizard civilization on Earth, and there is no reason to believe they existed, we can’t really prove that they didn’t exist either.