The Silurian Hypothesis is a thought experiment that was proposed in a paper by Frank and Schmidt in 2018. It discusses the ability of modern society to detect any evidence of prior advanced or industrial civilisation before us, even several million years ago. The paper was on the reading list for one of the seminars I took this term titles 'the coevolution if life and Earth'. The theme of the week overall was the Anthropocene which is a proposed geological epoch that we are currently living in where humanity has had enough of an impact on the planet for it to be seen in the rock record millions of years from now. This fits with the Silurian Hypothesis as we would be looking for similar signals of a society in the past to what signs we are leaving in the rocks today. Although there is a geological period called the Silurian, the hypothesis is named after a reptilian humanoid species seen in the TV show Doctor Who, and is nothing to do with the period. They were depicted as an ancient industrial civilisation hence giving name to this thought experiment.
There is an assumption that any intelligent civilisation would have developed some sort of industry, meaning that they have the ability to harness energy on a global scale. On Earth, there have been sufficient coal reserves to power an industrial society since the Carboniferous, so in theory an industrial society run on coal power could have existed anytime since then. The question is what traces would a hypothetical industrial society have left on the planet and how could we detect them today?
The thought experiment has direct relevance to the Drake equation, an argument used to estimate the number of active, communicative extraterrestrial civilisations in the Milky Way. The Drake equation is explained in the diagram below:
The equation assumes that each planet only plays host to one industrial/intelligent civilisation. If the Silurian Hypothesis were to be true then Earth would have played host to two intelligent civilisations, meaning that multiple civilisations may exist on any given planet. This changes the outcome of the equation.
So let's say this intelligent civilisation did exist, how might we detect them? Well we would want to look for markers in the rocks. One piece of evidence could be isotopes, specifically carbon isotopes. As humanity burns fossil fuels we are releasing a lot of Carbon-12 (or light carbon) that was locked up in biomass as fossil fuels. This is gradually changing thew C-13/C-12 ratio of the atmosphere, causing it to decrease as we add more C-12 to the system. It is likely that this change in carbon isotopes would be seen with any civilisation that was burning fossils fuels. However, we see these isotope records showing and increase in light carbon numerous times throughout geological history during episodes in time known as hyperthermals. During hyperthermals the Earth gets hotter due to changes to the global carbon cycle. These changes don't need to be artficial in origin, they can have many causes. One famous example of a hyperthemral is the PETM that occurred about 55 million years ago. It was likely triggered by volcanic activity. We might also see changes to isotope ratios of other elements. Humanity has altered the nitrogen cycle through the Haber-Bosch process used a lot in fertiliser production which artificially fixes nitrogen, so it is possible a similar thing would have been done by a past society.
Another piece of evidence to look out for in the rock record would be changes in sedimentation. Agriculture, deforestation, and canal building, amongst other things, have changed the way sediment gets to and flows through our rivers. Alongside this, a stronger water cycle due to increasing global temperatures causes changes in continental weathering which can be seen in the sediments through isotopes of elements such as strontium and osmium. It would also be worth looking for changes in fauna. Humanity, for example, has given the world a big increase in domesticated chicken, as well as rats, mice, cats, and dogs in places they wouldn't naturally be found in, if these were to survive to be fossils then this would show up in the fossil record. The same may be said for whatever animal the hypothetical Silurians decided to domesticate. If they had diplodoci on leads or something like that then we might see evidence of that in the fossil record.
More definitive evidence, however, would be the discovery of non-naturally occurring synthetic materials or isotopes in the rocks. If the ancient civilisation had discovered the use of nuclear power, like we have done today, then radioisotopes related to nuclear arms of artificial fission might be seen. This is one of the best arguments of evidence for the start of the Anthropocene. The atomic bomb testings back in 1945 would have released these artificial isotopes into the environment that can be detected worldwide. The only two synthetic radioactive isotopes that have long enough half lives for them to stick around in the rocks from an ancient civilisation would be plutonium-244 and curium-247. Attempts have been made to detect these two elements but none with overwhelming success. The other synthetic marker to look for would be plastics. We don't know if an ancient civilisation would have made plastics like we do today, but if they did we would be able to see them in the rocks due to their resistance to natural breakdown. Examples might be microplastics or plastiglomerates, which are aggregates we see on beaches today that form when plastics are brought to high temperatures and mixed with things. Were we to detect plastics, or another synthetic material in ancient rocks then maybe this.could suggest the presence of an ancient civilisation being around to make them.
In order to have definitive evidence of an ancient industrial civilisation - which the authors of the paper make very clear they do not think is very likely - we would need a lot of evidence that could not be attributed to natural processes. We are obviously yet to detect any of this evidence sufficient to prove the existence of previous civilisations, but if they did exist, why might this be the case? Well, the geological record itself has limitations in what it can preserve. The rock record is very incomplete, and if an industrial civilisation lasted for the same length of time that humanity has, or even a couple hundred years longer, it would only be a recorded as a few centimetres of rock, rock that can easily erode or changed. The quality of the geological record varies widely as a function of time and space. Short lived species, such as ourselves or the hypothetical Silurians, may not make to the fossil record at all, and if urbanisation by a past society was on similar levels to present day society (which is only 1% of the Earth's surface) then it is a great chance no technology, buildings, or other large synthetic structures would make it into the geological record either.
So how can we apply this to the Anthropocene? The Silurian Hypothesis is a thought experiment that not only gets us thinking about the past, but also about what impact we might be having on the planet today. It enables us to think about humanity as if we were some future civilisation looking back at ourselves and at evidence of our existence in the rocks. The fact that there might be evidence of us permanently in the Earth system is somewhat frightening. The existence, and definition, of the Anthropocene is a endlessly occurring debate across many academic fields, but it can be largely agreed that we are changing the Earth as a system. Is it likely that a Silurian civilisation did exist? No. I'm not saying definitely no, but it is likely not the case. But will a future civilisation looking back be able to detect us? And is this a worrying thing for the Earth as a system? Probably yes to both. These are very real questions worth thinking about today as we try and guide humanity to a more sustainable future.
There one part of the paper in particular that stood our to me. It explained how the longer a civilisation exists the larger the signal we would expect to see in the rocks. However, the longer a civilisation lasts, the more sustainable its practices would need to have become in order for it to survive, and a more sustainable society means a smaller footprint which then gives less of a signal in the geologic record. There is the chance that humanity becomes sustainable and we start to leave less of a mark. Or a chance that an ancient civilisation was so sustainable and got there so quickly that we can't see it's footprint in the rocks. So who knows? Maybe the Silurians were there after all.
Learn more:
If you're interested in reading the Silurian Hypothesis paper yourself I've linked it here.
If you want to learn more about the Anthropocene I've linked the London Natural History Museum website article about it here.
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