Hey, welcome to Stuff to Blow Your Mind. My name is Robert Lamb and I'm Joe McCormick, and it's Saturday. Time to dig into the vault for an older episode of the show. This is our episode on the Silurian hypothesis, originally published January. I hope you enjoy Welcome to Stuff to Blow Your Mind production of My Heart Radio. Hey, welcome to Stuff to Blow Your Mind. My name is Robert Lamb and I'm Joe McCormick, and today we're going to be talking about a subject that I've actually had on the radar for a while. This is something that was making the rounds on science blogs a few years back, and it has been suggested by a number of different listeners. So I'm glad we're finally coming around to it. I think I had some hesitation for a while that I want to briefly explain right at the beginning here. But today we're gonna be talking about an idea known as the Silurian hypothesis. Uh. And just to give you a little bit of background knowledge, we spent several minutes before recording today trying to look up how they pronounced silurian on Doctor Who because I was like, sure they might use some kind of British English variation where they say Silurian, but but a last we could not ever get the doctor to say it right. I watched I think, an entire scene where one of the more recent doctors was chatting with a Silurian or Silurian out out of your Will, and they they it was like they were trying not to say it, like if they said it, one would would pop up and and crawl out of the screen or something. Uh they used they referred to another alien race. There wasn't even in the scene, and I don't even think was part of that episode. And then in all these other terms, but yeah, they were just trying to mess with me. So this is a topic that I have been interested in covering for quite a while. It's it's been a few years now, but Rob, when you suggested it, I realized that I'd always been hesitating and not wanting to quite go ahead with it. And I think I realized the reason for that, which is that when I saw people mentioning this paper on the internet, it was clear to me that a lot of them were getting exactly the wrong takeaway from it, Like they were latching onto a very shallow understanding of the concept and and running off in a in a very different direction than the authors intended. Not only a different direction than they intend, but a direction they specifically say do not go in, and specifically say that they are not trying to to make yeah. Uh So, to to clarify what we're talking about here, the Silurian hypothesis paper begins with a fascinating question, in the words of the author's quote, if an industrial civilization had existed on Earth many millions of years prior to our own era, what traces would it have left and would they be detectable today? That that's the question at the heart of this paper. And obviously this is a tantalizing premise. You know, it sets your mind racing with images of impossibly weird organisms, you know, like land dwelling octopi and stuff in the in their own weird cities, and and what kind of technology would they have? Things that are as alien as anything you could imagine on another planet, except they would have all been from here, native to planet Earth, dating back millions of years into prehistory. But while this is a really attractive imaginative exercise, I think the first order of business when talking about this subject is to be clear that the Silurian hypothesis paper is about coming up with a framework for detecting physical traces of industrial civilizations and understanding how long those traces last. So it's about trying to say what are the right questions to ask when you're when you're looking at a planet and saying, how could we tell if there had been a civilization on this planet a long time ago. It is not a paper arguing that there was in fact a lost civilization deep in Earth's past. So it's not evidence for lizardman, ancient aliens, Graham Hancock, Junk Atlantis, or any of that stuff. But I would say in its true form, it is a really interesting question. Yeah, and at hard this episode is is not going to be about scientific evidence for lizardman civilizations in the Hollow Earth. So if you're looking for that, this is this is not the episode for you. But yeah, what I love about it is that it takes this sort of fantastic idea and then examines it reasonably, and that examination illuminates some very interesting geologic, climatic and astrophysical considerations. So, you know, setting aside pseudoscience and pseudo archaeology here. But on the other hand, I think if you if you're looking for some sci fi fund this topic in this episode, well also still engage you. Um. But it is interesting how from a certain perspective you can imagine people being drawn into it by just sort of this sci fi idea, this idea that does lean lend itself well to sort of conspiracy theorist mindsets, and then realizing actually, this paper is about geology and uh and uh and in the last year of our planet and also uh, I guess kind of you know, in many cases, kind of a downbeat message about the lasting impact of human technology on our planet. And on the other hand, too, how forgettable we may be from the standpoint of geological history. Yeah, so if you're on board for all of that, you've come to the right place. So real quick, I do want to just discuss the Doctor Who reference here, since since uh the author's Schmidt and Frank took the name for the hypothesis from the Doctor Who species the Silurians who first popped in the nineteen seventies series Doctor Who, and these Silarians, I think now you've got me saying Silarians. Oh, I'm sorry, I'm probably well. It gets even worse because so they take the name of the hypothesis from this Doctor Who series where these creatures show up. But then they say explicitly in the paper, you know, the range we would really be looking at would actually be after the Silurian geologic periods. Silurian period is something that's like a roughly twenty million year period. That's more than four hundred million years ago. I don't know, it's like four hundred and forty something to four hundred and twenty something, I think roughly. But if you were seriously looking for evidence of lost civilizations in Earth's ancient past, you'd probably be looking for things like after about four hundred million years ago, coming you know, forward in time from the Devonian period, when you could have the reasonable biological basis for land dwelling animals that might have evolved complex technological intelligence. Yes, uh, but at any rate. In Doctor Who, especially in that original nineteen seventy uh appearance, the Silurians are these kind of lizard men um. They factor into this plot with the Third Doctor played by John Perkway who lived nineteen nine six, and then they subsequently pop up again with the fifth Doctor played by Peter Davison and the eleventh Doctor played by Matt Smith, and then more recently the third teenth Doctor played by Jody Whittaker. So this is just the TV show. I can't speak to the various books and audio dramas that have come out, and their look has changed throughout the film. Um. You know, they are, in essence, this cold blooded, prehistoric reptile like species with significant technological advancement that they I think they end up entering various states of suspended animation to avoid uh you know, major changes on Earth, changes to the climate, etcetera. Um, and then they re emerge and encounter the Doctor. Um. So yeah, they're They're one of the many interesting alien and otherworldly species that pop up. Uh. Though I guess with the Silurians one of the key things is that they're they're not pure aliens. They're they're sort of the originals. There are their original terrans, original earthlings, uh, that are then encountered by these evolved apes that come much later. I mean to to them, we are the aliens, yeah, right, were like these weird future creatures. Uh the image you would tell And I gotta be honest, I'm not a hoovoid, so I don't know the lore. But the picture you're showing me of the Silurians, they look like they look like if the world was all creature from the Black Lagoon and there was a leather face of the creature from the Black Lagoon civilization. Yeah. I mean, they're definitely Doctor Who creatures of this era, which which I tend to love these costumes. I know they were working with with with with budget limitations here, but yeah, the the aliens and robot of this era really really called me. Now. You said they look kind of fish like. Bear in mind, and I'm sure some Doctor Who listeners Doctor Who viewers will will will chime in here. But I believe they are related to another species that pops up on the show that live in the water. I think they're like the sea devils or something. Um, but these guys are not aquatic in nature. I think I got that right. The seed of Oh wait, this is that like a bunch of intelligent euryptorids or something something like that. Yeah, alright, so this is two thousand eighteen paper, the Silurian hypothesis. Would it be possible to detect an industrial civilization in the Geological Record. This was published in the journal International Journal of Astrobiology. UH. It draws its name from that Doctor Who episode, and the authors here that they point out that they may be the first to seriously consider whether a technologically advanced civilization could have evolved prior to Homo sapiens on Earth, though the authors due stress that this is a to the best of the knowledge situation, So you know, it's entirely possible somebody was batting around the idea of previously, but this may be the first, and certainly this was this one really made a splash when it came out. Okay, So the two authors here would be Adam Frank and Gavin A. Schmidt right and Frank is a is a physicist and astronomer, and Schmidt is a climate scientist right right. Schmidt is a climatologist, climate model and director of the NASA Goddard Institute for Space Studies in New York and co founder of the award winning climate science blog Real Climate. Frank is a physicist, astronomer, and writer whose work has appeared in such publications as The New York Times and NPR. And I believe we've actually referenced his work on the podcast before. Uh. He also has a book. He has a few books, including The Constant Fire, The End of the Beginning, and Light of the Stars. Those are all nonfiction science books, of course. So to be clear, we're talking about two very legitimate scientists and science communicators, not you know, not a couple of of quacks who were staring into the hollow Earth or anything. So the authors here begin with a very reasonable consideration of the search for intelligent life elsewhere in the universe. Uh. And then this is something we've we've touched on the show plenty of times before. Ours is the only model of life, but we generally consider technological advancement to be a hallmark of intelligent life. And more to the point, something we can search for signs of, uh, concerning other worlds and other star systems. Uh. You know, and anytime you can you can figure out how to look for signs of of advanced and expansive energy harvesting or consumption. That might be a way for us to tell if there's something else out there that is significantly advanced, right, and it also might be a simple prerequisite for contact because uh, they're talking about so that They started off by looking at this as an astrobiology question. You know, you're you're looking for signs of life elsewhere in the universe, and of course, the the search for intelligent life in the universe, in practical terms, what's accessible to us, really boils down to the search for life capable of Harvard harnessing radio technology within our galaxy. You know, you could probably find maybe chemical biosignatures in in the atmospheres of exoplanets that would give you an indication that there's some kind of life there, maybe bacterial in nature or whatever. But if you're looking for intelligent life, you're you're probably talking about radio of some kind, right, And so the kinds of civilizations that develop radio communication technology would fall under the classification of industrial civilizations. And these are what author the authors define as civilizations that have the ability to harness energy on a global scale. And they bring up how this actually feeds into one of the the recurring characters in in the astrobiology literature, the Drake equation. That's right, Yeah, they bust out the Drake equation, and of course consider how how some of the takeaways relate to Earth, especially the notion that over the course of a planet's existence, multiple industrial civilizations can fear radically arise over the span of time that life exists there, And then we have to factor in questions over you know, how many times life itself may have evolved or started out on Earth before our last universal common ancestors got going, the possibility of a shadow biosphere, and the idea that species like dolphins may suggest independent evolutions of intelligence on Earth. Um, you know, so we're left with this idea that, yeah, theoretically, given the footprint of life on Earth, you could have had multiple intelligences uh evolved and arise during that time period. Because that's certainly what the Drake equation UH seems to allow for concerning other worlds. Well, yeah, and I you know, so I kind of love the Drake equation. You know, it's a famous tool that I really enjoy thinking about because because it does the job of taking a question that seems like we could not possibly answer it. The question is how many active technological civilizations are there in the Milky Way galaxy? And you know, if you're being honest with yourself, the correct answer to that is how the hell should I know? Like, there's no way to answer that question at all. But what the Drake equation does is break that unanswerable question down into a number of other questions that you then multiplied together to get an estimated number. And many of those smaller questions themselves could perhaps be answered, and in fact, some of them have been answered since the Drake equation was first formulated. Uh So, So it decomposes an unsolvable problem of are there aliens out there? And if so, how many, into a series of smaller problems, at least some of which are solvable, maybe all of which are. You know, you could come up with some kind of reasonable guess about. And so the classic formulation of the Drake equation is to get your number of civilizations in in the Milky way, you would multiply a bunch of different terms together. So one is the rate of average the average rate of star formation. You know, how much do you get stars times the fraction of stars that have planets times the average number of planets per star times the fraction of planets that develop life times the portion of those life systems that gain intelligence, times the portion of those intelligent life systems that develop technological means to communicate times And then here's a really interesting term, quote the length of time l over which such civilizations released detectable signals. And it's this very last term that I think very often gets overlooked by people who are thinking about, you know, are there aliens out there, and how could we know? I think we often tend to assume that, well, once there are aliens with technological means to communicate, that's just like a you know, progress only extends from their civilizations just continue to get bigger and their capabilities expand, and they spread out from there. But I don't know that there could be severe limitations on the length of a radio receptive or radio broadcasting civilist nation. Maybe they only exist for a few hundred years. Because one thing we know is that our technological civilization is just a tiny blip on the history of planet Earth, even a tiny blip on the history of life on planet Earth. Earth is four point five billion years old. There's been life on Earth for most of that time. Uh. The authors here estimate that there has been complex life on Earth's land surface for only about four hundred million years, So that's only a fraction of the entire history of Earth. But that but four hundred million years is still a gargantuan amount of time compared to the length of human civilization. They say, industrial civilization, you know, by their metric, has probably existed for only about three hundred years. This is since roughly the beginning of mass production methods for for things. And so if humans were wiped out by a global mass extinction of some kind in the near future, our industrial civilization would just be this tiny little splinter, this blip of three hundred years on a history of a currently four point five billion year old planet. Yeah, and so from there we get into the question, Okay, if if you have an industrial civilization like this it is just a blip, would we be able to see it? And if we could see it, what would we look for? And this is you know, this is pretty much the meat of the paper here analyzing this sort of question, which which is great because it again it gets into sort of uh, you know, sci fi friendly concepts. It's a useful in considering the evolution of life and the existence of intelligent life on other worlds. And it also shines a light on what we're doing now and where we are and and I think also, uh, you know, illustrates nicely illustrates this idea that um that that the technology is not just this um that this this this ramp to Star Trek, you know, or this ramp to the culture or any of our more optimistic sci fi dreams like there are there are severe challenges. Uh. And of course there's there's always the risk of extinction. That's exactly right. And one thing that's funny is we don't know whether the rise of technological civilization should generally be understood as, on average, a linear process where it just sort of goes in one direction and keeps going in that direction, or whether it should be understood as, on average a cyclical process where you get a rise in technological civilization and then it disappears for some reason. You can imagine what some of those reasons might be, um, and uh, and then maybe rises again out of the out of the same biosphere. I mean. Either one I think is is a perfectly plausible model to entertain is like what usually happens in the universe. Uh. And we just don't have the Uh, we don't have the evidence to really have an opinion on that. Yeah, I mean a lot of it just comes back to the fact that again we are the only model of intelligent life and certainly technologically advanced tech, um, intelligent life that we have to look at. So we have nothing to compare us to. Yeah, and we don't know what's going to happen to us in the long run, right, So, uh, well, let's get into the I guess the sort of the first part of the paper. And I do want to drive home that if you want to just go right to the paper yourself and dive in. Um, you just do a search for the title and you can find it hosted on NASA. They have a NASA has a has a PDF of this that's very easily accessible. Uh. You can also read it in full on the Cambridge University Press website, which I think is the press behind the journal, the International Journal Journal of Vester Biology. And yeah, so it's all on their end with the references hyperlinked and all that, which is nice. Yes, yeah, absolutely so if you hit a paywall, don't don't give up. It's out there. Um. And I believe Adam Frank also wrote a piece for What the Atlantic where he nicely summarizes some of the ideas here. Oh yeah, and he also tells a funny story about how they arrived at writing the paper because I think he says, uh, he showed up in Gevin Schmidt's office to talk about UM, to talk more about astrobiology, like Drake equation type questions, and he's like, okay, so we we we know we've got one industrial civilization on Earth. And then Schmidt responded by saying, how do we know we're the only one? I think just hitting that early like wall, They're like wow, And then that turned into the paper, Yeah, and it's it's it's quite a paper. Thank So let's see, let's get into the first part of it, which I think you can loosely think of it's just sort of a look at the limits of our vision. So they point out that for the last two point five million years, uh, there's widespread physical evidence of things like climate change, soil horizons, this is where one layer of soil differs from Belower above, speaking to changes recorded in the soil, as well as archaeological evidence of non Homo sapiens cultures such as the Neanderthals. And this two point five million year period is known as the Quaternary. Now, going back before the Quaternary again more than two point five million years ago, the land evidence is harder to come by. You have to depend on drilling, mining, and occasional exposed sections of the earth. Even in the ocean sentiment evidence apparently only goes back to around one seventy million years ago. Yeah, And I think for me this was actually one of the most interesting parts of the paper because I would say, if you just go by standard intuition, a person might think, uh, you know, if there had been a civilization on Earth, you know, a two million years ago or something like that. Uh, would wouldn't that just be completely obvious, Like we'd see evidence of it all around us. Would there be ruins and all that, you know, their their stone hinges, their skyscrapers and everything like that. Actually it's not. It might not be as obvious as you might think. In fact, the evidence of it could be rather scarce. And this runs counter to our side five imaginings, right, because when you encounter elder civilizations and in other works like there's usually some sort of a ruin or a vault or some sort of mysterious monolith or something like the idea that the elders would just be gone entirely, like just erased, not by some sort of a conspiracy or by some sort of a you know, alien shenanigans. But just because things don't last that long, that's a it's an alien concept from too many of our again, too many of our creative visions of of what the future in the past, maybe exactly so we think, well, you know, there are ruins of civilizations from thousands of years ago, but that's thousands of years ago. That's nothing in geological time. The surface of the I mean, look a look at what a map of the land formations on Earth just you know, sixty million years ago looked like it's like, you know, the the surface of the Earth is not fixed and constant. This is a geologically active planet. So would there be ruins all around us with evidence of a civilization from hundreds of millions of years ago just be totally obvious? I think I probably by the answer that the authors give here, which is that no, it would probably not be totally obvious. In fact, it might be incredibly difficult to find evidence of at all. Uh. And so one of the the interesting points the authors make here is that the the exposed land surface of Earth, of course, is geologically very young on average. They cite evidence from a study by Mattman at All in two thousand nine that the oldest large patch of land surface on Earth is probably in the Negev Desert, and that's only about one point eight million years old. At one point eight million years, I mean, compared to the history of civilization, is a long time. But that's again, it's like nothing in geological times, a tiny fraction of of Earth history. So if we wanted to find remnants of a civilization from say, hundreds of millions of years ago, you probably would not find that on the surface of the Earth. You'd have to look for it in in exposed geological strata from from previous eras. And even then you can't just count on the fact that you would be finding fossils of that civilization all over the place. Yeah, exactly. That they that they had done something we you've discussed in the show before, which is of course that the fossil record is inherently incomplete, because fossilization only occurs when conditions are just right. Um. They point out that that of all the dinosaurs that ever lived, and there were a ton of you know, uh, you know, they're the era of the dinosaurs taken as as one gigantic, gigantic monolith just dwarfs anything that that that that humanity has ever occupied. Uh. You know, it is a it is a cathedral, and and we're we're not even like a child's dollhouse, uh sort of situation here. Um. So, uh, you know, out of out of all of those dinosaurs that ever lived, there are only a few thousand near complete specimens. And so the authors here contend that given the rarity of fossilization, a species is short lived, as Homo sapiens m not make it into the fossil record at all. And of course, for fossilization to mean anything to us or to you know, anybody who's doing and doing, you know, some sort of an investigation of a planet or our planet, those fossilizations would have to survive and then they would have to be found. Yeah, exactly. And so again this might be pushing against your intuition. You would say, like, wait a minute, there's there's there's there's signs of human life all over the surface of Earth. And and we have you know, at least a few thousand, uh complete dinosaur fossils, enough to have museums of Natural history with dinosaur fossils in places all over the world. Uh, surely you'd expect more. But dinosaurs existed for almost two hundred million years. Human civilization again is like it's a few thousand years at this point. Yeah. So the problem here is like we have trouble comparing the odds because you're not realizing how many millions of times more the dinosaur bodies got to roll the dice than our cors would. Yeah. Absolutely. Another thing they touch on is that is the example of technology. Uh, and they point out how rarely complex early examples of human technology are ever found. So if you're thinking, well, surely we would find one of these factories or something that was made by one of these factories that a previous civilization might have had, well not necessarily. Yeah, sure, surely my rice cooker would be found hundreds of millions of years in the future. But yeah, that that they mentioned several reasons why that's maybe not as clear as you might assume. So they say urbanization that currently represents less than one percent of the Earth's surface, So that's a limitation on the deposition side of creating a fossil record of our current civilization. Only small parts of Earth's surface are actually inhabited by humans. That that sounds counterintuitive, but it's true. And then they point out that quote, exposed sections and drilling sites for pre quaternary surfaces are order of magnitude less as fractions of the original surface. So human civilization currently only feels a small portion of Earth's surface right now, and we only access tiny fractions of Earth's previous surface through through various kinds of drilling, and you know, access through exposure to rock strata. So there's just like extreme selection filters on both sides, on the deposition and on the excavation side. Yeah, it would be kind of like even if you knew somehow, with some certainty that there was that there was a technological civilization during this time and in the ancient ancient past, uh yeah, you would. You would have to you have to really know exactly where to drill down to hit them. You couldn't just expect to randomly do it unless you did a lot of drilling and digging and excavation. But but but so what we've been talking about here is challenging the the intuition that you would just be finding physical fossil remnants and artifacts of this civilization from hundreds of millions of years ago all over the place, and I think they do a very good job of knocking that down. But of course it is not hopeless, because they say, while our chance of finding the physical remains of a hypothetical Silarian civilization might be very low, there would be other traces of the existence of that civilization that would be preserved in the geologic record, and you you would have a very good chance of finding those traces. Yeah, and that's that's what most of the rest of the paper deals with. I do want to point out one other thing that they bring up in passing that I thought was interesting. They point out that that you could certainly make an argument for or against the evolution of intelligent life in a world based on the probable evolution of species that are in the fossil record, but that they would be focusing on physiochemical tracers for previous industrial civilization. So I hadn't really thought about this, but like the idea of like looking at say, dinosaur um fall souls and saying, well, we don't have evidence that they evolved in it into an intelligent technological species, but if we but we can make an argument based on this fossil and this fossil fossil that they were headed in that direction. I feel like even that's the kind of thing that probably wouldn't be quite as clear as your intuition might lead you to assign them, because I mean, like intelligence in mammals arose very rapidly in geologic time. Yeah, so again it reached this situation where the fossil record could just be missing that snapshot entirely. So this all leads you next to the next major question. Given the limits of what we can detect in the geochemical record, what exactly could we look for on a planet to see if an industrial society ever existed there? And yeah, that's what the bulk of the paper focuses on. So in the case of Earth, if an organized, intelligent society evolved during the pre Quaternary time but they didn't reach the level of an industrial society, there simply would be no heard off them as far as this paper is concerned. Right there, they're looking for the kinds of chemical material and climate type changes that would leave a trace in the geologic record, and that would primarily be a function of of industry, basically of energy production, of of material working things like metals and plastics, and the UH and the methods of harnessing energy for industrial use. I was reminded of our episodes on fire technology because if listeners may remember, we discussed well, could something that evolved in the water or or on a water world, could they ever really get any kind of advanced technology going if they didn't have access to the surface. And that seems to be a factor here as well, as they're only looking at the period during which something could have evolved on land. Yeah, so you could maybe have advanced intelligence in the water. But it's maybe this is just a lack of imagination on our party. You know, you always need to be aware of the limitation of your vision. But it does seem hard to imagine advanced technology under the water, because like, if you don't have fire, you can't do metal working, or metal working is very difficult. I don't I don't know. It just seems harder to imagine how technology like we understand it could come about in the water. But again, you know, limits of our vision. Yeah, so they say, quote, the focus is thus on the period between the emergence of complex life on land in the Devonian four million years ago in the Paleozoic era and the mid apply a scene. Uh, and that's around four million years ago. Yeah, because if it was much more recent, you you'd probably get into the area where you'd start to expect to actually see those kinds of remnants and artifacts that that we were talking about. Right, So they get into the discussion of what we might look for, and they have it nicely divided up. The first one is, uh, well, they basically have two broad categories and then some some details on that category. The first big one though, would be looking at the geological footprint of the anthroposcene. So, as we've discussed in the show before, there's an argument to be made that the impact of human civilization on the environment and the geologic record constitutes its own geologic era, the Anthropocene. So not all the changes would be recognizable millions of years later, but some would be. Right. So, human activity at this point is is large scale enough that we are making changes to the earth that that are that are widespread or you could even say global, and I was gonna say permanent, not permanent, but extremely long lived. You know, going way into the future, you will be able to find signs in the rocks and the ice and the settiment, you know, the things on Earth that persists over long periods of time that will leave records of what we did to the Earth in just the past three years or so. Right and uh, and as we've probably mentioned before, the anthroposcene is not a an official geological era, as much as any of these things can be, you know, officials. It seems like when you're talking about geologic terms, it's even more ridiculous. We can consider such a small part of geologic history that we occupy, but there's a lot of compelling evidence for it, and you often see it discussed, especially when we're talking about the changes that that humans have made to the planet and arem still making to the planet, and how they may show up in the geologic record. And just to be super clear, the majority of the changes we're talking about of this kind would not be changes like physical alterations of the Earth's surface. We're not talking about like records of people digging holes and building stuff. We're talking about records of like changes to the to the level of different carbon isotopes in in geological strata and things like that. Right now, they also hit upon something that that I thought was really interesting in in in a way almost almost encouraging, uh, the sustainability paradox. So the idea here is that, of course the longer human civilization lasts, especially technological civilization, the greater the geologic signal of its impact. Again, that that lasting those lasting signs in the environment, not the faces they carved into the mountains, but impacts again on their their geochemical in nature. Um, so that signal increases. But the longer human civilization lasts, the more sustainable it must become in order to survive. And this is of course the reality we're living in right now. If a civilization survives this test and becomes more sustainable, then that signal grows weaker. Right. So it's almost like the strength of the signal left for future people to discover is directly proportional to how suicidal that civilization is, right Like, the more it is just burning through fossil fuel resources and the rate of that, the stronger the signal will be. And so a civilization at some point, they say, well, will naturally tend to attenuate for a couple of reasons. Either it really says it can't keep going at that rate, or it's going to cause climate damage to itself, so it will naturally switch to more sustainable uh energy sources that are harder to detect in the future, or it of course does so much damage to itself that it's signal naturally is reduced, right right. So, so basically coming back to what you said earlier, it's not just that there there's gonna be a sign. There's gonna be the signal uh, this footprint of a civilization uh in the in the you know, the geochemical record. It's it's also that it may just be very short, it may be a little it's not going to be this Uh. It's not gonna be a symphony. It's going to maybe be a note or two. So basically the idea being that that that the real strong, much stronger argument for aliens existing and having some sort of role or some sort of advanced technology having some sort of role on Earth and they in the ancient history would not be look at the pyramids, I think aliens did this, or I think you know, ancient scientists did this. It would be pointing at say a blip and uh or an increase in global temperatures during a certain period of time and saying, I think the aliens did this? Or I think the advanced technology and question did this? I mean, even then, I think that would be a very speculative and and difficult to prove hypothesis. It would be kind of just like unfalsifiable speculation, but that that would perhaps be the more likely type of signal you would find if there had been alien intervention. Then you know, specific artifacts than al right, well, let's get into some of the specifics of the footprint that the authors lay out here. The first one is a stable isotope anomalies of carbon, oxygen, hydrogen, and nitrogen. And this is one of the big ones. An estimated point five trillion tons of fossil carbon via the burning of fossil fuels and warming of the planet um they quote, we we expect this temperature rise to be detectable and so FA ocean uh carbonates notably forminifera. This is a single celled organism with a with a with a chalky shell. UM. Organic biomarkers cave records, stuches such as stalactites, lake ostracods. These are minute aquatic crustaceans and high latitude ice cores, though only the first two of these will be retrievable in the time scales considered here un Right, So this thing about the the isotope anomalies of of carbon and these other elements is very interesting. So they say, you know, there are natural distributions that you would find records of in the different isotopes of carbon that are that are moving around in Earth's atmosphere. But when people suddenly start pulling huge amounts of fossil carbon, carbon of a biological origin out of the ground and burning it, you suddenly start throwing those isotopes out of whack, and that will be something that will leave records for millions of years to come. So you can look in the geological records, you know, the record of of strata from previous eras, and say hunh. For some reason, in this one period deep in history, suddenly the carbon isotopes got way out of whack, as if suddenly a bunch of fossil carbon like coal or oil or whatever had been burned at a at a hideous rate into the atmosphere. Right. And so you know, looking at our our time now of of of modern human civilization, you know this that we have this fossil fuel consumption and we have the invention of the haber Bosch process and the large scale use of of nitrogenous fertilizers and agriculture, which will also heavily impact the planet's nitrogen cycling. Yeah, the haber Bosch process, Yeah, as part of the changes in the nitrogen cycle that have come about as a result of of industrial civilization. As well. They also touched on sediment to logical records. The key causes here would be major soil erosion brought on by agriculture, but also by agriculture lated deforestation. Um. Now, this would be partially mitigated by dams, they point out, but erosion is also heightened by climate changes and thawing perma frost. Also sediment content changes due to just industrialization in general. Uh. Now here's a There's another big one that I think will be pretty obvious. Faunnel radiation and extinctions. Um. Basically, humans have brought about many extinctions already, and we're living in the midst of an extinction event. Uh. This will likely register in the fossil record. Yeah. Now, of course, previous major extinction events have usually been chalked up to two natural things like uh, oh, we can track massive vulcanisms as the cause of this one, or say a large space impact like the Katie extinction event. But there are other extinction events in Earth's history where the cause is not totally clear. You know, there are some speculations, but we don't know exactly why. Suddenly it seemed like there was a great reduction in marine biodiversity at this point in history. All right. The next area is non naturally occurring synthetics, So non naturally occurring chemicals generated by industrial activity that persists in the environment. Things persistent organic pollutants, chlorofluoral carbons, in related compounds. And they also point out that steroids, leaf waxes, alkanones, and lipids can be preserved in sediment for many millions of years. Now, that one naturally makes me think of King plastic baby. Yeah, yeah, And that's that's the next thing that they mentioned. And this one's you know, this one's disheartening but obvious. We've created tons of plastics, tons upon tons upon tons of plastics, and they sadly persist not only in heaps and floating masses, but inside the bodies of organisms, including ourselves, uh so quote. The potential for very long term persistence and detectability is high. Now. One of the things they point out about plastic that's interesting is that plastics may well proved to be a very long term signature of human civilization and the geologic record for you know, millions and millions of years to come. But the development of plastic is also something they would class under the I don't remember the umbrella term they use for this, but sort of chemical contingencies. A technological civilization does not have to use plastics. Plastics are just something that humans happen to use. There are other things that seem probably more universal, like almost any industrial civilization you would expect to burn lots of fossil fuels, But plastics, that's more of a question mark. Is that unusual that we did it, or is that a very common thing that the civilizations would do all right. The next area that they highlight transuranic elements. These are elements having a higher atomic number than uranium, which is ninety two. Most radioactive isotopes created via nuclear energy or weaponry have long half lives, but not long enough to be a factor on the time scale that they're talking about here. But the exceptions are plutonium to forty four and um curium two forty seven. So plutonium has a half life of eight point eight million years, and curium in this case we're talking about a half life of fifteen million years. So in sufficient quantities of disposal, these would these would pop up. And plutonium has no known natural causes outside of an actual supernova or something like that. This isotope of plutonium. Yes, this particularly yea plutonium two forty four so um. So, Yeah, if you found enough of this uh in the geologic record, that would be a sign that's that's something was at work, there was some sort of technological atomic uh enterprise that was in place. Now, I guess we've already mentioned earlier that the authors are not going to claim that there was in fact a a long lost civilization hundreds of millions of years ago. But they do actually look at the geologic record to say, are there anythings that that match these criteria we've been looking at, And they do find some interesting partial matches that have though of course nothing really comes close to evidence that would be conclusive that there actually was a civilization, but some of these matches raise interesting questions of their own. Yeah, they don't look at everything, and they point out that things like the kt extinction event. We know that that was not an industrial accident or anything. Um and uh and again they're not arguing that these are evidence of past pre human industrial civilizations on Earth, but merely point to them as the source of events we might look at. Yeah. And I would say the biggest one that they focus on in the paper is the event known as the Paleocene Eocene Thermal Maximum or p e t M. Yeah. This is an abrupt spike in carbon and oxygen isotopes near the Paleocene Eocene transition fifty six million years ago, resulting in a five to eight degree celsius global average temperature rise. This is widely thought to be due to well, I think they're there are different theories. One is that it's volcanic activity. Um. But there have also been hypotheses put forth that it could have been a common impact. It could be due to burning pete, methane being released, and a few other candidates. Um. It's also used as a means of of understanding and kind of like kind of modeling out the effects of climate change during our own era. Yeah. And and one of the reasons this one gets singled out is, uh, so it really looks like, Okay, here we're seeing, for example, these carbon isotope signature changes that would signal that huge amounts of biogenic carbon carbon that originally came from life forms, like the stuff you would find in fossil fuels, is being burned and released into the atmosphere. Now, how would that happen if if if it wasn't creatures from the Black Lagoon with leather face masks digging up a bunch of fossil fuels and burning them for their civilization. Well, no, you probably don't need to jump to that conclusion because there are other solutions on off for like, for example, there might have somehow been lots of access of volcanic magma two beds of fossil fuels. Maybe certain types of volcanic activity tended to set a light to a lot of natural reserves of fossil fuels and shale beds and things like that. And this almost acted as if the Earth itself, we're we're setting off an industrial revolution, but it was just volcanoes interacting with with these reservoirs of carbon in the ground. Yeah, all you need is geologic upheaval and volcanic activity and again um our Our planet has A has A has a very active geological life, so there's plenty of opportunity for this sort of thing to potentially have taken place. So it kind of comes back to a problem with the signal here, the signal you would be looking for in the geochemical record. In many cases, the very sort of signal we're looking for, especially concerning carbon and warming, could have also been caused by these naturally occurring causes, and so strong signals might be coming from something else, and more specific signals that we might look to just might be too weak to to ever possibly observe or to really make much out of. Oh yeah, this is an interesting paradox they talk about in their conclusion of all of the criteria they're able to come up with in this framework for for looking for past industrial civilizations, the stuff that you would expect any industrial civilization to do also has other explanations, and so so it's not conclusive that it was an industrial civilization that this would be things like you know, the carbon stuff. Meanwhile, the stuff that would be really strong evidence of an intelligent civilization origin, that stuff that civilizations might not do. It's more contingent things like plastics, and stuff. You know, you could have a civilization without plastics. That's not a necessary milestone in the in the progress of energy harnessing. And maybe it's even the sort of thing. Uh an advance civilization would move away from coming back to that the sustainability paradox, when one could hope, I imagine. Now the authors again, they're very clear about just how far you should take this hypothesis, stating that quote, the Silurian hypothesis cannot be regarded as likely merely because no other valid idea presents itself. Uh So they admit that this this sort of thing could easily get out of hand with folks pointing to any sort of signal in the geochemical record as being possible proof of pre human technological societies. If you're doing that, you're really taking it and running with it in the wrong direction. Yeah, I guess that's one of the frustrating things about about interesting work of this kind is so you can point out a lot of the ways that it's difficult to rule out past civilizations. But then for a lot of people who just want to have a theory that changes everything, you know, for a lot it's just like it's fun to believe that. So a lot of people just want to believe it. I want to believe, you know, that there was an Atlantis mother civilization that birthed everything, or I want to believe that there were alien on Earth before humans or anything like that, because that would change everything, and it feels so cool to believe it. Therefore becomes your default belief. And so thus a paper that says, well, it's more difficult to rule out that kind of thing than you might think. Uh, some people can erroneously conclude that that is in fact evidence for the thing they want to believe because it feels cool. It's not positive evidence for it, yes, yeah, absolutely, um yes, So so you know, they argue that that we need to to further research, you know, the likely signature left by our own um anthroposyne era, as well as a deeper exploration of the elemental and um compositional anomalies that we find in extant sediments. Basically, we look at past events mainly with stuff like impacts in mind, but perhaps the Silarian hypothesis needs to be at least on the table as well. Not because again we think it is, you know, actually a valid explanation for what has happened. Um, you know, ultimately it's an outside possibility, not a conclusion we should jump to, but perhaps it should just be part of sort of the spectrum of possibilities there. Again, not because we think it happened, but because, uh it gives us a little more of a sort of a robust spectrum and how to interpret these things. And and then moving forward to you know, potentially considering other worlds, looking at other planets like even Mars. Uh it gives us one more tool, one more uh way to look at the evidence. Yeah, exactly, they're not arguing this because they think there was a civilization. It's that we should consider these possibilities when looking at planets, even including our own, and know what we would look for if we wanted to consider that possibility, right, Yeah, because ultimately this is not a supernatural explanation. This is ultimately, you know, a natural hypothesis. But but it is admittedly an outside possibility. Now, of course we're here talking about reasons why you shouldn't just jump to the conclusion of a Silurian civilization, but but there are also some arguments against it in some of the specific events that they look at. For example, if you know, maybe the best possibility is this interesting event in Earth history, the Paleocene Eocene thermal maximum. More suddenly there was there was rapid global warming and UH and these in these chemical changes like with carbon isotopes. They even put some arguments back against considering UH civilization as a cause of this global warming in Earth's history, because they say, look, the kind of global warming caused by our civilization is happening in an incredibly rapid fashion over just a few hundred years. This actually, though it's it's relatively rapid in geologic terms. The the p et M actually happened probably over hundreds of thousands of years, which which is incredibly slow if you're imagining that civilization was the cause of it, right, if you're if you're comparing it to the model of of human industrial advancement, it's incredibly slow. So there's not only the point that you shouldn't just jump to the conclusion of there was a law civilization because it feels cool, but like in the specific instances they look at, there are some reasons for thinking that's probably not true. I don't know, I guess unless those civilizations were like just really lazy, Yeah, I mean, you can sort of, you know, pull out your sci fi hat and and put it on and come up with various ideas of you know, for why they might have been this way. Maybe they were super long lived. Yeah, they weren't very ambitious, and they're like this, note, this is the right level of industrialization, and we want to, uh, we need to keep going at this rate. I don't know. They didn't reproduce all that much. I don't know. I mean that that's ultimately one of the problems with with imagining uh, you know, other life forms. It's like it's just you know, it's you can you can make a case for any number of things, um and and try and make it fit your your hypothesis. And of course that's not really the way to go about it. I mean, not in from a scientific perspective, from a sci fi dreaming creative perspective. Yeah, go for it, um though, though I guess it does kind of come down to the conundrum two at the end of the day, like like when does when does mere creativity and um and dream weaving become this kind of corruption of our thought and uh and and a pollution of our ability to understand our place in the world and are where we're going in the future and where we were in the past. Well, you know, I feel like a thread without maybe intending intending to do so. That we've pursued A good bit on this podcast is understanding the ideally, the difference between your sort of interest and imagination and your epistemology. That like that an idea. You can like an idea because it's interesting and cool, and that doesn't necessarily mean it's true. You know that, like that, your your epistemology is probably best to based on evidence, and you should be skeptical of things that you want to believe because you like them and so forth. But it's still totally valid to, like, say, be interested in the bicameral mind or whatever because it's a fun idea, even if you know, you probably accepted as you know, there's not a lot of evidence for it, right right, You can you can altomly engage in a number of these ideas as as more as art than science, and there's certainly nothing wrong with that. Is when you start arguing that your art is science, that's where you can get into into some trouble. Um. I was reminded in all of this of Carl Sagan's approach to ancient aliens and ancient astronauts, particularly in the book that he co authored with Joseph Shklovsky, Intelligent Life in the Universe. Um. In this particular book, you know, they they examine this idea. They said, Okay, here's the speculative idea, and we don't have evidence that had ever happened. But if it were to have happened, what sorts of specific evidence might we look for? And in this case, we're talking about science ends that are evident in ancient religions and uh and so forth. Uh. And I thought that was a great treatment of that question. Uh. And against Sagan's treatment reminds me of the treatment given in this paper by these authors. But of course Segand had to come back and continue to argue with the ancient alien people who were, you know, very much going off in their own director is true, ye, yeah, and pushing pushing art as science, and Sagan having to remind them like, no, I love art as much as the next guy, but here's how we approach this from a scientific perspective. Well, I mean, I think the important thing about this stuff, like Sagan's work on that or or or the paper we're looking at here is it's good too when you're when you're exploring it like a tantalizing and juicy idea. It's a good idea to have criteria for what would be good evidence of such a thing before you're actually looking at individual evidence in cases, because if you look at the evidence first and then you try to come up with criteria, you're gonna have a tendency to want to fit your criteria or whatever evidence you've already got. The cherry picking model or what is it the other name of the barn wall fallacy or something like that. Remember, the idea is like, you know, somebody says, you know, I'm a great shot, and so they shoot at the side of a barn, and then they go up, they walk up to their bullet hole, and then they draw a bulls eye perfectly around it. That's that's a great point. That's that's a great way of looking at it. All Right, Well, I guess we're gonna go ahead and wrap this episode up. But we'd love to hear from everyone out there. I love to hear from any doctor who fans who have some additional information they want to share about the Silurians. And various related um species that have popped up in that show. And perhaps you have specific thoughts about about you know, just this this basic you know view, uh, and what it reveals about humanities uh place on Earth right now, and what technological civilization is doing to the planet, and just you know, ultimately, what kind of a you know, a small lip a week signal we may be in the future as opposed to this kind of lasting thing that we sometimes imagine that human civilization is. I'm going to say, I hope that I'll be optimistic, so I hope we do stick around. I hope we attenuate the kinds of geologic signal we leave due to climate change, in chemical alteration of the atmosphere and all you know stuff like that, uh, you know, heavy metal pollutions and things, and that we the signal of our civilization can always be charted against the geologic record because of the continuance of Doctor Who Seasons. So when we're on the like, you know, eventually we'll get into the exponential notation of the Doctor who Seasons. Yeah yeah, yeah. Well, one day some sort of ancient uh or rather some sort of far flung future civilization will look back and say, look, clearly they knew what they were doing. Uh, they were able to do you know, some three million years of doctor who though maybe at a certain point the doctor will be a robot and the enemy, as will be will be organics. And I don't know, Yeah, I wonder at what point do we get a robot doctor. I mean, they they're they've only recently really been been been mixing up the casting on that role. All right, Well, let us know in the meantime, if you want to listen to other episodes of Stuff to Blow Your Mind, we have core episodes on Tuesdays and Thursdays and the Stuff to Blow Your Mind podcast feed. On Monday's we do listener mail, where we hear from from you, the listeners, and we we read your various listener mails, always a good time. On Wednesday's we do a short form artifact or monster fact, and then on Fridays we do Weird How Cinema. That's our time to set aside most serious issues and just talk about a weird film. Huge thanks as always to our excellent audio producer Seth Nipolis Johnson. If you would like to get in touch with us with feedback on this episode or any other to suggest topic for the future, or just to say hello. You can email us at contact that Stuff to Blow Your Mind dot com. Stuff to Blow Your Mind is production of I Heart Radio. For more podcasts for my heart Radio, visit the i heart Radio app, Apple Podcasts, or wherever you listen to your favorite shows.