The Silurian Hypothesis: What Traces Of Humanity Will Be Left 50 Million Years From Now?

The Silurian Hypothesis: What Traces Of Humanity Will Be Left 50 Million Years From Now?

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This episode is sponsored by Audible. Our civilization is ancient, with ruins of its early incarnations going back many thousands of years, but that’s just an eyeblink against the age of our world. If we disappeared tomorrow, would the scientists of some successor race, millions of years from now, even know we existed, and if not, can we be sure we truly are the first civilization to arise on Earth? So today we will be looking at the Silurian Hypothesis and the general surrounding topics of if our civilization would leave any long-lasting evidence if we somehow disappeared tomorrow or if we would even know if some other civilization had existed here on Earth. That last one is a bit complicated by potentially coming in two forms, life that previously arose here – like if the dinosaurs had gotten high tech and then blown themselves up – or alien life that had colonized Earth at some point and that colony had failed. Now today is another Sci-fi Sunday episode here on SFIA, where we try to look at concepts from science fiction and ask how scientifically plausible they are, and since this has been a popular topic in sci-fi for over a century now, unsurprisingly, we’re not the first to look at this from a realism perspective.

Indeed the name Silurian Hypothesis was coined by Astrophysicist Adam Frank, and Gavin Schmidt, Director of the Goddard Institute for Space Studies, back in 2019 when they wrote a paper up, trying to analyze how long we could expect to see ruins of civilizations for. Silurian is a reference to Doctor Who’s race of lizardmen, who arose on Earth long before humanity did, not to be confused with the Silurian Period of Earth, a roughly 25 million year long period 419 to 444 Million years ago. That draws its name from a Celtic tribe in Wales, where the first rock strata of the period was identified by Roderick Murchison, and was inspired by his friend Adam Sedgwick who had named the Cambrian Period, 485 to 541 Million Years ago, after Cambria, the old Latinized name for Wales. The period between the Cambrian and Silurian, the Ordovician, is named after a Welsh tribe.

I’m probably botching some of these pronunciations incidentally, so don’t use me as your reference, though I’m never a good pronunciation reference for anything with the letters R or L in them anyway. The Silurians from Doctor Who are named-after the Silurian Period – making their first appearance way back in 1970 when we were just getting introduced to the Third Doctor. In that, he notes that they should really be called after the Eocene, the period 34 to 56 million years ago, post-dinos. And again, this isn’t a super-new idea. Almost as soon as we started getting strong evidence that our planet had been around a million times longer than humanity’s recorded history and that the Universe was huge and full of stars like our own Sun, folks began contemplating if civilization might predate humanity.

All at the same time, we were finding ever older ruins of human civilization that made the ‘Ancient’ era of around 1000 BC, prior to the rise of Classic Greece, seem downright recent. So sci-fi from the early 20th century had ruined human and alien civilizations that were older than the dirt in more books than I can count. The entire Cosmic Horror subgenre of authors like H.P.

Lovecraft is built around it and so are a lot of the Sword & Sorcery genre of folks, like Conan the Barbarian and Elric of Melnibone. It definitely saw a resurgence in the 1970s though, between the Space Race and books like Chariot of the Gods, that focused on the notion that our own ancient civilizations may have met advanced aliens and viewed them as deities. Now there are a lot of reasons that this is contraindicated, see our episode: Ancient Aliens, for more of those, but the one that always bugs me most is the implication that some of the great works of these ruined empires we dig up, can’t actually have been their own designs, and that they had to have borrowed or stolen them from aliens. Now to be fair, that’s not really an argument based on evidence, and indeed they probably did borrow or steal a lot from other civilizations. It just would have been their neighbors or predecessors, not advanced aliens.

And that’s kind of a critical point, folks often note parallels in ruined bits we recover, be it pyramids in Egypt and Mesoamerica, or a specific style of art or type of technology. And that has to contend with remembering our ancestors were quite mobile. They only had feet, theirs or maybe some animal’s, but they were pretty enthusiastic in using them. They were also more or less just as smart and curious as we are nowadays, so someone bringing a sketch of some sight they saw across half a continent of some cool building to show their king is not even vaguely unrealistic. What is a bit of a problem though, is that unlike our fossil record, which honestly is pretty scant itself, there’s not a million year chapter containing a trillion organisms that were on a step from the original critter to its new mutated form of which a few hundred fossils might realistical have been made and a few discovered. Art and Architecture certainly could be viewed as evolving with lots of early examples, but at the same time it is a much smaller sample.

Someone’s artistic style evolving from one period to another might only have included a couple dozen finished products showing their trajectory. And you probably don’t really get a lot of transition pieces showing the migration or evolution of many technologies or art styles even just a few millennia old. So we can have a bit of a problem trying to show the evolution of some building style or pottery style and how it migrated across Eurasia or down the Nile, which has a lot of history buried in its floodplain and delta. This would be even harder if it were millions of years, not thousands, and if entire continents were moving around and dipping under the sea, which is exactly what we think they were doing on those timelines. So how long does stuff last? Not long, but also a long time, and I should start from the outset by saying this one is a lot like how far you can see a radio signal. There’s no limit to how far radio signals can travel, excluding hubble shift, but you’ll hear people say we couldn’t hear one beyond a handful of light years, which kinda ignores all that radio astronomy we do even millions of light years away.

There’s no limit to how far those photons can travel, it is a question of how big the signal is, so to speak. A very high power signal that’s just pulsing at a low frequency just doesn’t blur much over distance, even over galaxies. Of course, nobody really would bother using that sort of signal for in-system communication because you could be cramming far more conversations in, like we do.

And those are the sorts of signals we expect to be mostly overhead signals, whereas the type I described is what you use when you want to be heard over long distances by people who don’t necessarily have mega-receivers or even know you’re talking, what we’d call a beacon. In that context we need to keep in mind that civilizations might be leaving time capsules behind that are built with the specific intent of being findable and still sturdy, millions or even billions of years later. In neither case, distant alien radio or ancient time capsules, are we assuming that this is anything like their day-to-day operations, but we have an entire niche industry for manufacturing time capsules which I’m assuming means we sell thousands of them a year.

Now they wouldn’t last millions of years but that’s a limit on our current technology, not intent. We have materials that would do the job incidentally, it’s just they tend to be way more expensive than a personal time capsule that someone is looking to buy on Amazon, for maybe a hundred bucks. We should assume that will change the longer we’re around, and we also need to acknowledge that we, thus far, do not have the technology to kill off all of humanity or even really set us back more than maybe a few centuries. Even at the height of the cold war we lacked enough ordinance for that, commentary about us having enough bombs to kill everybody ten times over again were utter hyperbole. We could build enough big bombs for that, but they’d get pricy to maintain and more importantly, they need expensive rockets to get where you want them to and bigger payloads equals more spending and the nuclear arsenals maintained by both sides were already painfully expensive on the national economies of those involved, boosting your arsenal up tenfold more wouldn’t have achieved much of strategic value and would have crippled their economies. Or basically, we can say with a decent deal of certainty that civilizations can’t wipe themselves out till they exceed our current technology level, and thus would have time capsule tech at least as good as ours.

Though of course technology need not proceed in unrelated fields at the same rate it did for us. Now we discussed the difficulty of keeping any sort of machine functioning for long times in our recent episode the Million Year Machine but proof of existence is a bit different. As an example, we have lots of things left over from the distant past in fossil format and if a tree can survive as a fossil for 386 Million Years – the current oldest tree fossil from a quarry in New York – then so can a lot of dimensional lumber and the big giveaway that you’re looking at the remnant of a technological civilization is all the weird right angles you would find in the geological record. Geometric shapes can occur naturally but mostly won’t. All those basements and foundations that would get flooded and eventually filled with sediment, some are going to end up getting dug up millions of years later.

We get banded deposits of iron leftover from the ancient pre-oxygen period of Earth’s atmosphere, those are intact bits of iron over a billion years old, because of course stable elements don’t decay with time. They’re still iron and they’re still titanium or aluminum, which doesn’t corrode, or gold, silver or many other metals that we have shaped into suspiciously unnatural forms. It's a bit of an unknown how long plastics would persist or how they’d fossilize and we really can’t say how easy it would be to figure out you were looking at a freeway someone built a billion years ago. For my part, I just really don’t see a scenario where all those right angled foundations in all those houses and skyscrapers, and all those wheel-shaped hubcaps and all those roads with driveways running perpendicular off them are going to somehow vanish universally and not seem really obvious later. Now critically, it is a bit of an academic issue anyway because we can pretty conclusively rule out historic high-tech civilizations on Earth, and as we’ll show in a moment, we can also flip it around and show why a post-human civilization wouldn’t have any problem seeing us, because it wouldn’t be long before they arose.

Now we cannot conclusively rule out aliens starting life here on Earth, but we can say they pretty much needed to have done it on Day 1. We do not have the DNA of anything fossilized, and we’re not optimistic about finding viable DNA in any ancient fossils, even using some of the techniques we contemplated in our De-extinction episode for recreating DNA from decayed or damaged DNA. Nonetheless, while our fossil record is scanty, it shows a pretty clear path of evolution and divergence. I mean we could be tricking ourselves into seeing what we want, that fossil record really is scanty, but you would expect to see some sort of organism sporting a pretty big brain in the fossil record during one of those eras where intelligence arose. They should have been pretty dominant.

We don’t really see anything breaking the general pattern back to that first single-celled life and you would think if aliens had arrived and colonized, then there’d be a sudden appearance of stuff that didn’t fit. Maybe not, we tend to assume anything we find does fit, so we could have some freaky alien fossil and just fit it in through determination and an assumption of what the right answer was by everyone looking at it. Still, it feels a stretch to assume we wouldn’t have at least one fossil that stuck out like a sore thumb, maybe it lacked bilateral symmetry or such. Nonetheless we do need to acknowledge that we cannot actually rule out either of two ancient civilization options. First, that large brains can evolve rather quickly and that one a bit smarter than humans might pop up and get to advanced technology in only a few thousand years after discovering fire. Remember, humans may have discovered fire a million years ago, not really very long after we got bigger brains, though that’s a bit hard to say, but we did kind of sit on it for 99% of the time we had it.

Pottery and metal-working are recent inventions, having been around for only about the last percent of time we’ve had fire for. We used it for warmth, food, safety, and light, and probably fire-sharpening sticks, but for 99% of the time we had it, that was the extent of our technology. If we’d been a bit smarter or more numerous, we may well have gotten pottery within a few hundred years of having fire and it only took us a handful of millenia with our current brains to get to space from there.

Had we been smarter, we may have gotten to space, and cheap doomsday devices, in under a thousand years. Additionally, we started burying our dead ritualistically as we got smart enough to realize what death was, and many opted for burning almost right away. We don’t want to rule out civilizations not leaving fossils because they had burial or body disposal methods that made finding fossils way less likely, meaning that almost as soon as they got big brains, they almost entirely disappeared from the record. In alien biology, that might include skulls that were not solid bone, as an adaptation to allow bigger brain growth, but which decayed quickly, or that that civilization engaged in ritualized cannibalism, bones and all, to take in the honored dead, so to speak, or there just happened to be very voracious critters when it comes to finding and eating bones. In that same context, if we’re suggesting bigger brains, and talking about civilizations blowing themselves up, one that goes from making pottery to designing nuclear bombs in a couple centuries might leave a lot less evidence they were around due to their abridged timeline.

If advanced technology almost inevitably kills off intelligent critters, then the smarter they are, the shorter their dwell time. Second, parallel biology and convergent evolution can’t be ignored, especially for choosing worlds to visit. Again, our fossil record is slim and our connections for missing links is necessarily hazy, and it wouldn’t be hard to have an alien civilization arrive that had picked Earth because it already had life and life not too different from theirs, especially in terms of fossil-preserved qualities, like physical shape. I really think that’s a stretch but there’s a number of extinction events where if those had actually been the result of an alien civilization arising and terraforming our world, we might have missed it simply by assuming it must be natural and thus connecting the few dots we have under the assumption that was the case.

As a brief tangent, that is always a worry when we're doing science and why I tend to emphasize the point, in Fermi Paradox phenomena discussions, that Intelligence is generally going to be blatantly obvious. You can confuse a single crude tool for a natural object, but not a workshop full of tools and electronics. So too, the ruins of a civilization might give off subtle telltales that might be harder to differentiate from nature, but a big old Dyson Swarm actively existing is not going to look any more natural than New York City does. However, it is a necessary assumption in science, when looking at the Universe, to greet any new phenomena or bit of evidence as being natural rather than of intelligent, paranormal or supernatural origin. I don’t think there’s a more valid approach scientifically, but we do need to be mindful that, just because it gives us the clearest vision overall, doesn’t mean it doesn’t blind us sometimes, and we may have built some houses of cards on some phenomena or evidence by assuming it was natural, and shoehorning everything in to fit our expectations. The Giant’s Causeway is an example that comes to mind, all those unusually hexagonal stones, it’s so tempting to assume it is artificial, much as a hexagonal beehive comb is, but it turns out to be a natural effect.

In that same way, if we dug up an actual road built by some ancient giants, we might trick ourselves into believing it was natural. By that same logic, we might trick ourselves into seeing some extinction event and shift in biology as a natural extinction and evolution process, not some alien showing up, or future time travelers, or extra-dimensional entities or deities. That said, again, probably not.

Even where missing links are missing, there’s other stuff missing, like higher lifeforms. We can definitely invent a scenario where that still works of course. Perhaps the aliens came long ago and didn’t believe in colonizing to eradicate existing life, so kept themselves in domes along with their own organisms, but some of their microbes got out, or a few organisms that resembled terrestrial ones so much, they didn’t notice a few snuck out and didn’t send out the drones to hunt them down. Or they engaged in some hybridization of alien and terrestrial life. And we don’t currently believe we have any DNA from back then, it could be that our lifeforms native to earth used something different and their hybrids were using DNA which was alien to Earth originally.

Or it really could be that DNA is just the norm in the Universe and convergent evolution in general is a thing, see our episode on Convergent Evolution, for more discussion of that. One other option is that civilizations actively erase themselves, and for instance, a science colony comes to study us, or make us, might release drones and nanobots to go and clean up any escaped lab samples and erase right angled ruins and such from the geological record. It also isn’t too much of a stretch to assume a civilization that felt like it really hurt its native world might decide to migrate off it and restore it to a near-pristine condition, there are trillions of empty worlds to colonize but only one beloved homeworld, and that could easily involve such erasures while simultaneously not engaging in too much de-extinction technologies, since resurrecting mammoths and dodos might be seen as just as bad to the new native ecology, so they just do it on artificial habitats they’ve moved to or made as nature preserves. That’s plausible enough, we can probably bring mammoths back, but putting them back into our ecology just messes things up again, so we build a space habitat just for them. It begs the question of what happened to them of course, these ancient predecessors, but the predecessor extinction path is less tricky in terms of the Fermi Paradox than the normal non-exclusivity issue of space aliens colonizing the galaxy, then somehow managing to have all of their colonies die off, so that we see an empty galaxy even though it has often been colonized by various aliens.

Because it’s actually easier for a world to evolve intelligence twice, after some partial doomsday, than to have two separate worlds kindle intelligent life independently. So we don’t have to assume a galaxy full of aliens for the Silurian Hypothesis to apply. So what we’re seeing thus far is that we can’t rule it out, which is nice for anyone wanting to write a story about it, but that it would take some plot conveniences or really contrived scenarios.

Not impossible ones though, and given that I just got done warning everyone about how we might trick ourselves into ignoring such events by assuming they were natural, those events might seem more contrived to us because we assume they’re not real. Reality is also often stranger than fiction and I’ve lost track of how many times real world events and decisions were truly made where, if they had been in a film or novel, the audience would have rolled their eyes at how unbelievable such behavior was. Which goes to the general point of how popular such theories have been down the years, in and out of fiction. To many of us, they sound interesting but implausible, good stories. To others they are very believable and while I strongly disagree, I can’t really just dub it crazy conspiracy theories only idiots believe in, even ignoring that I try not to say that about conspiracy theories in general.

Thus, much like the authors of the Silurian Hypothesis paper, I don’t really see any good reasoning to think an ancient civilization once dwelt on Earth millions of years ago, but there’s no really good way to eliminate the possibility entirely and there could be a lot of at least modestly plausible ways it could have happened and we just missed it. So, of course, the other big question is how long our ruins would be around and the big issue here is that we have to ask what the heck erased us, since it would be relevant to how long those ruins would last. If we did program some grey goo designed to erase all signs of technology, then the answer is not very long at all. And it is possible a civilization aiming for techno-primitivism, out of fear of the dangers of keeping technology, might contemplate something like that, possibly to police dangerous technologies and prevent anyone from building them, the nanobots just seek out and destroy higher tech, and perhaps that eventually runs amok or the civilization dies off and it eventually kicks over their traces. Otherwise though it’s very hard to imagine anything wiping us out completely, but leaving life behind that could eventually replace us, and needs many millions of years to get smart enough to engage in archeology. Give me a doomsday in which even a thousand people survive and I’ll give you a civilization that’s back on its feet before our cities and infrastructure have been covered with vegetation and sediment to hide them.

We’ve examined this in detail in our episodes like: Cyclic Apocalypses and Post-Apocalyptic Civilizations, and you almost need to do something like infect humans with a retrovirus that makes us fairly dumb, to have us not repopulate and build pretty quick. Again, I could see that, maybe we decide our tech is too dangerous and we unleash a virus to dumb us down, but then why are the cities going away as ruins? I can’t really see us deleting the pyramids and other big stone structures – of which we still make tons today – out of fears of dangerous technology and runaway AI. Of course, you could just have a super-plague, something made in a lab, kill off every single human, and then maybe ten million years from now, chimps, racoons or elephants finally get sapient and technological. And then there’s all that leftover stuff. Yeah all the electronics are gone though you’re likely to have buried computers who have suspicious corroded wafers full of semiconductor residue on them. Got an awful lot of abandoned mines that you encounter that probably all collapsed or filled with sediment and they are digging through looking for coal or metal, and call in the geologist who wonders why there’s a weirdly uniform corridor of sediment and fill running through geological layers and exploration finds tons of petrified wood and metal beams holding those tunnels up.

The continents move, so the South Pole currently on Antarctica won’t be in the same place a million years from now. No archaeologists scanning their south pole would see our old bases buried under the ice there. However, just as H.P. Lovecraft and others set ancient civilizations to have once lived in Antarctica before it got to the South Pole and got buried under ice, we need to remember that even 10 million years from now, Earth is going to look noticeably different, especially if we’re more technologically advanced at that time and really messed around with our planet’s landscape, building artificial islands and seawalls and other concepts we explored in the Earth 2.0 series.

Nonetheless, I just can’t imagine that future raccoon civilizations wouldn’t eventually find some remnant of us somewhere. Of course, even though archeology and paleontology are explorations of the ancient, they’re not really all that old of fields themselves. Perhaps given time, we will find the ruins of some ancient civilization that once dwelt here, and wonder who they were and what happened to them. That would be an important puzzle to solve too, since whatever befell them might befall us as well, if we’re not careful.

So I mentioned at the start that today’s topic gets its name from an episode of Doctor Who way back in the first year of the Third Doctor, Jon Pertwee, who was on from 1970-1974, and Doctor Who and the Silurians was the second serial he did for 7 episodes. They and their nominal cousins, the Sea Devils, who appeared a couple years later on the show, both disappeared from TV for over a decade before making a reappearance but they did show up in quite a lot of the doctor who novels. As you probably know, classic Doctor Who had a very long hiatus off TV between Doctors 7 and 9, but there were a lot of books written during that period along with audio dramas, often set during prior doctors and also often starring or entirely narrated by the original actor.

You also get some lost classics or unproduced episodes like Shada by Douglas Adams, that eventually became his book Dirk Gently’s Holistic Detective Agency. There are hundreds of Doctor Who Audio Dramas and Audiobooks out there, including Doctor Who and the Silurians, our Audible Audiobook of the Month, and they are all available on audible, as are many novels from other amazing series that got canceled before their time, also often narrated by the original cast members, so you can continue the journey with them. Audible has thousands of such audiobooks and dramas available and literally centuries worth of content for you to pick from, and more being added every day faster than you could listen to all of it.

But they don’t just have audiobooks, they also have many excellent podcasts, such as Science & Futurism with Isaac Arthur, where we have every single episode on Youtube, plus several audio-only exclusives I’ve made over the years. That’s just some of the great content in the Audible Plus Catalog, which also has sleep & meditation tracks available, as well as guided fitness programs for getting into shape this summer, and Audible Original’s like Impact Winter, from the folks who brought you Pacific Rim and the Walking Dead. The whole Audible Plus Catalog full of free books and other content, comes as a bonus when you join Audible, in addition to your usual 1 free audiobook each month and big member discounts on additional ones, and as always, new members can try Audible for free for the first month, just go to, or text isaac to 500-500.

All right, that wraps us up for today and we’ll be back next month on July 17th for our next Scifi Sunday episode looking at primitive aliens and how we should interact with them, But between now and then we have plenty of other episodes, starting this Thursday with Interstellar Probes, where we’ll begin our two-part story of traveling to an anomalous system to investigate it, concluding with Life as a Planetary Explorer on June 30th. We’ll pause between those two episodes to look at the concept of Crawl-onizing the Galaxy, how humanity can still settle the stars even if we are limited to spaceships moving at less than 1% of light speed, and what that will look like. We’ll also have our livestream Q&A two weeks from now on Sunday, June 26th. Then it is on to July for a look at Deep Space Habitats, then Extragalactic Sanctuaries. If you want alerts when those and other episodes come out, don’t forget to subscribe to the channel and hit the notifications bell.

And if you enjoyed today’s episode, and would like help support future episodes, you can got to our website,, which we’ve recently relaunched new and improved, and check out our donate tab for ways to help, and all those other options are linked in the episode description, along with our social media forums like facebook, reddit, and discord, where you can chat with others about today’s episode and many other futuristic ideas. Until next time, thanks for watching, and have a great week!

2022-06-13 22:38

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