From the Vault: The Silurian Hypothesis

If an advanced civilization existed on earth several million years ago, would we actually be able to detect that they were here? It may sound like conspiracy theorist bait, but a serious and balanced look at the question reveals a lot about geology, climate and the search for alien life. In this classic episode of the Stuff to Blow Your Mind podcast, Robert and Joe discuss the silurian hypothesis. (originally published 01/18/2022)

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Episode Transcript

Speaker 1 (00:05):
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

(00:28):
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

(00:49):
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

(01:11):
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

(01:34):
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

(01:56):
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,

(02:18):
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

(02:40):
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

(03:01):
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

(03:22):
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

(03:44):
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

(04:05):
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

(04:29):
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,

(04:49):
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,

(05:10):
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

(05:33):
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

(05:55):
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

(06:18):
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

(06:40):
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,

(07:00):
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

(07:23):
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

(07:45):
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

(08:05):
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

(08:27):
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

(08:49):
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

(09:10):
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.

(09:34):
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

(09:55):
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

(10:15):
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

(10:36):
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

(10:57):
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

(11:18):
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,

(11:39):
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

(12:03):
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

(12:23):
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.

(12:47):
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

(13:09):
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.

(13:33):
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

(13:53):
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

(14:14):
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

(14:36):
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

(14:59):
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

(15:20):
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

(15:42):
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

(16:03):
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

(16:24):
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.

(16:48):
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

(17:09):
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

(17:32):
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

(17:54):
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

(18:17):
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

(18:38):
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

(19:01):
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

(19:22):
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

(19:44):
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

(20:05):
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

(20:29):
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

(20:52):
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.

(21:15):
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,

(21:36):
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

(21:58):
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

(22:19):
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

(22:40):
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?

(23:02):
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

(23:23):
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.

(23:45):
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.

(24:07):
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

(24:27):
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

(24:49):
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

(25:11):
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

(25:32):
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

(25:53):
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

(26:15):
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

(26:37):
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

(26:59):
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

(27:20):
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

(27:41):
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

(28:04):
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

(28:25):
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

(28:47):
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

(29:10):
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

(29:30):
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

(29:52):
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

(30:13):
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

(30:35):
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

(30:56):
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,

(31:18):
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

(31:39):
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

(32:00):
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

(32:23):
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

(32:45):
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

(33:06):
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

(33:26):
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

(33:48):
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

(34:13):
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

(34:33):
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,

(34:55):
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

(35:18):
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

(35:43):
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,

(36:05):
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

(36:25):
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.

(36:47):
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

(37:12):
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.

(37:33):
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,

(37:55):
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

(38:17):
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

(38:37):
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,

(39:02):
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,

(39:22):
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

(39:45):
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

(40:08):
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,

(40:29):
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.

(40:53):
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

(41:14):
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

(41:34):
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

(41:55):
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

(42:18):
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,

(42:44):
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

(43:06):
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

(43:28):
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,

(43:53):
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,

(44:14):
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

(44:37):
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

(44:59):
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

(45:20):
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.

(45:42):
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

(46:05):
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

(46:25):
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

(46:47):
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

(47:09):
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

(47:29):
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.

(47:51):
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

(48:12):
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

(48:34):
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

(48:55):
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

(49:17):
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

(49:37):
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

(49:59):
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

(50:19):
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

(50:40):
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,

(51:01):
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

(51:22):
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

(51:44):
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

(52:04):
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

(52:26):
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.

(52:50):
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

(53:10):
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

(53:32):
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

(53:53):
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.

(54:13):
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

(54:34):
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

(54:56):
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,

(55:17):
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.

(55:40):
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

(56:02):
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

(56:22):
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

(56:42):
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

(57:04):
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,

(57:26):
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

(57:46):
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
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(58:07):
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