Thank you to Novium for supporting PBS We’re almost certainly the first technological
civilization on Earth. But what if we’re not? No. We are. But how sure are we, really? A favorite way to estimate the abundance of
life in the universe is the famous Drake equation, which figures that the number of intelligent
alien civilizations in our own galaxy who might overlap with us can be estimated by
multiplying a chain of factors, basically summarized as the number of habitable planets
times the probability of a planet forming life, then technological life, then factoring
in how long that civilization survives. We now know that there are billions of habitable
planets in our galaxy, but the other parameters are still extremely difficult to guess. And
that’s because we only have one example of the formation of life and civilization
to go off. And we have zero examples of a technological species going extinct, although
we’re on track to get at least one of those. If we detect just one more instance of life
or technology out there we’ll immediately be able to upgrade our guess at the probability
of any given planet reaching that stage of development. But you know what would increase
the probability even more? If we saw it happen twice on the same planet. There’s an extremely
loose piece of evidence, not even evidence, maybe just “teaser” that life may have
started independently twice on earth. There’s this zircon crystal found in the west of Australia
that contains a tiny speck of carbon that looks like it was filtered through a living
metabolism. It has the characteristic higher proportion of carbon-12 versus carbon-13 that
plants breath out. That speck of “biogenic” carbon was probably been perfectly sealed
in a zircon crystal probably for around 4.1 billion years old, dating it to before Earth’s
crust was probably re-liquified by massive asteroid bombardment that should have extinguished
all life. That’s a lot of probabilities, but this
is crazy given that the next earliest fossils– in Australia, and also in Greenland–date
to between 3.5 and 3.8 billion years ago after the Earth re-solidified following its pounding.
If so then abiogenesis–life arising from non-life–would have to have happened again.
And that’d mean that life really does start very quickly given the right conditions. We’d
have to update our Drake equation estimate of the probability of life forming given a
suitable planet. But all this is extremely speculative–and
one reason is that it’s really hard to verify the origin of life is because these fossils
are extremely scarce. Greenland and that bit of Western Australia are the only fragments
of land still left on the surface from that time. The entirety of the rest of the crust has been subducted–pulled
back into Earth’s mantle by tectonic activity–not once but many times. The crust is recycled
roughly every half billion years, meaning much of the early fossil record has been deleted,
which makes it extraordinarily difficult to track the early rise of life. But if the geological record is so crappy,
then in half a billion years Earth’s new inhabitants will have precious little evidence
that we ever existed. So could we even know that we are the first technological civilization on Earth? And if
we’re not the first, that would also have huge implications for our Drake equation, forcing us to update
both the probabilities of technological civilizations arising from life, and also our lifespan estimates
for those civilizations. I know this sounds like some lizard-illuminati rubbish, but
one effort to properly address an out-there proposal like this is a paper by NASA climatologist
Gavin Schmidt and physicist and astronomer Adam Frank from a few years ago. They propose
the Silurian hypothesis, which asks whether pre-human industrial civilizations might have
existed. The name is an old-school Doctor Who reference. I’ll leave you to google
that to see what we’re actually looking for. Let me be quite serious for a moment: I don’t
think such a civilization existed. Schmidt and Frank don’t think such a civilization
existed. The burden of proof of such a claim is massive, and massively on the claimant,
and currently there is no real evidence. The actual purpose of this video and of the
Silurian hypothesis is to frame a very clear scientific question around a very speculative idea. So let’s do just that. Broadly that question would be: Could a non-human industrial civilization
have existed before us even given our current lack of evidence of such? And if so, how long
ago would it have had to have existed in order to be completely erased by now? And finally
what traces of such might still exist that we could now look for? But first, to start thinking about what evidence
a past civilization may have left behind, let’s think about what our civilization
will look like in the far future. Our species has been around for 300-ish thousand
years, but has only had a major impact for the past 10-ish thousand years of the Anthropocene
era–by definition–it’s named as the era of our major impact. And that impact has
massively accelerated in the 300-ish years post-industrial revolution. When you look at the world now, you might
think that our constructions are the most telling signs of our presence once we’re
gone. But that won’t be the case for long. The pyramids may have lasted millenia, but
they won’t last millions of years. All of our great cities will erode to dust, be covered
up or washed away or end up as deserts, then ocean floors then deep sedimentary layers
then mountains then deserts again. Much will also be pulled beneath shifting tectonic plates–subducted.
It’ll be reforged into new rock in the mantle–its geological memory almost completely wiped
before it reemerges, if it ever does. So will any sign of these works be apparent to a far future? The recycling of the mantle means that anything
older than half a billion years is pretty much gone. But we do find plenty of fossilized
dinosaur bones of dinosaurs that are 100s of millions of years old. So surely we’d
find remnants of a civilization that’s only millions of years old. But that is not necessarily the case, because of the potential limited extent of the civilization, both spatially
and temporally. We have of course obtained samples of the
Earth’s surface from many millions of years ago. Some of those areas are still exposed,
but most we reach by digging or drilling. But here’s the thing, we’ve accessed only
a tiny fraction of a percent of the original Earth surface from before the Quaternary period,
2.6 million years ago. And modern urban land coverage is less than 1% of Earth’s surface
today. So, if a civilization just like us existed a few million years ago, it’s extremely
unlikely that we, today, would have stumbled on their physical remnants. That includes
cities and artifacts, but also fossils. The fossilization rate is so low that we’ve
only discovered a small fraction of the species that ever existed. Remember that the dinosaurs roamed the Earth
from around 240 to 65 million years ago. They’ve been gone for a third of the time they were
around. And across those 180 million years we have just a handful of specimens. By comparison,
our 10,000 years of civilization is barely a blip in the fossil record, our industrial
era not even a blip. We can find T-rexes, but we can’t find T-rexes from a particular
few centuries. I’ll elaborate on the extent of the Anthropocene
blip: our best way to trace geological time is in sedimentary rock. This is rock that
originally formed from the precipitation of dead ocean things onto the ocean floor, building
up over millions of years. From sedimentary rock that has since been exposed, or from
deep cores drilled from the oceans, we can analyze these layers to track changes in ocean
chemistry, temperature, and biological content from these layers, which in turn reflect those
same properties on a global scale. The entire Anthropocene will be represented
by a layer only several centimeters thick in kilometers of sedimentary layers, with
the industrial age a proportionally tiny fraction of that–perhaps millimeters at best. Although a far-future civilization probably
wouldn't find our bones or artifacts, they might find and recognize this wafer thin layer
in the geological record. As our structures and cities grew, so did our broader footprint.
“Anthropocene” is the word for the geological era dominated by human activity. It’s not
quite textbook yet, but probably will be soon. Geological eras are periods of time reflected
by distinct changes in the geological record. The Anthropocene certainly qualifies. In fact,
the geological marker currently being laid down in new sedimentary layers due to our
activities bears an eerie resemblance to some of the transitions in the geological record
of millions of years ago. So to assess whether any of those previous
events might be connected to industrial activity, let’s take a look at the specifics of how
we’re going to confuse future geologists and paleontologists with the crap we’re
sprinkling over the surface of the Earth that are already seen in the newly-laid sedimentary
layers. First there are the more direct chemical and
isotopic imbalances due to us making a bunch of weird stuff. Industrial pollutants like
heavy metals and chemicals such as CFCs and their long-lived byproducts. Consumer waste
like rare earth elements and plastics. Nitrogenous fertilizers and even steroids from large-scale
farming. Radioactive isotopes deposited globally from nuclear weapons testing that will last
tens of millions of years. Then there’s the effect of habitat destruction and species
extinction, which won’t be so much a distinct layer in the geological record as a sudden
drop in the markers of biodiversity. But our main geological impact is, of course,
climate change. Remember that speck of biogenic carbon in the Aussie crystal? This is more
than a spec. Over the industrial revolution, around half a trillion tons of carbon that’s
rich in the C-12 isotope has been pumped into the air. This is increasing the ratio of C-12
to C-13 in the atmosphere, which is then reflected in the soil and oceans, and ultimately in
the sedimentary layers currently being built. The rise in temperature associated with this new atmospheric carbon will influence the geological record in a number of ways. Increased rainfall and rising sea levels increase
erosion even further. Warmer oceans give up more of their dissolved CO2, increasing their
acidity; this and the temperature change itself alters the fauna that can live in it and ultimately
layer on the ocean floor. There are many other known and unknown effects beyond these. So from all this it sounds like it’ll be
easy for a future civilization to see where we scrawled “we was here” in the geological
record. And if so, we should be able to find a prior one. Except that so many of these signals can be duplicated by natural phenomena. We’ll come back to those natural explanations, but
first let’s see if there are any interesting past geological markers that look anything
at all like the one we’re currently forging. Let’s go hunting for pre-human technological
civilizations. Our geological record contains several candidate
“weird layers” that also represent massive environmental shifts. Some we understand well–like
the Cretaceous–Paleogene boundary, which is connected to a dinosaur-hating asteroid
hitting the Earth. But some still don’t have broadly accepted
explanations. And here we get to the Silurian hypothesis, which asks if any of these as-yet-unexplained
abrupt shifts in the geological record could have been due to an industrial civilization. Schmidt and Frank focus on two broad types,
which have a lot overlap. We have hyperthermals–largely in the Eocene from 56 to 34 million years
ago. These are characterized by rapid increases in global temperatures and are often accompanied
by significant shifts in carbon isotope ratios. The latter suggest that the temperature shift
is due to a rapid injection of CO2 from burning of some organic fuel. We are triggering a
hyperthermal now, but could some of the past hyperthermals also have been triggered by
something like us? Not impossible, but stand-by for the debunkings. Then we have ocean anoxic events, a few of
which are found in the earlier Cretaceous and Jurassic periods. These are characterized
by a rapid drop in the oxygenation of the oceans, accompanied by a great dying of ocean
life. We’re seeing the beginning of a potential “OAE” right now. As in our case, the ancient
OAEs are often accompanied by shifts in the CO2 isotopic ratios, and so could also have
been triggered by climate change. Going back further, there have been several
abrupt shifts since around 500 million years ago involving combinations of the signals
I’ve already described. And the further back we go the harder it is to pin down the
definite cause, so there’s more and more room for our flights of fancy about dinosaur
empires. The biggest challenge to assessing the Silurian
hypothesis is that natural climate shifts can duplicate so many of the current anthropogenic
signals–from the changes in temperature and the carbon situation to the different
types of minerals being deposited on the ocean floor due to changes in ocean life. And there
are many potential natural causes for climate change–for example, the periodic shifts
in Earth’s orbit characterized by the Milankovic cycles, which we covered once upon a time,
and which are correlated with past climate shifts. And if a signal is not caused by the climate
change itself, then it might be share a cause with that climate change. Spikes in heavy
metals or rare earth elements can come from technology manufacturing, or be produced by
volcanic eruptions which spew vast quantities of minerals and metals into the atmosphere
and oceans. Radioactive isotopes could be due to a nuclear program or due to a nearby
supernova explosion. Layers of soot and particulates in sediment could indicate combustion processes
from an ancient industry, or come from widespread wildfires or an asteroid impact. These can
send soot and dust into the upper atmosphere, eventually settling around the globe. And
more importantly, can trigger a shift in the climate if enough CO2 is released. There’s
good evidence that the Paleocene-Eocene Thermal Maximum–the first and greatest hyperthermal
of the Eocene, was caused by a giant magma zone intruding on a giant fossil fuel bed.
And these sorts of catastrophes and the associated climate change can also trigger the type of
mass extinction that we expect to accompany our own geological book-end. That said, there are some things we’re doing
to our own geological layer that would be hard to explain naturally. Some of the long-lived
synthetic chemicals really have no known natural source, like some of these industrial fluorides.
There are things like the chirality of molecules–the left- or right-handedness of their symmetry–that
is strongly one way in nature, but random in our industrial production. I should also point out that past sudden climate
shifts appear to have been significantly less sudden than our own. So if we found a shift
that occurred on the timescale of centuries rather than 10s of thousands of years, that
might be an indication. But it’s extremely difficult to get that sort of time resolution
in the geological record. In general this points to a bit of a paradox: a civilization
which perished due to climate change would leave such a narrow record it might be invisible.
On the other hand, a much longer lived and so presumably environmentally sustainable
advanced civilization might produce minimal ecological disruption over millenia, and so
would similarly be almost invisible. In any case, distinguishing between these
artificial and natural scenarios requires extremely careful analysis of the context,
distribution, and composition of these markers and their association with each other, all
while piecing together clues that could span geological epochs. So have we detected signs of industry in ancient geological transitions? Nope. But our search has also been limited. And this is the true
value of the Silurian hypothesis, as Schmidt and Frank themselves emphasize. It’s not
to propose pre-human industrial civilizations as a likely explanation for past geological
events, rather its to refine our understanding of what to look for if we wanted to find evidence
of such. And although it seems extraordinarily unlikely,
the implications of discovering such a pre-human civilization would be impossible to overstate.
Both for our sense of our own place on this planet, for our understanding of the likely
abundance of civilization in the universe. It can even guide us in looking for past life–even
past civilizations on other worlds, starting with Mars. But most importantly, discovering
an extinct people who reached the same or higher level of advancement as ourselves would
give us a stunning new perspective on our own future and a profound reminder of our
fragility. Perhaps enough of a reminder to allow us to avoid that hypothetical predecessor’s
fate, and avoid becoming just another local geological blip that had once dreamed of exploring
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