November 7, 2018 • 43 mins
The Great Filter hypothesis says we’re alone in the universe because the process of evolution contains some filter that prevents life from spreading into the universe. Have we passed it or is it in our future? Humanity’s survival may depend on the answer. (Original score by Point Lobo.)
Interviewees: Robin Hanson, George Mason University economist (creator of the Great Filter hypothesis); Toby Ord, Oxford University philosopher; Donald Brownlee, University of Washington astrobiologist (co-creator of the Rare Earth hypothesis); Phoebe Cohen, Williams College paleontologist.
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Episode Transcript
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Speaker 1 (00:00):
What exactly does figuring out if aliens exist have to
do with the end of the world. Well, it turns
out that the odd emptiness that we find in the
universe can give us clues about what may have gone
on or didn't go on before we humans came along.
Did something bad happen? And if so, might it happen
(00:22):
to us two? Looking around for signs of whether we're
the only intelligent life to have ever evolved can help
answer those questions. And when you look at it, the
universe does seem amazingly large for Earth to be the
only planet with life on it. Consider it like this.
(00:43):
Let's say that you're a photon, a tiny packet of light,
and one day you had the wherewithal this set out
to travel across the universe. You would find that, perhaps
to your great surprise, such a trip would take you
around fifty billion years. Yes, you, light, which can travel
at the speed of light, would take fifty billion years
(01:05):
to cross from one side of the universe to the other.
At least that's how long it would appear to take
you to as humans. And this is just the observable universe.
The amount of the universe that light like you has
had time to travel across since the Big Bang. Within
that vast space, there are anywhere from one billion to
two trillion galaxies by our current causes, at least of
(01:27):
which our own Milky Way is on the larger side
of the spectrum. There are larger but there are also
a lot of smaller ones too. Within these billions or
trillions of galaxies are billions and billions and billions of stars,
and probably exponentially more planets. The total number of planets
and stars in our universe, the total number of places
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for life to exist, is mind bogglingly large. And so
you packet of light or wavelength, depending on your mood,
might think to yourself, as you traveled across the verse
and saw that the Earth is the only planet that
is home to intelligent life. Out of the attend to
the who knows what power planets that could host life,
(02:09):
You might think to yourself, in your little photon voice,
what waste are we alone in the universe? And if
we are alone, why? These are the questions at the
heart of the Fermi paradox, and they continue to nag
at us. The answer is plainly obvious if you look
at it, but it depends on how you look at it.
(02:30):
With the Fermi paradox, the same thing can look very
different to any two people. And it's not just the
paradox itself. Even the evidence is equally ambiguous. It's all
like one big contradem words that have two meanings that
are the opposite of one another, like how weather can
mean both to wear away and to withstand something. The
(02:51):
size of our empty universe can mean that we are
both alone or one of many. Another example of this
ambiguity is the very presence of life on Earth, something
that people who believe that we're not alone in the
universe point to. His evidence is that life here on
Earth seems to have emerged the first chance that it had.
(03:12):
The famous astronomer and science writer Carl Sagan was one
of those people. He was an optimist when it came
to the family paradox. He believed that life was out there,
we just hadn't found it yet. Sagan pointed to evidence
from the fossil record that here on Earth life began
as early as five hundred million years after the Earth form.
(03:32):
It was almost like it was waiting to emerge, and
since it emerged quickly here on Earth, it stands to
reason that life should emerge wherever it gets the chance,
anywhere in our universe. When you take into account the
idea that there are perhaps three hundred billion stars in
the Milky Way alone, even if some small fraction of
those have habitable planets that could host life, then we
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should expect to encounter it sometime soon as we spread
out to explore the country side around planet Earth. But
there's a problem with basing our view of the rest
of the universe on our own existence. The idea that
we can gain insight into our universe from our existence
is called the anthropic principle, and it's vulnerable to a
logical fallacy called selection bias. Being the only intelligent life
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in the universe, we're the only data point in our
data set, and so we tend to skew the results
a little bit. It's hard to resist the temptation of
cherry picking the data when there's only one cherry. Yes,
of course, life can arise. Our very existence proves that fact.
But what it does not prove is that the emergence
(04:40):
of intelligent life or any life, really is easy or inevitable.
What if, instead, life emerging in our universe is really,
really really hard. Perhaps the existence of living, breathing, intelligent
things here on Earth doesn't show the emergence of life
is inevitable. Perhaps as shows that it was the singularly
(05:01):
most unlikely event in the history of our entire universe.
If you could crack open a strand of your DNA
and read the pairs of adenine, guanine, cite of scene
and dimin the ones and zeros of your genetic code,
(05:21):
you would find a history of life on Earth written
into it. Not only does your DNA contain the blueprints
for making a full version of you, but if you
look at it correctly, it also bears the marks of
those who have come before you. You'll find your parents genes,
of course, and their parents. But as you go further
back in time, you'll also find the contributions of all
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of the animals in bacteria that ever reproduced along the
last several billion years to form a connected chain of
life that eventually led to you. But you'll find that
you run into a wall the further you go back.
There's a point beyond which we can no longer read
the taves of our DNA. It ends right before we
get to the emergence of life here on Earth the
(06:05):
very beginning. That is to say, no one is sure
how life began as it stands now, the general consensus
among sciences the concept of a biogenesis, that life emerged
from nothing nothing living anyway, let's go back to the
early Earth. About five million years after it formed, the
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surfaces just begun to cool enough that solid clay ground
has begun to form, and with the cooling off, the
muggy atmosphere cooled as well, condensing in the rain that
began to collect in pools which would become the peatree
dishes where life took its first steps. But to get
from here to life, something extremely unlikely had to happen.
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Plain old, lifeless molecules had to spontaneously arrange themselves into
new forms that became the building blocks of life. At
this point, there was nothing but raw materials on Earth,
and we have to get from here to living organisms.
That means that not only do you have a functioning organism,
it has to carry with it the encoded instructions to
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make a copy of itself, and some way to read
those instructions and actually make that copy. It's like the
idea of putting some plastic pellets and metal shavings into
a bucket and expecting an autonomous three D printer to
form from it, or more to the point, it would
be like the idea of rotting meat growing maggots. For
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a long time, humans thought that this kind of thing
spontaneous generation, was how some life arose. Prior to when
science took up the mantle of explaining our universe, people
relied on their everyday observations to explain occurrence is like
maggots growing on rotting meat, seeming to appear out of nowhere.
It seemed just as likely as anything that maggots could
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spontaneously generate, or baby mice from grain, which was another
common folk belief. But eventually scientists figured out a way
to disprove this idea, which really gained support when we
realized that tiny, unseen life lived around us everywhere and
that it had a hand in a lot of the
things that we saw. Germ theory was developed and the
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concept of spontaneous generation was abandoned. That is until the
nineteen fifties, when researchers started working hard on figuring out
how life on Earth might have come about. Spontaneous generation
made an unexpected comeback. A biogenesis holds that quadrillions upon
quintillions of simple molecules present in Earth's early atmosphere and
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oceans randomly configured themselves into a mind bending number of
different combinations. Some of these abominably large number of combinations
happen to create useful complex molecules like amino acids, which
are the precursors for proteins. Now, it's one thing to
form simple molecules to randomly combined to form more complex molecules,
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but there is still the issue of replication. Ship If
those molecules don't make a younger copy of themselves, that
innovative chemical chain is broken and the new molecule loses
the chance to continue to evolve in a new and
even more complex things. And this is the point where
science is currently stuck. Somehow, they say, some of those
molecules managed to form a stable string of nuclear times,
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probably r N, a ribonucleic acid which is capable of
doing two very important things. It can encode information in
its nuclear tide chain, and it can also transcribe those
nuclear tides to produce proteins. And once you have proteins,
you can do all sorts of things that supports life.
Back in two organic chemists Stanley Miller and Herald Urray
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saw it to show that this was possible by recreating
the conditions of earlier Earth. In a flask. They simulated
a primitive ocean and built an atmosphere out of the
gases thought back in the fifties to have been present
on Earth soon after it formed. They mimicked why in
storms with the flickering electrical current, and when Miller inspected
the broth that resulted, he found that nineteen amino acids
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and amans, the precursors to proteins, had assembled spontaneously. It
seems that Miller had shown that when the conditions were right,
the foundations for life would arise. But lately the idea
that a biogenesis just happened randomly is falling out of favor. Instead,
some scientists have begun to suspect that there is some
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set of organizing principles that serves as a driving force
for life to emerge. Just like how gravity will draw
a ball downhill, or how magnets will always repel or
attract one another when they're close together, there is some
fundamental governing force of nature that causes life to assemble
along predictable lines. We just haven't figured out what that
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force or those lines are yet. This is a pretty
surprising idea if you think of it. One of the
laws of the universe, the second law of therm mom dynamics,
is that things tend towards disorder, not order. The idea that,
when presented with the right conditions, dead molecules in the
universe will organize themselves into something living and breathing runs
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totally counter to that, and this new view also includes
evolutionary biology as one stretch of these organizing principles of life.
So the idea is that molecules arrange themselves into self replicating,
metabolizing parts that form increasingly complex beings that eventually include
you and me, which makes you wonder what the end
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point is. To some people, the idea that life arose
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from simple dead molecules that just happened to randomly assemble
themselves in the living things is just too unlikely to accept,
And even if we do accept that this is precisely
how life arose on Earth, the idea that it could
ever happen again anywhere else is too improbable. That virtually
proves that we humans are alone in the universe. One
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issue people raise is time. They say that Earth just
simply hasn't been around long enough for all of that
random chemical trial and error to have taken place. The
idea that life organizes along some unknown universal principles definitely
addresses that idea of time, and so does pant spermia.
Pant Spermia is a concept from astrobiology, and it says
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that the seeds of life are all over the universe
in abundance everywhere. They can be found on board asteroids
and other celestial objects, and that these seeds of life
are constantly bombarding planets all over the universe. If the
conditions on the planet happen to be right, well, then
those sea needs of life will germinate and grow into
something new and living. This certainly addresses the issue of time.
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Life could have evolved elsewhere in the universe, which is
billions of years older than Earth, and then spread to
our planet aboard an asteroid. We've recently found that some
chemical precursors to life can be found on celestial objects
like asteroids, and that they're able to survive re entry
into an atmosphere, which can get pretty hot. This is
important because it's widely accepted that an atmosphere is a
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precondition for life to emerge. If you take pants bermia,
and you take the recent view that life follows some
organizing principles as immutable as the laws of physics, then
you arrive at a conclusion that Earth is just another
place that happened to have the right conditions when a
rock bearing the precursors to life landed around four billion
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years ago. In this view, then of course life is
abundant in the universe. But then we find ourselves right
back to where we started. Where is everybody? Perhaps the
best answer to that comes not from an astrobiologist or
an astronomer, but from an economist named Robin Hansen, who
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proposed that there must be something, some incredibly difficult step
between the point where dead matter forms life and the
point where intelligent life becomes a galactically colonizing civilization that
no species has ever been able to overcome. He calls
it the great filter. Every piece of matter in the
universe is the sort of thing that could have started
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that process, started life, and then advanced life, etcetera. But
so far nothing out there has done that. So the
great filter is whatever is in the way, whatever makes
it hard for any one piece of ordinary dead matter
to produce expanding, lasting life. There's surely a countless number
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of steps along the path from dead matter to the
emergence of a galactically visible civilization. But the Great Filter
high Path says, supposes that a handful of them are really,
really hard, and that one of them in particular must
be so hard that is thus far prevented any life
from reaching galactic proportions. This is Oxford University philosopher Toby
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ord if there were a hundred pieces you needed to
get into the right order in order to create something
that obeyed natural selection and and was it the basic
level needed to actually bootstrap up towards complex life, then
there are a hundred factorial ways you could arrange those pieces.
That's that's more than uh tends to the power of
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a hundred different possibilities. And then it just turns out
you need an incredibly rare event to get there. The
Great Filter offers two possible solutions to the question posed
by the Fermi paradox. Whereas everybody everybody never existed, or
everybody is dead, if the Great Filter is in our past,
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it says pretty strongly that we are the first and
only intelligent life that exists in the universe. If that's true,
then by the Great Filter, there is something, some step,
some right of passage you could call it, that has
prevented every other life from reaching the point that we're at.
And if that's the case, then we are the only
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life to have made it through the Great Filter. That
means that we can be optimistic about our future. We
made it through that right of passage that has kept
every other attempt at life from evolving. It means our big,
vast universe isn't wasted on us. It's just waiting there
for us to use it, and guilt free too. Since
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no one else is around to use it, we have
a responsibility to put it to use. You could even say.
But there's another possibility to the Great Filter too. It
may also lie just ahead in our future. Maybe intelligent
life is a dime a dozen in the universe. But
the reason we don't see other civilizations is because they've
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all died off. And if all of those intelligent civilizations
all died before any any of them could make it
off of their home planet and spread throughout the universe,
the Great Filter is a big red flag for us.
It tells us that we should expect to meet the
same fate that every other intelligent life has. The Great
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Filter will spell the end of the world for us too.
To answer the question of whether we face imminent doom
or not, we have to look at the evolution of life,
and to do that we have no choice but to
turn to the one place where we know life evolved.
At the risk of falling victim to the selection bias,
we have to look to Earth for clues. We've already
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talked about how utterly improbable the origin of life appears
to have been, but it's probably best to start looking
even further back than that. If life emerged on Earth,
that means that the conditions were right for life on Earth.
That's the one day to point we have ipso fact, though,
so the best way to find out what conditions life
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requires is to look at the conditions of our planet.
And it turns out that Earth has some spectacularly peculiar
characteristics that make it ripe for life. So peculiar, in fact,
that a pair of researchers named Peter Ward and Donald
Brownlee rolled all of them up into what they call
the rare Earth hypothesis. It's just what it sounds like
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when you add up all of its peculiarities. Earth is
not like other planets. First, Earth happened to form around
the right kind of star. Our son is a main
sequence star, which means it produces light and heat by
fusing hydrogen into helium. Main Sequence stars aren't rare. They
make up about nine of stars. But our Son also
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happened to be of the right size too. It's not
so large that it will use up its fuel in
just a few billion years. That means that the Sun's slow,
steady burn would go on long enough to give life
time to develop. The Sun also isn't too small to
support life either. It is you could say, just right
for life. Our Sun has also placed in a really
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great spot in our galaxy. We happen to be located
out in the country, in a bit of a backwater
when it comes to the Milky Way, about twenty eight
thousand light years from the galactic center, in a galaxy
that's one thousand light years across. For decades, study presumed
that the center of the galaxy would be the most
happening spot. That's where the most stars tend to be,
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and more stars would be more potentially habitable planets. Civilizations
that arose in the center might even be in contact
with one another, forming something of a galactic urban area.
But recently it's become clear that the galactic center might
not be so flourishing. After all. There are more stars, sure,
but more stars also means that there are more collapsing stars,
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which release bursts of energy that can burn away the
atmosphere of any planets in the vicinity, So the galactic
center might actually be less of a bustling urban area
and more like sterile and dead. Our Sun is pretty
far away from the galactic center, way out past the
suburbs where nothing much happens. In other words, that's good
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for life on Earth because it means that it cuts
down on the number of sterilization events as life developed
over Earth's history, giving it a good chance of succeeding.
And Earth just so happens to be located within the
Sun's goldilocks zone that I mentioned in the last episode.
If it was a little nearer the Sun and about
one and a half million kilometers closer, it would be
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too hot to support life, and much further away Earth
would be too cold for it. It also turns out
that where Earth is positioned in the Solar System is
hugely important to giving life a fighting chance. Earth is
the third rock from the Sun, with five others between
us and the rest of the galaxy. Six if the
math that suggests there's a planet nine out there turns
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out to be correct, or if you continue to count
hapless Pluto, the Sun and the planets in our Solar
System formed from a massive cloud of cosmic dust, that
same stuff that might have blown up so many alien
pilots traveling between the stars. We still aren't quite sure
exactly how the planets formed. One model astoundingly suggested that
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they could have formed in as little as a thousand years,
but the birth of the Solar System was likely a
free for all grab of the elements that make up
the planets today. Initially, astronomers presumed that the planets formed
in their current arrangement, but lately it's become clear that
that probably wasn't the case. The planets may have actually
moved around and migrated from one spot to another in
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the early life of the Solar System before settling into
the arrangement that we see them in today. If that's correct,
then we were astoundingly lucky that Jupiter ended up where
it didn't. Jupiter is a gas giant made up largely
of hydrogen and helium with a metal and ice core.
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It's like a Sun that never started burning because it
lacked the mass needed for gravity to kick start fusion.
But Jupiter is massive, truly, you could fit around within it.
It's the most massive planet in our Solar System, and
because of its mass and its position between Earth and
the chaos of the interstellar space outside of our Solar System,
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Jupiter acts as a huge defensive guard for our planet.
When asteroids or comets or other flotsam and jetsam bent
on destruction in oer our Solar System, Jupiter's extraordinary gravitational
pull draws them into its orbit and slingshots them out
back into space. Without Jupiter to run interference for Earth,
it would have a steady diet of life ending bombardments
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from space. So thanks Jupiter. This is astronomer Donald brown Lee.
He's one of the guys who came up with the
rare Earth hypothesis. Jupers are actually pretty rare uh planets
and um uh so uh in terms of rare Earth.
You know whether it was juper is good or bad.
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Typical planet as systems probably don't have jupers. Our moon, too,
seems to have played a number of factors and fostering
life on Earth. Our moon's pretty unusual itself as far
as moons go. It's enormous. It's about one point to
percent the mass of the Earth and doesn't sound like much.
But other planets moons like Phobos and Demos, which or
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a bit Mars, are closer to asteroids in size. Our
moon more resembles a small planet, and because it's so big,
it has some very peculiar effects on Earth. For one,
it stabilizes our planet. The Earth doesn't sit upright as
it spins around on its axis. It's tilted actually at
about a twenty three degree angle. Because of this tilt,
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we have seas which create predictable variations in the temperature
of regions on Earth over the course of the year.
So when you think about the difference between winter and summer,
you get a good idea how much variation a tilt
can create. And add more of an angle to the
tilt and the Earth and the temperature variations would become
more severe. Since during winter or hemisphere would be further
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away than it is now from the Sun and much
closer in the summer. And if you added in some wobble,
like if the tilt of the Earth wasn't stable and fluctuated,
all of these wild swings and temperature could make it
very difficult for life to take hold. The Moon's mass
actually exerts gravity over Earth and keeps it stable, not
wobbling or tilting more than it does, and allowing for
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a nice, gradual, predictable and not two varying seasonal temperature
shifts that we even have a moon appears to be
a fluke itself. The current view is that the Moon
was calved off from Earth following a head on collision
in the Earth's early history with a planetoid about the
size of Mars called Thea, which the Earth likely absorbed.
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This giant impact hypothesis explains a lot. The Moon is
unusually large and unusually close to us because it was
created of a mixture of Earth and that fateful planetoid.
Because the Moon is so close to Earth, it's tidally
locked and orbit around us. It doesn't spin on its axis,
which is why we always see the same side of
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the Moon as it orbits us. This is an important
feature because it also creates the tide you're on Earth.
As the Moon orbits Earth, it pulls the oceans toward it,
stretching them out on the ends and narrowing them in
the middle. We hear on our planet experience this as
low tide or high tide, depending on where you are.
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And it was in these tidal pools of young Earth's
oceans where some people think life began. When those ancient
tides came in, they deposited a flood of molecules into
the tidle pools, and as they withdrew and the water
evaporated in the sun, the increasing concentration of salt could
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have provided just the right laboratory conditions where those earliest
chemicals could combine. Without a moon as large and as
close as ours is, these tides could not have existed
on Earth, and those early proteins would have lacked that
kind of natural peatrie dish. So the moon itself is
a collection of exquisite coincidences that supported life on Earth.
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But perhaps the most peculiar aspect of Earth is that
it has massive plates that make up the crust, which
slide along a molten bed underneath that. It, in other words,
features plate tectonics. It thought that this is a major
reason why the Earth has actually had an amazingly stable
climate for most of you know, temperatures for for most
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of its uh age. So it's drastically different than Venus
or or Mars, which are famously unstable over tri lage
uh time scope. So so we really owe a lot
uh to this. We don't really know why I played
tectonics works on our on Earth. It doesn't work on Mars,
it doesn't work on vines, it doesn't work on mercury,
(27:12):
doesn't work on any Yeah, I mean they're there, are
you know, movements of rocks rolled to each other, but
not I played tectonics of the type that we that
we have here, So so we think it's really important.
As far as we know, Earth is the only planet
to feature plate tectonics, which actually is a massive thermostat
for the planet. When oceanic plates slide underneath the lighter
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continental plates, massive amounts of the oceanic crusts is crushed
and absorbed into magma. The rock in those oceanic plates
contains huge stores of carbon dioxide, so when volcanoes release
magma from beneath the crust, it also contains some of
that CEO two, which travels as a gas up into
the atmosphere and there it hangs around and it absorbs sunlight,
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which in term warms the atmosphere and eventually the planet below.
And when the planet warms, more of the oceans evaporate,
warming the atmosphere even more. When you have a warm,
wet atmosphere, you have lots of rain that rain brings
dissolves c O two back down to Earth. Rocks on
Earth are an excellent store of carbon, and when CO
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two rich, rain falls on them as the knock on
effect of weathering them, meaning the wearing down definition, not
the opposite one. All of that carbon in the rocks
and rain makes its way to the sea, where it
dissolves and eventually sinks to the ocean's bottom. As more
carbon is removed from the atmosphere and stored, the planet
begins to cool again. Over time, the carbon at the
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bottom of the ocean forms new oceanic plate crusts, and
eventually it will find itself along an ocean ridge where
it collides with the continental plate and the whole process
begins again, and the Earth starts to warm once more.
This unique property of Earth has kept the planet's climate
stable more or less constantly for more than four billion years,
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which has allowed life on Earth to grow and flourish.
When you take all of these details together, a weird
picture of Earth emerges. It's almost freakishly perfect for life.
That's so many different variables, each at just the right temperature,
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just the right location, heat time, whatever would come together
to form a stable whole seems to make Earth a
staggeringly improbable place. You couldn't ask for a better place
for life to emerge. And maybe that's the point. Maybe
the seeds of life are commonplace in the universe, but
Earth is unique. We simply don't know. We still know
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so little about space, our galaxy, and the universe that
we can't say if Earth is freakishly unique or one
of many. And as we get better at deducing the
existence of habit of planets with our space telescopes, they
seem to pop out of the cosmos like a magic
eye poster does when you lose focus in just the
right way. Maybe planets that can sustain life are more
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abundantly the universe than we realize perhaps assembling those seeds
into life is where the filter lies. The farther back
we look in time, the less we understand. So if
you're going to look for something that might be really hard,
harder than it seems, farther back in time is the
plausible place to look, because that's where we don't understand things. Uh,
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And the very first step from completely dead matter to
some proto life that has to be the earliest step
on the one we have the least knowledge of, and
more plausibly it's the hardest step. Or perhaps not. Perhaps
right now the universe is teeming with primitive life. Perhaps
the great filter lies somewhere after that. There were, after all,
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some enormous steps to get from the emergence of life
here on Earth in the moment we're sharing between us
right now, m back on the early Earth, tucked away
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just so, in a solar system, tucked away just so,
in the galaxy with its volcanoes chugging away at producing
a warm atmosphere and its oceans producing a warm liquid
medium for molecules to organize into life. At some point,
one particular moment in Earth's history, all of those separate
components came together to form a full fledged living cell.
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As far as we can tell, this moment happened around
three point eight billion years ago. At first, these simple,
single celled organisms were nothing more than gooey bags, with
the parts for replicating themselves and the parts for converting
food into energy all slashing together inside of the cell.
They spread by dividing into exact replicas of themselves. But
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over time, over a very very long time, some new
versions of these simple cells began to appear, and they
had new specializations. Their interiors became compartmentalized, no longer slashing together,
which meant that the processes they carried out, like converting
that food to energy, became vastly more efficient. So processes
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like photosynthesis were able to develop. And after they did,
the oxygen that this early cellular life excreted as waste
started to settle into the atmosphere, creating an entirely new
one that would eventually support the rise of new types
of life. Then comes the invention of sex, a new
type of reproduction where two entirely distinct individuals combined to
(32:46):
form a new third version of themselves. And it's about
here that natural selection cracks its knuckles and comes aboard
as the driving force of evolution here on Earth. When
natural selection is presented with new options rather than rough
copies of the same thing, new adaptations emerge much more quickly,
which kicks evolution into hyper drive. The simple celled organisms
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that made up life on Earth got better at being
living things, and then for a long time nothing much changed.
It seems as if life had reached the stasis, maxed out,
come upon some invisible wall, and take into coasting. Rather
than progressing toward ever more complexity, Earth seemed content with
(33:33):
its fast seas teeming with specialized, extremely well adapted, single
celled organisms. After that first three hundred million years of
ceaseless innovation, life on earths stayed the same for the
next three billion years. But around five hundred million years
ago something huge happened. Life suddenly developed new ambitions, and
(33:56):
it exploded into new, extremely complicated and sophisticated forms. And
in geological terms, it happened overnight. Basically go from nothing
to everything. This is Dr Phoebe Cohen. She's a paleontologist
at Williams College in Massachusetts. So the vast majority of
(34:16):
the history of life on Earth is that of microscopic organisms.
Before the Cambrian almost all life was microscopic or at
least very very small, and ecosystems were dominated by things
like amiba and bacteria. And after the Cambrian ecosystems are
(34:37):
dominated by animals, um and so it's a really, really
huge shift in the biological evolution of our planet. This
sudden surge in complexity is called the Cambrian Explosion. We
aren't quite sure why it happened. It's possible it was
triggered when the widespread glaciation that covered Earth just before
it began to melt. Or perhaps it was the oxygen
(35:00):
and levels in the atmosphere released by photosynthesis slowly displacing
the Earth's atmosphere of carbon dioxide, ammonia, and methane. We
need lots of oxygen to power the conversion of food
to energy, and perhaps the Cambrian explosion was triggered when
atmospheric oxygen reached a critical threshold that could support more
complex life. So if you go into low oxygen areas
(35:22):
of the ocean today, there's plenty of animals living with
almost no oxygen, but they're not doing anything. They're very boring.
They kind of just lay there because they don't have
enough oxygen to do anything exciting. So one idea is
that oxygen did reach some sort of threshold around the
Cambrian that enabled organisms to start doing fun things like
chasing after each other and ripping each other apart, and
(35:44):
that that played a big role in changing sort of
the structure of ecosystems that lead to a huge diversification
um within the animal claid. Whatever the reason, half a
billion years ago, most of the complex body plans that
are still around on Earth to day suddenly arrived. It's
as if those organizing principles of life entered a new phase.
(36:07):
Within just thirty million years, plants made their debut on land,
and just forty million years after that, animals followed them
out of the water. It's astounding to think of, but
within a span of just seventy million years, life on
Earth went from nothing but single celled aquatic organisms to
animals that lived and moved and walked around on land.
(36:31):
Because life had remained so simple for so long before it,
the Cambrian explosion makes an excellent candidate for the Great Filter.
It could be so unlikely that it universally denies intelligent
life from forming evolutionarily speaking, there's never been a more
important event in the history of Earth. Following the emergence
(36:51):
of life, twenty of the thirty six body plans that
exist on Earth today, the plans that give shape to
squid and the ish, and humans and worms, all suddenly
appeared on Earth, and those new body plans led to
a riot of evolution. The dinosaurs rose and fell, and
small mammals emerged from their burrows and climbed the trees.
(37:14):
Tree dwelling apes found their way out of the savannah
and started walking upright, becoming the first contours of humans.
And it's about here that we arrive at another step
in the long span between the emergence of life on
Earth and us, the evolution of intelligent life. Humans tend
(37:36):
to think of intelligence is what differentiates us from other
life on Earth, but instead we seem more like the
current endpoint in an evolution of intelligence. Signs of intelligence,
in some form or fashion are all around us. In
two thousand eight, Japanese researchers showed that slime mold, a
(37:57):
unicellular organism, can learn a schedule of electric shocks when
they shocked the mold at regular intervals, and yes, they
shocked mold. The mold learned to anticipate the next shock
and recoil from it before it was delivered. The Moosta
plants have shown that they can learn to differentiate between
being dropped and being touched. Where initially the plants responded
(38:20):
to both experiences in the same way by curling their leaves,
after being dropped several times, they learned to keep their
leaves unfurled, and they retain this learned behavior even after
they haven't been dropped or touched for months. The use
of tools, which we imagine is quintessentially human, isn't unique
to us either. Chimpanzees use tools to make gathering food easier,
(38:43):
like sticks to draw termites from their mountains by the fistful,
and rocks to bash open hard shelled nuts to get
to the meat inside. But at some point we drew
away from the rest of life. In the development of
our intelligence, we became the first animals to use tools
to make other tools. This is the birth of technology.
(39:04):
No longer were we relegated to using only what was
found in nature. We learned to use nature to fashion
new tools to better suit our needs. We made spearheads
and axes, out of stone and learned to hunt large animals.
Meat is more energy dense than plants, and we learned
to cook food about eight hundred thousand years ago. When
(39:24):
we did, we unlocked a tremendous amount of nutrients and
energy that hadn't been available to us before. We developed language,
which allowed us to better coordinate ourselves and hunt together
and interact with one another more effectively. We learned to
make clothes to keep us warm as we spread out
beyond the subtropical climate we evolved in. We learned to
(39:44):
make boats to carry us to new places. We learned
to make ceramics to store our food. We learned to
grow crops, which led to cities in the foundation of
the modern era. And there was another quirk of that
chance collision between the planetoid THEA, and Earth, which produced
the Moon. It also produced massive deposits of minerals and
metals near the surface that humans could easily get to.
(40:06):
Over time, we abandon those stone tools in favor of
more reliable metal ones, and eventually we put all of
those millions of years of accumulated intelligence and technology into
ships that broke the bonds of Earth and launched the
first of our species into space. Within the astronomically short
period of about a hundred thousand years, humans left the
(40:29):
wild and went to space. But perhaps as unlikely as
the emergence of human intelligence may seem, it may simply
be the expected outcome of those organizing principles of life.
There are two options before us. Then, either we humans
are unique in our universe and utterly alone, or we
are not. And if there is other life elsewhere, then
(40:51):
that means that the great filter, the hardest step, lies
not in our past, but in our future. It means
that the challenge that lies ahead of us is more difficult,
more improbable to overcome than dead molecules organizing themselves into
living cells or apes learning to build ships to the moon,
And rather than having millions of years to try and
(41:12):
fail before succeeding, we will have one shot to get
it right. If the great filter lies in our future,
then it appears that we are entering it right now,
now here in the twenty one century, four point three
billion years after life emerged on Earth. We are entering
the evolutionary step that no life in the universe has
(41:32):
ever managed to survive. Carl Sagan had this this great
phrase about humanity has grown powerful before it's grown wise
um and our power through technology has been increasing exponentially,
but our wisdom has been maybe it's been increasing a
little bit, but suddenly not exponentially, and it's it's getting
(41:55):
these two things have got out of check with each other.
We've got an unsustainable level of risk. The technology that
got us to this point is taking a new shape,
one that we haven't encountered before, and it is presenting
new risks to the survival of our species and indeed
life on Earth. You right now are living in what
maybe the beginning of the most dangerous period in the
(42:18):
history of the human race. On the next episode of
The End of the World with Josh Clark, if we
are the only intelligent life, humans could have a bright,
(42:39):
long future ahead of us, a triple less civilization based
on super intelligence, super happiness, and super longevity. But between
us and that bright future lay existential risks, and they're
like nothing we've ever encountered before.
What exactly does figuring out if aliens exist have to
do with the end of the world. Well, it turns
out that the odd emptiness that we find in the
universe can give us clues about what may have gone
on or didn't go on before we humans came along.
Did something bad happen? And if so, might it happen
(00:22):
to us two? Looking around for signs of whether we're
the only intelligent life to have ever evolved can help
answer those questions. And when you look at it, the
universe does seem amazingly large for Earth to be the
only planet with life on it. Consider it like this.
(00:43):
Let's say that you're a photon, a tiny packet of light,
and one day you had the wherewithal this set out
to travel across the universe. You would find that, perhaps
to your great surprise, such a trip would take you
around fifty billion years. Yes, you, light, which can travel
at the speed of light, would take fifty billion years
(01:05):
to cross from one side of the universe to the other.
At least that's how long it would appear to take
you to as humans. And this is just the observable universe.
The amount of the universe that light like you has
had time to travel across since the Big Bang. Within
that vast space, there are anywhere from one billion to
two trillion galaxies by our current causes, at least of
(01:27):
which our own Milky Way is on the larger side
of the spectrum. There are larger but there are also
a lot of smaller ones too. Within these billions or
trillions of galaxies are billions and billions and billions of stars,
and probably exponentially more planets. The total number of planets
and stars in our universe, the total number of places
(01:48):
for life to exist, is mind bogglingly large. And so
you packet of light or wavelength, depending on your mood,
might think to yourself, as you traveled across the verse
and saw that the Earth is the only planet that
is home to intelligent life. Out of the attend to
the who knows what power planets that could host life,
(02:09):
You might think to yourself, in your little photon voice,
what waste are we alone in the universe? And if
we are alone, why? These are the questions at the
heart of the Fermi paradox, and they continue to nag
at us. The answer is plainly obvious if you look
at it, but it depends on how you look at it.
(02:30):
With the Fermi paradox, the same thing can look very
different to any two people. And it's not just the
paradox itself. Even the evidence is equally ambiguous. It's all
like one big contradem words that have two meanings that
are the opposite of one another, like how weather can
mean both to wear away and to withstand something. The
(02:51):
size of our empty universe can mean that we are
both alone or one of many. Another example of this
ambiguity is the very presence of life on Earth, something
that people who believe that we're not alone in the
universe point to. His evidence is that life here on
Earth seems to have emerged the first chance that it had.
(03:12):
The famous astronomer and science writer Carl Sagan was one
of those people. He was an optimist when it came
to the family paradox. He believed that life was out there,
we just hadn't found it yet. Sagan pointed to evidence
from the fossil record that here on Earth life began
as early as five hundred million years after the Earth form.
(03:32):
It was almost like it was waiting to emerge, and
since it emerged quickly here on Earth, it stands to
reason that life should emerge wherever it gets the chance,
anywhere in our universe. When you take into account the
idea that there are perhaps three hundred billion stars in
the Milky Way alone, even if some small fraction of
those have habitable planets that could host life, then we
(03:55):
should expect to encounter it sometime soon as we spread
out to explore the country side around planet Earth. But
there's a problem with basing our view of the rest
of the universe on our own existence. The idea that
we can gain insight into our universe from our existence
is called the anthropic principle, and it's vulnerable to a
logical fallacy called selection bias. Being the only intelligent life
(04:20):
in the universe, we're the only data point in our
data set, and so we tend to skew the results
a little bit. It's hard to resist the temptation of
cherry picking the data when there's only one cherry. Yes,
of course, life can arise. Our very existence proves that fact.
But what it does not prove is that the emergence
(04:40):
of intelligent life or any life, really is easy or inevitable.
What if, instead, life emerging in our universe is really,
really really hard. Perhaps the existence of living, breathing, intelligent
things here on Earth doesn't show the emergence of life
is inevitable. Perhaps as shows that it was the singularly
(05:01):
most unlikely event in the history of our entire universe.
If you could crack open a strand of your DNA
and read the pairs of adenine, guanine, cite of scene
and dimin the ones and zeros of your genetic code,
(05:21):
you would find a history of life on Earth written
into it. Not only does your DNA contain the blueprints
for making a full version of you, but if you
look at it correctly, it also bears the marks of
those who have come before you. You'll find your parents genes,
of course, and their parents. But as you go further
back in time, you'll also find the contributions of all
(05:43):
of the animals in bacteria that ever reproduced along the
last several billion years to form a connected chain of
life that eventually led to you. But you'll find that
you run into a wall the further you go back.
There's a point beyond which we can no longer read
the taves of our DNA. It ends right before we
get to the emergence of life here on Earth the
(06:05):
very beginning. That is to say, no one is sure
how life began as it stands now, the general consensus
among sciences the concept of a biogenesis, that life emerged
from nothing nothing living anyway, let's go back to the
early Earth. About five million years after it formed, the
(06:27):
surfaces just begun to cool enough that solid clay ground
has begun to form, and with the cooling off, the
muggy atmosphere cooled as well, condensing in the rain that
began to collect in pools which would become the peatree
dishes where life took its first steps. But to get
from here to life, something extremely unlikely had to happen.
(06:49):
Plain old, lifeless molecules had to spontaneously arrange themselves into
new forms that became the building blocks of life. At
this point, there was nothing but raw materials on Earth,
and we have to get from here to living organisms.
That means that not only do you have a functioning organism,
it has to carry with it the encoded instructions to
(07:11):
make a copy of itself, and some way to read
those instructions and actually make that copy. It's like the
idea of putting some plastic pellets and metal shavings into
a bucket and expecting an autonomous three D printer to
form from it, or more to the point, it would
be like the idea of rotting meat growing maggots. For
(07:31):
a long time, humans thought that this kind of thing
spontaneous generation, was how some life arose. Prior to when
science took up the mantle of explaining our universe, people
relied on their everyday observations to explain occurrence is like
maggots growing on rotting meat, seeming to appear out of nowhere.
It seemed just as likely as anything that maggots could
(07:52):
spontaneously generate, or baby mice from grain, which was another
common folk belief. But eventually scientists figured out a way
to disprove this idea, which really gained support when we
realized that tiny, unseen life lived around us everywhere and
that it had a hand in a lot of the
things that we saw. Germ theory was developed and the
(08:13):
concept of spontaneous generation was abandoned. That is until the
nineteen fifties, when researchers started working hard on figuring out
how life on Earth might have come about. Spontaneous generation
made an unexpected comeback. A biogenesis holds that quadrillions upon
quintillions of simple molecules present in Earth's early atmosphere and
(08:36):
oceans randomly configured themselves into a mind bending number of
different combinations. Some of these abominably large number of combinations
happen to create useful complex molecules like amino acids, which
are the precursors for proteins. Now, it's one thing to
form simple molecules to randomly combined to form more complex molecules,
(08:58):
but there is still the issue of replication. Ship If
those molecules don't make a younger copy of themselves, that
innovative chemical chain is broken and the new molecule loses
the chance to continue to evolve in a new and
even more complex things. And this is the point where
science is currently stuck. Somehow, they say, some of those
molecules managed to form a stable string of nuclear times,
(09:21):
probably r N, a ribonucleic acid which is capable of
doing two very important things. It can encode information in
its nuclear tide chain, and it can also transcribe those
nuclear tides to produce proteins. And once you have proteins,
you can do all sorts of things that supports life.
Back in two organic chemists Stanley Miller and Herald Urray
(09:45):
saw it to show that this was possible by recreating
the conditions of earlier Earth. In a flask. They simulated
a primitive ocean and built an atmosphere out of the
gases thought back in the fifties to have been present
on Earth soon after it formed. They mimicked why in
storms with the flickering electrical current, and when Miller inspected
the broth that resulted, he found that nineteen amino acids
(10:07):
and amans, the precursors to proteins, had assembled spontaneously. It
seems that Miller had shown that when the conditions were right,
the foundations for life would arise. But lately the idea
that a biogenesis just happened randomly is falling out of favor. Instead,
some scientists have begun to suspect that there is some
(10:28):
set of organizing principles that serves as a driving force
for life to emerge. Just like how gravity will draw
a ball downhill, or how magnets will always repel or
attract one another when they're close together, there is some
fundamental governing force of nature that causes life to assemble
along predictable lines. We just haven't figured out what that
(10:51):
force or those lines are yet. This is a pretty
surprising idea if you think of it. One of the
laws of the universe, the second law of therm mom dynamics,
is that things tend towards disorder, not order. The idea that,
when presented with the right conditions, dead molecules in the
universe will organize themselves into something living and breathing runs
(11:13):
totally counter to that, and this new view also includes
evolutionary biology as one stretch of these organizing principles of life.
So the idea is that molecules arrange themselves into self replicating,
metabolizing parts that form increasingly complex beings that eventually include
you and me, which makes you wonder what the end
(11:35):
point is. To some people, the idea that life arose
(11:55):
from simple dead molecules that just happened to randomly assemble
themselves in the living things is just too unlikely to accept,
And even if we do accept that this is precisely
how life arose on Earth, the idea that it could
ever happen again anywhere else is too improbable. That virtually
proves that we humans are alone in the universe. One
(12:19):
issue people raise is time. They say that Earth just
simply hasn't been around long enough for all of that
random chemical trial and error to have taken place. The
idea that life organizes along some unknown universal principles definitely
addresses that idea of time, and so does pant spermia.
Pant Spermia is a concept from astrobiology, and it says
(12:42):
that the seeds of life are all over the universe
in abundance everywhere. They can be found on board asteroids
and other celestial objects, and that these seeds of life
are constantly bombarding planets all over the universe. If the
conditions on the planet happen to be right, well, then
those sea needs of life will germinate and grow into
something new and living. This certainly addresses the issue of time.
(13:07):
Life could have evolved elsewhere in the universe, which is
billions of years older than Earth, and then spread to
our planet aboard an asteroid. We've recently found that some
chemical precursors to life can be found on celestial objects
like asteroids, and that they're able to survive re entry
into an atmosphere, which can get pretty hot. This is
important because it's widely accepted that an atmosphere is a
(13:30):
precondition for life to emerge. If you take pants bermia,
and you take the recent view that life follows some
organizing principles as immutable as the laws of physics, then
you arrive at a conclusion that Earth is just another
place that happened to have the right conditions when a
rock bearing the precursors to life landed around four billion
(13:50):
years ago. In this view, then of course life is
abundant in the universe. But then we find ourselves right
back to where we started. Where is everybody? Perhaps the
best answer to that comes not from an astrobiologist or
an astronomer, but from an economist named Robin Hansen, who
(14:11):
proposed that there must be something, some incredibly difficult step
between the point where dead matter forms life and the
point where intelligent life becomes a galactically colonizing civilization that
no species has ever been able to overcome. He calls
it the great filter. Every piece of matter in the
universe is the sort of thing that could have started
(14:32):
that process, started life, and then advanced life, etcetera. But
so far nothing out there has done that. So the
great filter is whatever is in the way, whatever makes
it hard for any one piece of ordinary dead matter
to produce expanding, lasting life. There's surely a countless number
(14:52):
of steps along the path from dead matter to the
emergence of a galactically visible civilization. But the Great Filter
high Path says, supposes that a handful of them are really,
really hard, and that one of them in particular must
be so hard that is thus far prevented any life
from reaching galactic proportions. This is Oxford University philosopher Toby
(15:16):
ord if there were a hundred pieces you needed to
get into the right order in order to create something
that obeyed natural selection and and was it the basic
level needed to actually bootstrap up towards complex life, then
there are a hundred factorial ways you could arrange those pieces.
That's that's more than uh tends to the power of
(15:37):
a hundred different possibilities. And then it just turns out
you need an incredibly rare event to get there. The
Great Filter offers two possible solutions to the question posed
by the Fermi paradox. Whereas everybody everybody never existed, or
everybody is dead, if the Great Filter is in our past,
(15:57):
it says pretty strongly that we are the first and
only intelligent life that exists in the universe. If that's true,
then by the Great Filter, there is something, some step,
some right of passage you could call it, that has
prevented every other life from reaching the point that we're at.
And if that's the case, then we are the only
(16:19):
life to have made it through the Great Filter. That
means that we can be optimistic about our future. We
made it through that right of passage that has kept
every other attempt at life from evolving. It means our big,
vast universe isn't wasted on us. It's just waiting there
for us to use it, and guilt free too. Since
(16:40):
no one else is around to use it, we have
a responsibility to put it to use. You could even say.
But there's another possibility to the Great Filter too. It
may also lie just ahead in our future. Maybe intelligent
life is a dime a dozen in the universe. But
the reason we don't see other civilizations is because they've
(17:02):
all died off. And if all of those intelligent civilizations
all died before any any of them could make it
off of their home planet and spread throughout the universe,
the Great Filter is a big red flag for us.
It tells us that we should expect to meet the
same fate that every other intelligent life has. The Great
(17:25):
Filter will spell the end of the world for us too.
To answer the question of whether we face imminent doom
or not, we have to look at the evolution of life,
and to do that we have no choice but to
turn to the one place where we know life evolved.
At the risk of falling victim to the selection bias,
we have to look to Earth for clues. We've already
(17:49):
talked about how utterly improbable the origin of life appears
to have been, but it's probably best to start looking
even further back than that. If life emerged on Earth,
that means that the conditions were right for life on Earth.
That's the one day to point we have ipso fact, though,
so the best way to find out what conditions life
(18:11):
requires is to look at the conditions of our planet.
And it turns out that Earth has some spectacularly peculiar
characteristics that make it ripe for life. So peculiar, in fact,
that a pair of researchers named Peter Ward and Donald
Brownlee rolled all of them up into what they call
the rare Earth hypothesis. It's just what it sounds like
(18:33):
when you add up all of its peculiarities. Earth is
not like other planets. First, Earth happened to form around
the right kind of star. Our son is a main
sequence star, which means it produces light and heat by
fusing hydrogen into helium. Main Sequence stars aren't rare. They
make up about nine of stars. But our Son also
(18:54):
happened to be of the right size too. It's not
so large that it will use up its fuel in
just a few billion years. That means that the Sun's slow,
steady burn would go on long enough to give life
time to develop. The Sun also isn't too small to
support life either. It is you could say, just right
for life. Our Sun has also placed in a really
(19:16):
great spot in our galaxy. We happen to be located
out in the country, in a bit of a backwater
when it comes to the Milky Way, about twenty eight
thousand light years from the galactic center, in a galaxy
that's one thousand light years across. For decades, study presumed
that the center of the galaxy would be the most
happening spot. That's where the most stars tend to be,
(19:37):
and more stars would be more potentially habitable planets. Civilizations
that arose in the center might even be in contact
with one another, forming something of a galactic urban area.
But recently it's become clear that the galactic center might
not be so flourishing. After all. There are more stars, sure,
but more stars also means that there are more collapsing stars,
(19:59):
which release bursts of energy that can burn away the
atmosphere of any planets in the vicinity, So the galactic
center might actually be less of a bustling urban area
and more like sterile and dead. Our Sun is pretty
far away from the galactic center, way out past the
suburbs where nothing much happens. In other words, that's good
(20:19):
for life on Earth because it means that it cuts
down on the number of sterilization events as life developed
over Earth's history, giving it a good chance of succeeding.
And Earth just so happens to be located within the
Sun's goldilocks zone that I mentioned in the last episode.
If it was a little nearer the Sun and about
one and a half million kilometers closer, it would be
(20:40):
too hot to support life, and much further away Earth
would be too cold for it. It also turns out
that where Earth is positioned in the Solar System is
hugely important to giving life a fighting chance. Earth is
the third rock from the Sun, with five others between
us and the rest of the galaxy. Six if the
math that suggests there's a planet nine out there turns
(21:02):
out to be correct, or if you continue to count
hapless Pluto, the Sun and the planets in our Solar
System formed from a massive cloud of cosmic dust, that
same stuff that might have blown up so many alien
pilots traveling between the stars. We still aren't quite sure
exactly how the planets formed. One model astoundingly suggested that
(21:23):
they could have formed in as little as a thousand years,
but the birth of the Solar System was likely a
free for all grab of the elements that make up
the planets today. Initially, astronomers presumed that the planets formed
in their current arrangement, but lately it's become clear that
that probably wasn't the case. The planets may have actually
moved around and migrated from one spot to another in
(21:46):
the early life of the Solar System before settling into
the arrangement that we see them in today. If that's correct,
then we were astoundingly lucky that Jupiter ended up where
it didn't. Jupiter is a gas giant made up largely
of hydrogen and helium with a metal and ice core.
(22:07):
It's like a Sun that never started burning because it
lacked the mass needed for gravity to kick start fusion.
But Jupiter is massive, truly, you could fit around within it.
It's the most massive planet in our Solar System, and
because of its mass and its position between Earth and
the chaos of the interstellar space outside of our Solar System,
(22:31):
Jupiter acts as a huge defensive guard for our planet.
When asteroids or comets or other flotsam and jetsam bent
on destruction in oer our Solar System, Jupiter's extraordinary gravitational
pull draws them into its orbit and slingshots them out
back into space. Without Jupiter to run interference for Earth,
it would have a steady diet of life ending bombardments
(22:53):
from space. So thanks Jupiter. This is astronomer Donald brown Lee.
He's one of the guys who came up with the
rare Earth hypothesis. Jupers are actually pretty rare uh planets
and um uh so uh in terms of rare Earth.
You know whether it was juper is good or bad.
(23:16):
Typical planet as systems probably don't have jupers. Our moon, too,
seems to have played a number of factors and fostering
life on Earth. Our moon's pretty unusual itself as far
as moons go. It's enormous. It's about one point to
percent the mass of the Earth and doesn't sound like much.
But other planets moons like Phobos and Demos, which or
(23:37):
a bit Mars, are closer to asteroids in size. Our
moon more resembles a small planet, and because it's so big,
it has some very peculiar effects on Earth. For one,
it stabilizes our planet. The Earth doesn't sit upright as
it spins around on its axis. It's tilted actually at
about a twenty three degree angle. Because of this tilt,
(23:59):
we have seas which create predictable variations in the temperature
of regions on Earth over the course of the year.
So when you think about the difference between winter and summer,
you get a good idea how much variation a tilt
can create. And add more of an angle to the
tilt and the Earth and the temperature variations would become
more severe. Since during winter or hemisphere would be further
(24:21):
away than it is now from the Sun and much
closer in the summer. And if you added in some wobble,
like if the tilt of the Earth wasn't stable and fluctuated,
all of these wild swings and temperature could make it
very difficult for life to take hold. The Moon's mass
actually exerts gravity over Earth and keeps it stable, not
wobbling or tilting more than it does, and allowing for
(24:44):
a nice, gradual, predictable and not two varying seasonal temperature
shifts that we even have a moon appears to be
a fluke itself. The current view is that the Moon
was calved off from Earth following a head on collision
in the Earth's early history with a planetoid about the
size of Mars called Thea, which the Earth likely absorbed.
(25:05):
This giant impact hypothesis explains a lot. The Moon is
unusually large and unusually close to us because it was
created of a mixture of Earth and that fateful planetoid.
Because the Moon is so close to Earth, it's tidally
locked and orbit around us. It doesn't spin on its axis,
which is why we always see the same side of
(25:25):
the Moon as it orbits us. This is an important
feature because it also creates the tide you're on Earth.
As the Moon orbits Earth, it pulls the oceans toward it,
stretching them out on the ends and narrowing them in
the middle. We hear on our planet experience this as
low tide or high tide, depending on where you are.
(25:46):
And it was in these tidal pools of young Earth's
oceans where some people think life began. When those ancient
tides came in, they deposited a flood of molecules into
the tidle pools, and as they withdrew and the water
evaporated in the sun, the increasing concentration of salt could
(26:06):
have provided just the right laboratory conditions where those earliest
chemicals could combine. Without a moon as large and as
close as ours is, these tides could not have existed
on Earth, and those early proteins would have lacked that
kind of natural peatrie dish. So the moon itself is
a collection of exquisite coincidences that supported life on Earth.
(26:28):
But perhaps the most peculiar aspect of Earth is that
it has massive plates that make up the crust, which
slide along a molten bed underneath that. It, in other words,
features plate tectonics. It thought that this is a major
reason why the Earth has actually had an amazingly stable
climate for most of you know, temperatures for for most
(26:50):
of its uh age. So it's drastically different than Venus
or or Mars, which are famously unstable over tri lage
uh time scope. So so we really owe a lot
uh to this. We don't really know why I played
tectonics works on our on Earth. It doesn't work on Mars,
it doesn't work on vines, it doesn't work on mercury,
(27:12):
doesn't work on any Yeah, I mean they're there, are
you know, movements of rocks rolled to each other, but
not I played tectonics of the type that we that
we have here, So so we think it's really important.
As far as we know, Earth is the only planet
to feature plate tectonics, which actually is a massive thermostat
for the planet. When oceanic plates slide underneath the lighter
(27:34):
continental plates, massive amounts of the oceanic crusts is crushed
and absorbed into magma. The rock in those oceanic plates
contains huge stores of carbon dioxide, so when volcanoes release
magma from beneath the crust, it also contains some of
that CEO two, which travels as a gas up into
the atmosphere and there it hangs around and it absorbs sunlight,
(27:57):
which in term warms the atmosphere and eventually the planet below.
And when the planet warms, more of the oceans evaporate,
warming the atmosphere even more. When you have a warm,
wet atmosphere, you have lots of rain that rain brings
dissolves c O two back down to Earth. Rocks on
Earth are an excellent store of carbon, and when CO
(28:19):
two rich, rain falls on them as the knock on
effect of weathering them, meaning the wearing down definition, not
the opposite one. All of that carbon in the rocks
and rain makes its way to the sea, where it
dissolves and eventually sinks to the ocean's bottom. As more
carbon is removed from the atmosphere and stored, the planet
begins to cool again. Over time, the carbon at the
(28:42):
bottom of the ocean forms new oceanic plate crusts, and
eventually it will find itself along an ocean ridge where
it collides with the continental plate and the whole process
begins again, and the Earth starts to warm once more.
This unique property of Earth has kept the planet's climate
stable more or less constantly for more than four billion years,
(29:04):
which has allowed life on Earth to grow and flourish.
When you take all of these details together, a weird
picture of Earth emerges. It's almost freakishly perfect for life.
That's so many different variables, each at just the right temperature,
(29:26):
just the right location, heat time, whatever would come together
to form a stable whole seems to make Earth a
staggeringly improbable place. You couldn't ask for a better place
for life to emerge. And maybe that's the point. Maybe
the seeds of life are commonplace in the universe, but
Earth is unique. We simply don't know. We still know
(29:49):
so little about space, our galaxy, and the universe that
we can't say if Earth is freakishly unique or one
of many. And as we get better at deducing the
existence of habit of planets with our space telescopes, they
seem to pop out of the cosmos like a magic
eye poster does when you lose focus in just the
right way. Maybe planets that can sustain life are more
(30:10):
abundantly the universe than we realize perhaps assembling those seeds
into life is where the filter lies. The farther back
we look in time, the less we understand. So if
you're going to look for something that might be really hard,
harder than it seems, farther back in time is the
plausible place to look, because that's where we don't understand things. Uh,
(30:32):
And the very first step from completely dead matter to
some proto life that has to be the earliest step
on the one we have the least knowledge of, and
more plausibly it's the hardest step. Or perhaps not. Perhaps
right now the universe is teeming with primitive life. Perhaps
the great filter lies somewhere after that. There were, after all,
(30:53):
some enormous steps to get from the emergence of life
here on Earth in the moment we're sharing between us
right now, m back on the early Earth, tucked away
(31:16):
just so, in a solar system, tucked away just so,
in the galaxy with its volcanoes chugging away at producing
a warm atmosphere and its oceans producing a warm liquid
medium for molecules to organize into life. At some point,
one particular moment in Earth's history, all of those separate
components came together to form a full fledged living cell.
(31:40):
As far as we can tell, this moment happened around
three point eight billion years ago. At first, these simple,
single celled organisms were nothing more than gooey bags, with
the parts for replicating themselves and the parts for converting
food into energy all slashing together inside of the cell.
They spread by dividing into exact replicas of themselves. But
(32:02):
over time, over a very very long time, some new
versions of these simple cells began to appear, and they
had new specializations. Their interiors became compartmentalized, no longer slashing together,
which meant that the processes they carried out, like converting
that food to energy, became vastly more efficient. So processes
(32:23):
like photosynthesis were able to develop. And after they did,
the oxygen that this early cellular life excreted as waste
started to settle into the atmosphere, creating an entirely new
one that would eventually support the rise of new types
of life. Then comes the invention of sex, a new
type of reproduction where two entirely distinct individuals combined to
(32:46):
form a new third version of themselves. And it's about
here that natural selection cracks its knuckles and comes aboard
as the driving force of evolution here on Earth. When
natural selection is presented with new options rather than rough
copies of the same thing, new adaptations emerge much more quickly,
which kicks evolution into hyper drive. The simple celled organisms
(33:11):
that made up life on Earth got better at being
living things, and then for a long time nothing much changed.
It seems as if life had reached the stasis, maxed out,
come upon some invisible wall, and take into coasting. Rather
than progressing toward ever more complexity, Earth seemed content with
(33:33):
its fast seas teeming with specialized, extremely well adapted, single
celled organisms. After that first three hundred million years of
ceaseless innovation, life on earths stayed the same for the
next three billion years. But around five hundred million years
ago something huge happened. Life suddenly developed new ambitions, and
(33:56):
it exploded into new, extremely complicated and sophisticated forms. And
in geological terms, it happened overnight. Basically go from nothing
to everything. This is Dr Phoebe Cohen. She's a paleontologist
at Williams College in Massachusetts. So the vast majority of
(34:16):
the history of life on Earth is that of microscopic organisms.
Before the Cambrian almost all life was microscopic or at
least very very small, and ecosystems were dominated by things
like amiba and bacteria. And after the Cambrian ecosystems are
(34:37):
dominated by animals, um and so it's a really, really
huge shift in the biological evolution of our planet. This
sudden surge in complexity is called the Cambrian Explosion. We
aren't quite sure why it happened. It's possible it was
triggered when the widespread glaciation that covered Earth just before
it began to melt. Or perhaps it was the oxygen
(35:00):
and levels in the atmosphere released by photosynthesis slowly displacing
the Earth's atmosphere of carbon dioxide, ammonia, and methane. We
need lots of oxygen to power the conversion of food
to energy, and perhaps the Cambrian explosion was triggered when
atmospheric oxygen reached a critical threshold that could support more
complex life. So if you go into low oxygen areas
(35:22):
of the ocean today, there's plenty of animals living with
almost no oxygen, but they're not doing anything. They're very boring.
They kind of just lay there because they don't have
enough oxygen to do anything exciting. So one idea is
that oxygen did reach some sort of threshold around the
Cambrian that enabled organisms to start doing fun things like
chasing after each other and ripping each other apart, and
(35:44):
that that played a big role in changing sort of
the structure of ecosystems that lead to a huge diversification
um within the animal claid. Whatever the reason, half a
billion years ago, most of the complex body plans that
are still around on Earth to day suddenly arrived. It's
as if those organizing principles of life entered a new phase.
(36:07):
Within just thirty million years, plants made their debut on land,
and just forty million years after that, animals followed them
out of the water. It's astounding to think of, but
within a span of just seventy million years, life on
Earth went from nothing but single celled aquatic organisms to
animals that lived and moved and walked around on land.
(36:31):
Because life had remained so simple for so long before it,
the Cambrian explosion makes an excellent candidate for the Great Filter.
It could be so unlikely that it universally denies intelligent
life from forming evolutionarily speaking, there's never been a more
important event in the history of Earth. Following the emergence
(36:51):
of life, twenty of the thirty six body plans that
exist on Earth today, the plans that give shape to
squid and the ish, and humans and worms, all suddenly
appeared on Earth, and those new body plans led to
a riot of evolution. The dinosaurs rose and fell, and
small mammals emerged from their burrows and climbed the trees.
(37:14):
Tree dwelling apes found their way out of the savannah
and started walking upright, becoming the first contours of humans.
And it's about here that we arrive at another step
in the long span between the emergence of life on
Earth and us, the evolution of intelligent life. Humans tend
(37:36):
to think of intelligence is what differentiates us from other
life on Earth, but instead we seem more like the
current endpoint in an evolution of intelligence. Signs of intelligence,
in some form or fashion are all around us. In
two thousand eight, Japanese researchers showed that slime mold, a
(37:57):
unicellular organism, can learn a schedule of electric shocks when
they shocked the mold at regular intervals, and yes, they
shocked mold. The mold learned to anticipate the next shock
and recoil from it before it was delivered. The Moosta
plants have shown that they can learn to differentiate between
being dropped and being touched. Where initially the plants responded
(38:20):
to both experiences in the same way by curling their leaves,
after being dropped several times, they learned to keep their
leaves unfurled, and they retain this learned behavior even after
they haven't been dropped or touched for months. The use
of tools, which we imagine is quintessentially human, isn't unique
to us either. Chimpanzees use tools to make gathering food easier,
(38:43):
like sticks to draw termites from their mountains by the fistful,
and rocks to bash open hard shelled nuts to get
to the meat inside. But at some point we drew
away from the rest of life. In the development of
our intelligence, we became the first animals to use tools
to make other tools. This is the birth of technology.
(39:04):
No longer were we relegated to using only what was
found in nature. We learned to use nature to fashion
new tools to better suit our needs. We made spearheads
and axes, out of stone and learned to hunt large animals.
Meat is more energy dense than plants, and we learned
to cook food about eight hundred thousand years ago. When
(39:24):
we did, we unlocked a tremendous amount of nutrients and
energy that hadn't been available to us before. We developed language,
which allowed us to better coordinate ourselves and hunt together
and interact with one another more effectively. We learned to
make clothes to keep us warm as we spread out
beyond the subtropical climate we evolved in. We learned to
(39:44):
make boats to carry us to new places. We learned
to make ceramics to store our food. We learned to
grow crops, which led to cities in the foundation of
the modern era. And there was another quirk of that
chance collision between the planetoid THEA, and Earth, which produced
the Moon. It also produced massive deposits of minerals and
metals near the surface that humans could easily get to.
(40:06):
Over time, we abandon those stone tools in favor of
more reliable metal ones, and eventually we put all of
those millions of years of accumulated intelligence and technology into
ships that broke the bonds of Earth and launched the
first of our species into space. Within the astronomically short
period of about a hundred thousand years, humans left the
(40:29):
wild and went to space. But perhaps as unlikely as
the emergence of human intelligence may seem, it may simply
be the expected outcome of those organizing principles of life.
There are two options before us. Then, either we humans
are unique in our universe and utterly alone, or we
are not. And if there is other life elsewhere, then
(40:51):
that means that the great filter, the hardest step, lies
not in our past, but in our future. It means
that the challenge that lies ahead of us is more difficult,
more improbable to overcome than dead molecules organizing themselves into
living cells or apes learning to build ships to the moon,
And rather than having millions of years to try and
(41:12):
fail before succeeding, we will have one shot to get
it right. If the great filter lies in our future,
then it appears that we are entering it right now,
now here in the twenty one century, four point three
billion years after life emerged on Earth. We are entering
the evolutionary step that no life in the universe has
(41:32):
ever managed to survive. Carl Sagan had this this great
phrase about humanity has grown powerful before it's grown wise
um and our power through technology has been increasing exponentially,
but our wisdom has been maybe it's been increasing a
little bit, but suddenly not exponentially, and it's it's getting
(41:55):
these two things have got out of check with each other.
We've got an unsustainable level of risk. The technology that
got us to this point is taking a new shape,
one that we haven't encountered before, and it is presenting
new risks to the survival of our species and indeed
life on Earth. You right now are living in what
maybe the beginning of the most dangerous period in the
(42:18):
history of the human race. On the next episode of
The End of the World with Josh Clark, if we
are the only intelligent life, humans could have a bright,
(42:39):
long future ahead of us, a triple less civilization based
on super intelligence, super happiness, and super longevity. But between
us and that bright future lay existential risks, and they're
like nothing we've ever encountered before.
The End Of The World with Josh Clark News
Host
Josh Clark
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