0:00

What exactly does figuring

0:02

out if aliens exist have to do with

0:04

the end of the world. Well,

0:07

it turns out that the odd emptiness

0:09

that we find in the universe can give

0:11

us clues about what may have gone

0:13

on or didn't go on before

0:16

we humans came along. Did

0:18

something bad happen? And

0:20

if so, might it happen to us

0:22

two? Looking around

0:25

for signs of whether we're the only intelligent

0:27

life to have ever evolved can

0:29

help answer those questions. And

0:32

when you look at it, the universe does

0:34

seem amazingly large for

0:37

Earth to be the only planet with life

0:39

on it. Consider it

0:41

like this. Let's say

0:44

that you're a photon, a tiny

0:46

packet of light, and one

0:48

day you had the wherewithal this set out

0:50

to travel across the universe. You

0:53

would find that, perhaps to your great surprise,

0:55

such a trip would take you around fifty

0:58

billion years. Yes, you, light,

1:01

which can travel at the speed of light, would

1:04

take fifty billion years to cross from

1:06

one side of the universe to the other. At

1:08

least that's how long it would appear to take you to

1:10

as humans. And this is

1:13

just the observable universe. The amount

1:15

of the universe that light like you has had time

1:17

to travel across since the Big Bang. Within

1:20

that vast space, there are anywhere

1:22

from one billion to two trillion

1:24

galaxies by our current causes,

1:26

at least of which our own Milky

1:28

Way is on the larger side of the spectrum.

1:31

There are larger but there are also a lot of

1:33

smaller ones too. Within

1:35

these billions or trillions of galaxies

1:37

are billions and billions and billions

1:40

of stars, and probably exponentially

1:43

more planets. The total number

1:45

of planets and stars in our universe, the

1:47

total number of places for life to exist,

1:50

is mind bogglingly large. And

1:53

so you packet of light or wavelength,

1:56

depending on your mood, might think

1:58

to yourself, as you traveled across the verse

2:00

and saw that the Earth is the only

2:02

planet that is home to intelligent life.

2:05

Out of the attend to the who knows what power

2:07

planets that could host life, You

2:09

might think to yourself, in your little photon

2:12

voice, what waste

2:16

are we alone in the universe? And

2:18

if we are alone, why? These

2:20

are the questions at the heart of the Fermi paradox,

2:23

and they continue to nag at us. The

2:25

answer is plainly obvious if you look at

2:27

it, but it depends on how you look

2:29

at it. With the Fermi paradox,

2:32

the same thing can look very different to any

2:34

two people. And it's not just the

2:36

paradox itself. Even the evidence

2:39

is equally ambiguous. It's

2:41

all like one big contradem words

2:43

that have two meanings that are the opposite

2:45

of one another, like how weather

2:48

can mean both to wear away and to withstand

2:50

something. The size of our empty universe

2:53

can mean that we are both alone or

2:56

one of many. Another

2:58

example of this ambiguity is the very

3:00

presence of life on Earth, something

3:03

that people who believe that we're not alone in the universe

3:06

point to. His evidence is that life

3:08

here on Earth seems to have emerged the first

3:10

chance that it had. The famous

3:12

astronomer and science writer Carl Sagan

3:15

was one of those people. He was an optimist

3:18

when it came to the family paradox. He

3:20

believed that life was out there, we just

3:23

hadn't found it yet. Sagan

3:25

pointed to evidence from the fossil record that here

3:27

on Earth life began as early as

3:30

five hundred million years after the Earth form.

3:32

It was almost like it was waiting to emerge,

3:35

and since it emerged quickly here on Earth,

3:38

it stands to reason that life should emerge

3:40

wherever it gets the chance, anywhere in our

3:43

universe. When you take

3:45

into account the idea that there are perhaps three

3:47

hundred billion stars in the Milky Way alone,

3:50

even if some small fraction of those

3:52

have habitable planets that could host life,

3:55

then we should expect to encounter it sometime

3:57

soon as we spread out to explore

3:59

the country side around planet Earth. But

4:02

there's a problem with basing our view of the

4:04

rest of the universe on our own existence.

4:08

The idea that we can gain insight into

4:10

our universe from our existence is called the

4:12

anthropic principle, and it's

4:14

vulnerable to a logical fallacy called

4:16

selection bias. Being

4:18

the only intelligent life in the universe, we're

4:21

the only data point in our data set,

4:23

and so we tend to skew the results

4:26

a little bit. It's hard to resist

4:28

the temptation of cherry picking the data

4:30

when there's only one cherry. Yes,

4:33

of course, life can arise. Our

4:35

very existence proves that fact. But

4:37

what it does not prove is that the emergence

4:40

of intelligent life or any life,

4:42

really is easy or inevitable.

4:45

What if, instead, life emerging

4:47

in our universe is really, really

4:50

really hard. Perhaps

4:52

the existence of living, breathing, intelligent

4:55

things here on Earth doesn't show the emergence

4:57

of life is inevitable. Perhaps as

5:00

shows that it was the singularly most unlikely

5:02

event in the history of our entire

5:04

universe. If

5:11

you could crack open a strand of your DNA

5:14

and read the pairs of adenine, guanine,

5:17

cite of scene and dimin the

5:19

ones and zeros of your genetic code, you

5:21

would find a history of life on

5:24

Earth written into it. Not

5:26

only does your DNA contain the blueprints

5:28

for making a full version of you, but

5:30

if you look at it correctly, it also

5:33

bears the marks of those who have come before you.

5:36

You'll find your parents genes, of course, and their

5:38

parents. But as you go further back

5:40

in time, you'll also find the contributions

5:42

of all of the animals in bacteria that

5:44

ever reproduced along the last several

5:47

billion years to form a connected

5:49

chain of life that eventually led

5:51

to you. But you'll

5:53

find that you run into a wall the further you

5:55

go back. There's a point beyond

5:58

which we can no longer read the taves

6:00

of our DNA. It ends

6:02

right before we get to the emergence of life

6:04

here on Earth the very beginning. That

6:06

is to say, no one is sure how

6:09

life began as

6:12

it stands now, the general consensus

6:14

among sciences the concept of a biogenesis,

6:18

that life emerged from nothing nothing

6:20

living anyway, let's

6:22

go back to the early Earth. About five

6:25

million years after it formed, the

6:27

surfaces just begun to cool enough that

6:30

solid clay ground has begun to form,

6:32

and with the cooling off, the muggy atmosphere

6:34

cooled as well, condensing in the rain

6:37

that began to collect in pools which would

6:39

become the peatree dishes where life

6:41

took its first steps. But

6:43

to get from here to life, something

6:46

extremely unlikely had to happen.

6:49

Plain old, lifeless molecules had

6:52

to spontaneously arrange themselves

6:54

into new forms that became the

6:56

building blocks of life. At

6:58

this point, there was nothing but raw materials

7:01

on Earth, and we have to get from here to

7:03

living organisms. That

7:05

means that not only do you have a functioning organism,

7:08

it has to carry with it the encoded instructions

7:10

to make a copy of itself, and some way

7:13

to read those instructions and actually make that copy.

7:16

It's like the idea of putting some plastic

7:18

pellets and metal shavings into a bucket

7:20

and expecting an autonomous three D printer

7:23

to form from it, or

7:25

more to the point, it would be like

7:27

the idea of rotting meat growing maggots.

7:31

For a long time, humans thought

7:33

that this kind of thing spontaneous generation,

7:35

was how some life arose. Prior

7:38

to when science took up the mantle of explaining

7:40

our universe, people relied on their

7:42

everyday observations to explain occurrence

7:45

is like maggots growing on rotting meat, seeming

7:47

to appear out of nowhere. It

7:49

seemed just as likely as anything that maggots

7:51

could spontaneously generate, or

7:54

baby mice from grain, which was another

7:56

common folk belief. But eventually

7:59

scientists figured out a way to disprove this idea,

8:01

which really gained support when we realized

8:04

that tiny, unseen life lived around

8:06

us everywhere and that it had a hand

8:08

in a lot of the things that we saw. Germ

8:11

theory was developed and the concept

8:13

of spontaneous generation was abandoned.

8:16

That is until the nineteen fifties,

8:19

when researchers started working hard on figuring

8:21

out how life on Earth might have come

8:23

about. Spontaneous generation

8:26

made an unexpected comeback. A

8:29

biogenesis holds that quadrillions

8:32

upon quintillions of simple molecules

8:34

present in Earth's early atmosphere and

8:36

oceans randomly configured themselves

8:39

into a mind bending number of different combinations.

8:42

Some of these abominably large number

8:44

of combinations happen to create useful complex

8:47

molecules like amino acids, which

8:49

are the precursors for proteins. Now,

8:52

it's one thing to form simple molecules

8:55

to randomly combined to form more complex

8:57

molecules, but there is still the issue

8:59

of replication. Ship If those

9:01

molecules don't make a younger copy of themselves,

9:04

that innovative chemical chain is broken

9:06

and the new molecule loses the chance to

9:08

continue to evolve in a new and even more

9:10

complex things. And

9:12

this is the point where science is currently stuck.

9:15

Somehow, they say, some of those molecules

9:18

managed to form a stable string of

9:20

nuclear times, probably r N,

9:22

a ribonucleic acid which

9:25

is capable of doing two very important

9:27

things. It can encode information

9:29

in its nuclear tide chain, and it

9:32

can also transcribe those nuclear tides

9:34

to produce proteins. And once you

9:36

have proteins, you can do all sorts

9:38

of things that supports life. Back

9:41

in two organic

9:43

chemists Stanley Miller and Herald Urray

9:45

saw it to show that this was possible by recreating

9:48

the conditions of earlier Earth. In a flask. They

9:51

simulated a primitive ocean and built

9:53

an atmosphere out of the gases thought back

9:55

in the fifties to have been present on Earth soon

9:57

after it formed. They mimicked

9:59

why in storms with the flickering electrical

10:01

current, and when Miller inspected the

10:03

broth that resulted, he found

10:06

that nineteen amino acids and amans,

10:08

the precursors to proteins, had assembled

10:10

spontaneously. It

10:12

seems that Miller had shown that when the conditions

10:15

were right, the foundations for life would

10:17

arise. But

10:19

lately the idea that a biogenesis

10:21

just happened randomly is falling out

10:24

of favor. Instead, some

10:26

scientists have begun to suspect that there

10:28

is some set of organizing principles

10:30

that serves as a driving force for life

10:33

to emerge. Just like how

10:35

gravity will draw a ball downhill, or

10:37

how magnets will always repel or attract

10:40

one another when they're close together, there

10:42

is some fundamental governing force of nature

10:45

that causes life to assemble along

10:47

predictable lines. We just

10:49

haven't figured out what that force or those lines

10:52

are yet. This is

10:54

a pretty surprising idea if you think of

10:56

it. One of the laws of the universe,

10:59

the second law of therm mom dynamics, is

11:01

that things tend towards disorder, not

11:04

order. The idea that, when presented

11:06

with the right conditions, dead molecules

11:08

in the universe will organize themselves

11:10

into something living and breathing runs

11:13

totally counter to that, and

11:15

this new view also includes evolutionary

11:17

biology as one stretch of these

11:19

organizing principles of life. So

11:22

the idea is that molecules arrange themselves

11:25

into self replicating, metabolizing

11:27

parts that form increasingly complex

11:29

beings that eventually include you and

11:32

me, which makes you wonder

11:34

what the end point is. To

11:53

some people, the idea that life arose

11:55

from simple dead molecules that just happened

11:57

to randomly assemble themselves in the

12:00

living things is just too

12:02

unlikely to accept, And

12:04

even if we do accept that this is precisely

12:07

how life arose on Earth, the

12:09

idea that it could ever happen again anywhere

12:11

else is too improbable.

12:14

That virtually proves that we humans

12:16

are alone in the universe. One

12:19

issue people raise is time. They

12:22

say that Earth just simply hasn't been around

12:24

long enough for all of that random

12:26

chemical trial and error to have taken

12:28

place. The idea that life

12:31

organizes along some unknown universal

12:33

principles definitely addresses that idea

12:35

of time, and so does pant spermia.

12:38

Pant Spermia is a concept from astrobiology,

12:41

and it says that the seeds of life are

12:44

all over the universe in abundance everywhere.

12:46

They can be found on board asteroids

12:48

and other celestial objects, and

12:50

that these seeds of life are constantly bombarding

12:53

planets all over the universe.

12:56

If the conditions on the planet happen to be right,

12:59

well, then those sea needs of life will germinate

13:01

and grow into something new and living.

13:04

This certainly addresses the issue of time.

13:07

Life could have evolved elsewhere in the universe,

13:09

which is billions of years older than Earth, and

13:12

then spread to our planet aboard an asteroid.

13:15

We've recently found that some chemical

13:18

precursors to life can be found on celestial

13:20

objects like asteroids, and that they're

13:22

able to survive re entry into an atmosphere,

13:24

which can get pretty hot. This is

13:27

important because it's widely accepted that

13:29

an atmosphere is a precondition for life

13:31

to emerge. If you

13:33

take pants bermia, and you take the recent view

13:35

that life follows some organizing principles

13:38

as immutable as the laws of physics, then

13:40

you arrive at a conclusion that Earth

13:43

is just another place that happened to have the

13:45

right conditions when a rock bearing

13:47

the precursors to life landed around

13:49

four billion years ago. In

13:52

this view, then of course life

13:54

is abundant in the universe. But

13:57

then we find ourselves right back to where we

13:59

started. Where is everybody?

14:02

Perhaps the best answer to that comes not

14:04

from an astrobiologist or an astronomer,

14:07

but from an economist named Robin

14:09

Hansen, who proposed that

14:12

there must be something, some incredibly

14:14

difficult step between the point where

14:16

dead matter forms life and the point

14:18

where intelligent life becomes a galactically

14:21

colonizing civilization that

14:23

no species has ever been able

14:25

to overcome. He calls

14:27

it the great filter. Every

14:30

piece of matter in the universe is the sort of thing that could

14:32

have started that process, started

14:34

life, and then advanced life, etcetera. But

14:38

so far nothing out there has done

14:40

that. So the great filter is whatever is

14:42

in the way, whatever makes

14:45

it hard for any one piece of ordinary

14:47

dead matter to produce expanding, lasting

14:49

life. There's surely a countless

14:52

number of steps along the path from dead

14:54

matter to the emergence of a galactically

14:56

visible civilization. But the

14:58

Great Filter high Path says, supposes

15:00

that a handful of them are really, really

15:03

hard, and that one of them in particular

15:06

must be so hard that is thus

15:08

far prevented any life from reaching

15:10

galactic proportions.

15:13

This is Oxford University philosopher

15:15

Toby ord if there

15:17

were a hundred pieces you needed to get into the

15:19

right order in order to create something that

15:22

obeyed natural selection and and was

15:25

it the basic level needed

15:27

to actually bootstrap up towards

15:29

complex life, then there are a hundred

15:31

factorial ways you could arrange

15:34

those pieces. That's that's more than uh

15:36

tends to the power of a hundred different possibilities.

15:39

And then it just turns out you need an incredibly

15:41

rare event to get there. The Great

15:43

Filter offers two possible solutions

15:45

to the question posed by the Fermi paradox.

15:48

Whereas everybody everybody

15:50

never existed, or everybody

15:53

is dead, if the

15:55

Great Filter is in our past, it says

15:57

pretty strongly that we are the first and

16:00

only intelligent life that exists

16:02

in the universe. If that's

16:04

true, then by the Great Filter, there

16:07

is something, some step, some

16:09

right of passage you could call it, that

16:12

has prevented every other life from

16:14

reaching the point that we're at. And

16:17

if that's the case, then we are the only

16:19

life to have made it through the Great Filter.

16:23

That means that we can be optimistic about our future.

16:27

We made it through that right of passage that has

16:29

kept every other attempt at life from evolving.

16:32

It means our big, vast universe isn't

16:34

wasted on us. It's just waiting

16:36

there for us to use it, and

16:38

guilt free too. Since no one else

16:41

is around to use it, we have a

16:43

responsibility to put it to use.

16:45

You could even say. But

16:47

there's another possibility to the Great

16:49

Filter too. It may also

16:51

lie just ahead in our future.

16:55

Maybe intelligent life is a dime a dozen

16:57

in the universe. But the reason we don't

16:59

see other civilizations is

17:01

because they've all died off. And

17:04

if all of those intelligent civilizations

17:07

all died before any

17:09

any of them could make it off

17:11

of their home planet and spread throughout the universe,

17:15

the Great Filter is a big red flag

17:17

for us. It

17:19

tells us that we should expect to meet the same

17:21

fate that every other intelligent

17:24

life has. The Great Filter

17:26

will spell the end of the world for

17:28

us too. To

17:30

answer the question of whether we face imminent

17:32

doom or not, we have to look at the

17:35

evolution of life, and to

17:37

do that we have no choice but to

17:39

turn to the one place where we know life

17:41

evolved. At the risk of

17:43

falling victim to the selection bias, we

17:45

have to look to Earth for clues. We've

17:49

already talked about how utterly improbable

17:51

the origin of life appears to have been, but

17:54

it's probably best to start looking even

17:56

further back than that. If

18:01

life emerged on Earth, that means that the conditions

18:03

were right for life on Earth. That's

18:05

the one day to point we have ipso fact,

18:08

though, so the best way to find out

18:10

what conditions life requires is

18:12

to look at the conditions of our planet. And

18:15

it turns out that Earth has

18:17

some spectacularly peculiar characteristics

18:20

that make it ripe for life. So

18:22

peculiar, in fact, that a pair

18:24

of researchers named Peter Ward and Donald

18:27

Brownlee rolled all of them up into

18:29

what they call the rare Earth hypothesis. It's

18:31

just what it sounds like when you add up

18:34

all of its peculiarities. Earth

18:36

is not like other planets. First,

18:39

Earth happened to form around the right kind of star.

18:42

Our son is a main sequence star, which

18:45

means it produces light and heat by

18:47

fusing hydrogen into helium.

18:49

Main Sequence stars aren't rare. They

18:51

make up about nine of stars.

18:53

But our Son also happened to be of the right size

18:56

too. It's not so large that it

18:58

will use up its fuel in just a few billion

19:00

years. That means that the Sun's

19:02

slow, steady burn would go on long enough

19:05

to give life time to develop. The

19:07

Sun also isn't too small to support life

19:09

either. It is you could say,

19:12

just right for life. Our

19:14

Sun has also placed in a really great spot in our

19:17

galaxy. We happen to be located

19:19

out in the country, in a bit of a backwater

19:21

when it comes to the Milky Way, about

19:23

twenty eight thousand light years from the galactic

19:26

center, in a galaxy that's one thousand

19:28

light years across. For decades,

19:31

study presumed that the center of the galaxy

19:33

would be the most happening spot. That's

19:35

where the most stars tend to be, and more

19:37

stars would be more potentially habitable

19:40

planets. Civilizations

19:42

that arose in the center might even be in contact

19:44

with one another, forming something of a galactic

19:46

urban area. But recently

19:49

it's become clear that the galactic center

19:51

might not be so flourishing. After all. There

19:54

are more stars, sure, but more

19:56

stars also means that there are more collapsing

19:58

stars, which release bursts of

20:00

energy that can burn away the atmosphere of any

20:02

planets in the vicinity, So

20:05

the galactic center might actually be less

20:07

of a bustling urban area and more

20:09

like sterile and dead. Our

20:12

Sun is pretty far away from the galactic center,

20:14

way out past the suburbs where nothing

20:17

much happens. In other words, that's

20:19

good for life on Earth because it means that

20:21

it cuts down on the number of sterilization

20:23

events as life developed over Earth's

20:25

history, giving it a good chance of

20:27

succeeding. And Earth

20:30

just so happens to be located within the

20:32

Sun's goldilocks zone that I mentioned in the last

20:34

episode. If it was a little nearer

20:36

the Sun and about one and a half million kilometers

20:39

closer, it would be too hot to support

20:41

life, and much further away

20:43

Earth would be too cold for it.

20:46

It also turns out that where Earth is positioned

20:48

in the Solar System is hugely

20:50

important to giving life a fighting chance.

20:53

Earth is the third rock from the Sun, with

20:56

five others between us and the rest of the galaxy.

20:59

Six if the math that suggests there's a

21:01

planet nine out there turns out to be correct,

21:04

or if you continue to count hapless Pluto,

21:07

the Sun and the planets in our Solar System

21:09

formed from a massive cloud of cosmic

21:11

dust, that same stuff that might have

21:13

blown up so many alien pilots traveling

21:16

between the stars. We still

21:18

aren't quite sure exactly how the planets

21:20

formed. One model astoundingly

21:22

suggested that they could have formed in as little

21:24

as a thousand years, but

21:26

the birth of the Solar System was likely a

21:28

free for all grab of the elements that

21:30

make up the planets today. Initially,

21:33

astronomers presumed that the planets formed

21:36

in their current arrangement, but lately

21:38

it's become clear that that probably wasn't

21:40

the case. The planets may have actually

21:42

moved around and migrated from one

21:44

spot to another in the early life of the Solar

21:47

System before settling into the arrangement

21:49

that we see them in today. If

21:52

that's correct, then we were astoundingly

21:54

lucky that Jupiter ended up where it didn't.

22:02

Jupiter is a gas giant made up

22:04

largely of hydrogen and helium with a metal

22:06

and ice core. It's like a Sun that

22:08

never started burning because it lacked the

22:10

mass needed for gravity to kick start

22:13

fusion. But Jupiter is

22:15

massive, truly, you could

22:17

fit around within it.

22:20

It's the most massive planet in our Solar

22:22

System, and because of its mass

22:25

and its position between Earth and the chaos

22:27

of the interstellar space outside of

22:29

our Solar System, Jupiter acts

22:31

as a huge defensive guard for our

22:33

planet. When asteroids or comets

22:36

or other flotsam and jetsam bent on destruction

22:38

in oer our Solar System, Jupiter's

22:41

extraordinary gravitational pull draws

22:43

them into its orbit and slingshots

22:45

them out back into space. Without

22:48

Jupiter to run interference for Earth, it

22:50

would have a steady diet of life ending

22:52

bombardments from space. So

22:55

thanks Jupiter. This

22:58

is astronomer Donald brown Lee. He's

23:00

one of the guys who came up with the rare Earth hypothesis.

23:03

Jupers are actually pretty

23:05

rare uh planets

23:09

and um uh so uh

23:12

in terms of rare Earth. You know whether

23:14

it was juper is good or bad. Typical

23:16

planet as systems probably don't have jupers.

23:19

Our moon, too, seems to have played a number

23:21

of factors and fostering life on

23:23

Earth. Our moon's pretty

23:26

unusual itself as far

23:28

as moons go. It's enormous. It's

23:30

about one point to percent the mass of the

23:32

Earth and doesn't sound like much. But

23:34

other planets moons like Phobos and Demos,

23:36

which or a bit Mars, are closer to

23:38

asteroids in size. Our moon

23:41

more resembles a small planet, and

23:43

because it's so big, it has some very

23:45

peculiar effects on Earth. For

23:47

one, it stabilizes our planet. The

23:50

Earth doesn't sit upright as it spins around

23:52

on its axis. It's tilted actually

23:55

at about a twenty three degree angle.

23:58

Because of this tilt, we have seas which

24:00

create predictable variations in the temperature

24:03

of regions on Earth over the course of

24:05

the year. So when you think

24:07

about the difference between winter and summer,

24:09

you get a good idea how much variation a

24:12

tilt can create. And add

24:14

more of an angle to the tilt and the Earth

24:16

and the temperature variations would become more

24:18

severe. Since during winter or hemisphere

24:21

would be further away than it is now from the

24:23

Sun and much closer in the summer.

24:26

And if you added in some wobble, like if

24:28

the tilt of the Earth wasn't stable and fluctuated,

24:31

all of these wild swings and temperature

24:33

could make it very difficult for life to take

24:35

hold. The Moon's mass

24:38

actually exerts gravity over Earth and keeps

24:40

it stable, not wobbling or tilting

24:42

more than it does, and allowing for a

24:44

nice, gradual, predictable

24:46

and not two varying seasonal temperature

24:49

shifts that we even have

24:51

a moon appears to be a fluke itself. The

24:54

current view is that the Moon was calved off

24:56

from Earth following a head on collision in

24:58

the Earth's early history with a planetoid

25:01

about the size of Mars called Thea, which

25:03

the Earth likely absorbed. This

25:05

giant impact hypothesis explains

25:08

a lot. The Moon is unusually

25:10

large and unusually close to us

25:12

because it was created of a mixture of Earth and

25:14

that fateful planetoid. Because

25:17

the Moon is so close to Earth, it's tidally

25:19

locked and orbit around us. It doesn't

25:22

spin on its axis, which is why we

25:24

always see the same side of the Moon as

25:26

it orbits us. This is an important

25:28

feature because it also creates the tide

25:31

you're on Earth. As

25:33

the Moon orbits Earth, it pulls the oceans

25:36

toward it, stretching them out on the

25:38

ends and narrowing them in the middle. We

25:41

hear on our planet experience this as low

25:43

tide or high tide, depending on where you are.

25:46

And it was in these tidal pools of young

25:48

Earth's oceans where some people think

25:50

life began. When

25:55

those ancient tides came in, they deposited

25:58

a flood of molecules into the tidle pools,

26:01

and as they withdrew and the water evaporated

26:03

in the sun, the increasing concentration

26:05

of salt could have provided just the right

26:08

laboratory conditions where those earliest

26:10

chemicals could combine. Without

26:12

a moon as large and as close as ours is,

26:15

these tides could not have existed on Earth,

26:17

and those early proteins would have lacked that

26:19

kind of natural peatrie dish.

26:22

So the moon itself is a collection

26:24

of exquisite coincidences that supported

26:26

life on Earth. But perhaps the

26:28

most peculiar aspect of Earth is

26:31

that it has massive plates that make up the

26:33

crust, which slide along a molten

26:35

bed underneath that. It, in

26:37

other words, features plate tectonics.

26:39

It thought that this is a major

26:42

reason why the Earth has actually

26:44

had an amazingly stable

26:47

climate for most of you know, temperatures

26:49

for for most of its uh

26:51

age. So it's drastically different

26:54

than Venus or or

26:56

Mars, which are famously unstable

26:59

over tri lage uh time scope.

27:01

So so we really owe a lot uh

27:03

to this. We don't really know why I played

27:06

tectonics works on

27:08

our on Earth. It doesn't work on Mars, it

27:10

doesn't work on vines, it doesn't work on mercury,

27:12

doesn't work on any Yeah, I

27:14

mean they're there, are you know, movements

27:16

of rocks rolled to each other, but not I played tectonics

27:18

of the type that we that

27:21

we have here, So so we think it's really important.

27:23

As far as we know, Earth is the only planet

27:25

to feature plate tectonics, which

27:27

actually is a massive thermostat for the planet.

27:31

When oceanic plates slide underneath

27:33

the lighter continental plates, massive

27:35

amounts of the oceanic crusts is

27:38

crushed and absorbed into magma.

27:40

The rock in those oceanic plates contains

27:43

huge stores of carbon dioxide, so

27:45

when volcanoes release magma from

27:47

beneath the crust, it also contains some of

27:49

that CEO two, which travels as

27:51

a gas up into the atmosphere and

27:54

there it hangs around and it absorbs

27:56

sunlight, which in term warms the atmosphere

27:59

and eventually the planet below. And

28:01

when the planet warms, more of the oceans

28:03

evaporate, warming the atmosphere even

28:06

more. When you have a warm, wet

28:08

atmosphere, you have lots of

28:10

rain that rain brings

28:13

dissolves c O two back down to Earth.

28:15

Rocks on Earth are an excellent store of carbon,

28:18

and when CO two rich, rain falls

28:20

on them as the knock on effect of weathering

28:23

them, meaning the wearing down

28:25

definition, not the opposite one. All

28:27

of that carbon in the rocks and rain makes

28:30

its way to the sea, where it dissolves

28:32

and eventually sinks to the ocean's bottom.

28:35

As more carbon is removed from the atmosphere

28:37

and stored, the planet begins to cool

28:39

again. Over time, the carbon

28:41

at the bottom of the ocean forms new oceanic

28:44

plate crusts, and eventually it

28:46

will find itself along an ocean ridge where

28:49

it collides with the continental plate and

28:51

the whole process begins again, and the

28:53

Earth starts to warm once more. This

28:57

unique property of Earth has kept the planet's

28:59

climate stable more or less constantly

29:02

for more than four billion years, which

29:04

has allowed life on Earth to grow and

29:07

flourish.

29:14

When you take all of these details together,

29:16

a weird picture of Earth emerges. It's

29:19

almost freakishly perfect for life.

29:22

That's so many different variables, each

29:24

at just the right temperature, just the

29:26

right location, heat time, whatever

29:29

would come together to form a stable

29:31

whole seems to make Earth a

29:34

staggeringly improbable place.

29:36

You couldn't ask for a better place for life to emerge.

29:39

And maybe that's the point. Maybe

29:42

the seeds of life are commonplace in the universe,

29:44

but Earth is unique. We

29:47

simply don't know. We still

29:49

know so little about space, our galaxy,

29:52

and the universe that we can't say if

29:54

Earth is freakishly unique or one of

29:56

many. And as we get better at

29:58

deducing the existence of habit of planets

30:00

with our space telescopes, they seem

30:02

to pop out of the cosmos like a magic

30:05

eye poster does when you lose focus in

30:07

just the right way. Maybe planets

30:09

that can sustain life are more abundantly the

30:11

universe than we realize perhaps

30:13

assembling those seeds into life is

30:15

where the filter lies. The farther

30:18

back we look in time, the less we understand.

30:21

So if you're going to look for something that

30:23

might be really hard, harder than it seems,

30:26

farther back in time is the plausible place to

30:28

look, because that's where we don't understand things.

30:32

Uh, And the very first step from

30:34

completely dead matter to some proto

30:36

life that has to be the

30:38

earliest step on the one we have the

30:41

least knowledge of, and more plausibly it's

30:43

the hardest step. Or perhaps

30:45

not. Perhaps right now the universe is teeming

30:47

with primitive life. Perhaps the great

30:49

filter lies somewhere after that. There

30:52

were, after all, some enormous steps

30:54

to get from the emergence of life here on Earth

30:57

in the moment we're sharing between us right

31:00

now, m

31:14

back on the early Earth, tucked away just

31:16

so, in a solar system, tucked away

31:18

just so, in the galaxy with its

31:21

volcanoes chugging away at producing a warm

31:23

atmosphere and its oceans producing a

31:25

warm liquid medium for molecules

31:28

to organize into life. At some point,

31:30

one particular moment in Earth's history,

31:33

all of those separate components came

31:36

together to form a full fledged

31:38

living cell. As

31:40

far as we can tell, this moment happened around

31:42

three point eight billion years ago. At

31:45

first, these simple, single celled organisms

31:48

were nothing more than gooey bags,

31:50

with the parts for replicating themselves and

31:52

the parts for converting food into energy

31:55

all slashing together inside of the cell.

31:58

They spread by dividing into exact

32:00

replicas of themselves. But over

32:02

time, over a very very long time,

32:05

some new versions of these simple cells began

32:07

to appear, and they had new specializations.

32:11

Their interiors became compartmentalized,

32:13

no longer slashing together, which

32:15

meant that the processes they carried out, like

32:18

converting that food to energy, became

32:20

vastly more efficient. So

32:23

processes like photosynthesis were

32:25

able to develop. And after they did,

32:27

the oxygen that this early cellular

32:29

life excreted as waste started

32:32

to settle into the atmosphere, creating

32:34

an entirely new one that would eventually

32:37

support the rise of new types of life.

32:39

Then comes the invention of sex, a

32:42

new type of reproduction where two entirely

32:44

distinct individuals combined to form a

32:46

new third version of themselves. And

32:49

it's about here that natural selection

32:51

cracks its knuckles and comes aboard

32:54

as the driving force of evolution here

32:56

on Earth. When natural

32:58

selection is presented with new options

33:00

rather than rough copies of the same thing, new

33:03

adaptations emerge much more quickly,

33:06

which kicks evolution into hyper

33:08

drive. The simple celled

33:10

organisms that made up life on Earth got

33:13

better at being living things, and

33:16

then for a long time nothing

33:19

much changed. It seems

33:21

as if life had reached the stasis,

33:23

maxed out, come upon some invisible

33:26

wall, and take into coasting.

33:28

Rather than progressing toward ever more complexity,

33:32

Earth seemed content with its fast

33:34

seas teeming with specialized, extremely

33:37

well adapted, single celled organisms.

33:40

After that first three hundred million years

33:42

of ceaseless innovation, life

33:44

on earths stayed the same for the next three

33:47

billion years. But

33:49

around five hundred million years ago something

33:52

huge happened. Life suddenly

33:54

developed new ambitions, and

33:56

it exploded into new, extremely

33:58

complicated and sophisticated forms.

34:01

And in geological terms, it happened

34:04

overnight. Basically go from

34:06

nothing to everything. This is Dr

34:09

Phoebe Cohen. She's a paleontologist

34:11

at Williams College in Massachusetts.

34:14

So the vast majority of

34:16

the history of life on Earth is that

34:19

of microscopic organisms.

34:22

Before the Cambrian almost

34:24

all life was microscopic or

34:27

at least very very small, and

34:29

ecosystems were dominated by things

34:32

like amiba and bacteria.

34:34

And after the Cambrian ecosystems

34:37

are dominated by animals, um

34:39

and so it's a really, really huge shift in

34:42

the biological evolution

34:44

of our planet. This sudden surge

34:47

in complexity is called the Cambrian Explosion.

34:50

We aren't quite sure why it happened. It's

34:52

possible it was triggered when the widespread

34:54

glaciation that covered Earth just before

34:56

it began to melt. Or perhaps

34:59

it was the oxygen and levels in the atmosphere

35:01

released by photosynthesis slowly

35:03

displacing the Earth's atmosphere of carbon

35:05

dioxide, ammonia, and methane. We

35:08

need lots of oxygen to power the conversion

35:10

of food to energy, and perhaps

35:12

the Cambrian explosion was triggered when

35:14

atmospheric oxygen reached a critical

35:17

threshold that could support more complex

35:19

life. So if you go into low oxygen

35:21

areas of the ocean today, there's plenty of animals

35:23

living with almost no oxygen, but they're not doing

35:26

anything. They're very boring. They kind of just lay there

35:29

because they don't have enough oxygen to do anything

35:31

exciting. So one idea

35:33

is that oxygen did reach some sort of threshold

35:36

around the Cambrian that enabled organisms

35:38

to start doing fun things like chasing

35:41

after each other and ripping each other apart, and

35:44

that that played a big role in changing

35:46

sort of the structure of ecosystems that lead

35:48

to a huge diversification um

35:51

within the animal claid. Whatever

35:54

the reason, half a billion years ago,

35:56

most of the complex body plans that are

35:58

still around on Earth to day suddenly

36:01

arrived. It's as if those

36:03

organizing principles of life entered

36:05

a new phase. Within

36:08

just thirty million years, plants

36:10

made their debut on land, and just

36:12

forty million years after that, animals

36:14

followed them out of the water. It's

36:17

astounding to think of, but within

36:19

a span of just seventy million

36:21

years, life on Earth went from

36:23

nothing but single celled aquatic

36:25

organisms to animals

36:27

that lived and moved and walked around on land.

36:31

Because life had remained so simple for

36:33

so long before it, the Cambrian explosion

36:36

makes an excellent candidate for the Great

36:38

Filter. It could be so

36:40

unlikely that it universally denies

36:43

intelligent life from forming evolutionarily

36:46

speaking, there's never been a more important

36:48

event in the history of Earth. Following

36:51

the emergence of life, twenty

36:53

of the thirty six body plans that

36:55

exist on Earth today, the plans

36:58

that give shape to squid and the ish,

37:00

and humans and worms, all suddenly

37:02

appeared on Earth, and those

37:04

new body plans led to a riot

37:07

of evolution. The dinosaurs

37:09

rose and fell, and small mammals

37:11

emerged from their burrows and climbed the trees.

37:14

Tree dwelling apes found their way out of the

37:16

savannah and started walking upright,

37:19

becoming the first contours of humans.

37:22

And it's about here that we arrive

37:24

at another step in the long span

37:26

between the emergence of life on Earth and

37:29

us, the evolution

37:31

of intelligent life. Humans

37:36

tend to think of intelligence is what differentiates

37:38

us from other life on Earth, but

37:41

instead we seem more like the current

37:43

endpoint in an evolution of intelligence.

37:49

Signs of intelligence, in some form or fashion

37:51

are all around us. In two

37:53

thousand eight, Japanese researchers

37:56

showed that slime mold, a unicellular

37:58

organism, can learn a schedule

38:00

of electric shocks when they

38:02

shocked the mold at regular intervals,

38:05

and yes, they shocked mold. The

38:07

mold learned to anticipate the next shock

38:09

and recoil from it before it was delivered.

38:12

The Moosta plants have shown that they can learn

38:14

to differentiate between being dropped and being

38:17

touched. Where initially the plants

38:19

responded to both experiences in the same

38:21

way by curling their leaves, after

38:24

being dropped several times, they learned to keep

38:26

their leaves unfurled, and they retain

38:28

this learned behavior even after they haven't

38:30

been dropped or touched for months. The

38:34

use of tools, which we imagine is quintessentially

38:36

human, isn't unique to us either.

38:39

Chimpanzees use tools to make gathering

38:42

food easier, like sticks to draw

38:44

termites from their mountains by the fistful, and

38:46

rocks to bash open hard shelled nuts

38:49

to get to the meat inside. But

38:51

at some point we drew away from

38:53

the rest of life. In the development

38:55

of our intelligence, we became

38:57

the first animals to use tools

38:59

to make other tools. This

39:02

is the birth of technology. No

39:04

longer were we relegated to using

39:06

only what was found in nature. We

39:08

learned to use nature to fashion new tools

39:10

to better suit our needs. We

39:13

made spearheads and axes, out of stone

39:16

and learned to hunt large animals. Meat

39:18

is more energy dense than plants, and

39:20

we learned to cook food about eight hundred thousand

39:23

years ago. When we did, we unlocked

39:25

a tremendous amount of nutrients and energy that

39:27

hadn't been available to us before. We

39:30

developed language, which allowed us

39:32

to better coordinate ourselves and hunt together

39:35

and interact with one another more effectively.

39:38

We learned to make clothes to keep us warm

39:40

as we spread out beyond the subtropical

39:42

climate we evolved in. We learned

39:44

to make boats to carry us to new places.

39:47

We learned to make ceramics to store our food.

39:49

We learned to grow crops, which led to cities

39:51

in the foundation of the modern era. And

39:54

there was another quirk of that chance collision

39:56

between the planetoid THEA, and Earth, which

39:58

produced the Moon. It also produced

40:01

massive deposits of minerals and metals

40:03

near the surface that humans could easily get

40:05

to. Over time, we

40:07

abandon those stone tools in favor

40:10

of more reliable metal ones, and

40:12

eventually we put all of those millions

40:14

of years of accumulated intelligence

40:16

and technology into ships that broke

40:19

the bonds of Earth and launched the

40:21

first of our species into space. Within

40:24

the astronomically short period of about

40:26

a hundred thousand years, humans

40:29

left the wild and went to space.

40:32

But perhaps as unlikely as the emergence

40:34

of human intelligence may seem, it

40:36

may simply be the expected outcome of

40:38

those organizing principles of life. There

40:41

are two options before us. Then, either

40:43

we humans are unique in our universe and utterly

40:46

alone, or we are not. And

40:49

if there is other life elsewhere, then that

40:51

means that the great filter, the hardest

40:53

step, lies not in our past, but

40:55

in our future. It means

40:57

that the challenge that lies ahead of us is more

41:00

difficult, more improbable to overcome

41:02

than dead molecules organizing themselves

41:05

into living cells or apes

41:07

learning to build ships to the moon, And

41:10

rather than having millions of years to try

41:12

and fail before succeeding, we will

41:14

have one shot to get it right.

41:17

If the great filter lies in our future, then

41:19

it appears that we are entering it right

41:21

now, now here in the

41:24

twenty one century, four point three

41:26

billion years after life emerged on Earth.

41:28

We are entering the evolutionary step that

41:31

no life in the universe has ever managed

41:33

to survive. Carl Sagan had

41:35

this this great phrase about

41:38

humanity has grown powerful before

41:40

it's grown wise um and our power

41:43

through technology has been increasing

41:45

exponentially, but our wisdom

41:47

has been maybe it's been increasing

41:50

a little bit, but suddenly not exponentially,

41:53

and it's it's getting these two

41:55

things have got out of check with each other. We've got an unsustainable

41:58

level of risk. The technology

42:00

that got us to this point is taking a new shape,

42:03

one that we haven't encountered before, and

42:05

it is presenting new risks to the survival

42:07

of our species and indeed life

42:10

on Earth. You right

42:12

now are living in what maybe

42:15

the beginning of the most dangerous period

42:17

in the history of the human race.

42:32

On the next episode of The End of the World

42:34

with Josh Clark, if

42:36

we are the only intelligent life, humans

42:38

could have a bright, long future ahead of

42:41

us, a triple less civilization

42:43

based on super intelligence,

42:46

super happiness, and super longevity.

42:48

But between us and that bright future lay

42:51

existential risks, and they're like

42:53

nothing we've ever encountered before.