Latest / Talking Science / Extinction – The Five Great Dyings

Transcript

Extinction – The Five Great Dyings · Talking Science

0:00
More than 99 % of all the species that have ever
0:02
existed on Earth are just, well, gone. They're
0:05
completely extinct. Yeah, it's a staggering number
0:08
when you really stop and think about it. Right.
0:10
I mean, when you look at the incredible diversity
0:12
of the living world today, you know, the elephants,
0:16
redwood trees, deep sea corals, or even just
0:19
you and me, we're looking at a tiny fractional
0:21
remnant of a much grander, much more volatile
0:24
story. It really forces a complete perspective
0:27
shift. Because we tend to think of the biosphere
0:30
as this permanent fixture. But the fossil record
0:35
tells us that life on Earth is essentially a
0:37
series of these massive sweeping resets. Which
0:41
is exactly the core mission of our deep dive
0:43
today. We're pulling from a really fascinating
0:45
stack of sources outlining the big five mass
0:48
extinctions in Earth's history. Right, the big
0:50
ones. Yeah, and we want to understand the mechanisms
0:52
of how our planet wipes the slate clean, you
0:55
know, and what those ancient catastrophes can
0:57
actually tell us about the environmental pressures
1:00
we're facing right now. The mechanics of those
1:02
resets are crucial to understand because learning
1:05
how a planet sheds its biosphere, it isn't just
1:08
a history lesson. It's, well, it's a lesson in
1:11
the physical limits of biology. Absolutely. Yeah.
1:13
And, you know, you don't need a science degree
1:15
to follow this. We're going to break it all down
1:17
together. I actually found an analogy really
1:19
helpful. While reading through the material.
1:22
Oh, yeah. What was it? Okay, so imagine a massive
1:26
global library containing every single book ever
1:29
written in the history of the world. Just a true
1:32
archive of everything. Okay, I'm picturing it.
1:35
Now, imagine that 99 % of those books have been
1:38
destroyed. The surviving 1 % is what we see outside
1:41
our windows today. But to understand how that
1:44
library lost so many books, we have to distinguish
1:46
between... you know, a few books naturally wearing
1:49
out over centuries versus the entire building
1:52
catching fire. That's a great way to look at
1:54
it. The natural wearing out of the books, that's
1:57
what biologists call the background rate of extinction.
1:59
Right, the background rate. Yeah, because species
2:02
are not permanent. Just as individual organisms
2:05
are born and eventually die, a species as a whole
2:08
emerges, adapts to its environment. persists
2:11
for, you know, perhaps a few million years and
2:14
eventually fades away. Wait, so a species dying
2:16
out is just a natural part of how life turns
2:19
over. It's not always this huge disaster. Exactly.
2:22
It gets out competed or its specific niche changes.
2:25
That slow, steady trickle of loss is just the
2:28
baseline rhythm of evolution. So sort of a zero
2:31
sum game in slow motion. As one species fades
2:34
out, a new one is usually evolving to take its
2:37
place. Right. The shelves of your library are
2:39
constantly being restocked as the old books crumble.
2:42
And as long as the rate of new species emerging
2:45
keeps pace with that background extinction rate,
2:48
the biosphere remains totally robust. Okay, but
2:51
we're talking about the library catching fire
2:52
today. Yes. Five times in the last half billion
2:55
years, that dynamic completely fractured. The
2:57
rate of loss skyrocketed so fast that evolution
3:00
just couldn't keep up. The library caught fire.
3:02
And that's the technical definition of a mass
3:04
extinction, right? A sharp worldwide spike where
3:08
a massive share of all species across, you know,
3:10
oceans, land, plants, animals, they all just
3:13
collapse together in a short span of geological
3:15
time. Yes, in a geological blink of an eye. Now,
3:19
a blink of an eye in geology can still be thousands
3:21
of years, but in the context of deep time, it's
3:25
virtually instantaneous. But how could we possibly
3:28
know this if it happened hundreds of millions
3:30
of years ago? Like, how do we read that history?
3:33
We read it in what we can call the stone diary,
3:35
the sedimentary rock record. OK, the stone diary.
3:38
Right. Over millions of years, sand, mud and
3:41
minerals settle and compress into distinct rock
3:43
layers. And the simple rule of superposition
3:46
means that the deeper layers are older. And those
3:48
layers act as pages, I assume, trapping the fossils
3:51
of whatever was living when that mud was laid
3:54
down. Precisely. If the background rate is normal,
3:56
you just see a fluid, gradual transition in the
3:59
types of fossils as you read the layers from
4:01
bottom to top. But a mass extinction leaves a
4:04
stark, sudden boundary. Like a literal line in
4:07
the rock. Exactly. You'll have a layer of limestone
4:10
teeming with a vast, vibrant array of marine
4:13
fossils. Then, directly above it, a thin band
4:16
of barren clay. And above that, a drastically
4:19
impoverished layer where most of those previous
4:22
life forms are simply gone. Wow. Replaced by
4:24
completely different organisms. Yeah, exactly.
4:26
So that line in the rock is basically the fire
4:29
alarm. Let's open up the earliest chapters, the
4:31
big five. I want to walk through the mechanisms
4:34
of each because the sources show it's never as
4:36
simple as just everybody died. No, it's usually
4:39
a very specific cascade of events. Right. So
4:41
the first one happens around 444 million years
4:44
ago at the end of the Ordovician period. And
4:46
keep in mind, this was a marine world. Almost
4:48
all complex life was in the oceans back then.
4:51
And this first great dying was driven by a dramatic
4:54
climate swing. The supercontinent, Gondwana,
4:57
drifted over the South Pole and a massive ice
5:00
age began. But looking at the sources, the cold
5:02
itself wasn't the only killer, was it? It was
5:04
what the ice actually did to the water. Yeah,
5:07
the formation of massive continental ice sheets
5:09
locked up huge amounts of the planet's water.
5:12
And this caused global sea levels to just plummet
5:15
drastically. Which is bad news if you live in
5:17
shallow water. Exactly. Most life thrived in
5:20
the shallow, warm, sunlit waters of the continental
5:23
shelves. When the ocean retreated, those vital
5:26
habitats were exposed to the air and destroyed.
5:29
So it was a massive loss of real estate. The
5:32
shallow seas just... drained away. And then reading
5:36
further, the climate kind of whiplashed back
5:38
the other way. It did. It's a brutal double punch.
5:40
Volcanic activity was pumping carbon dioxide
5:42
into the atmosphere. But because the rocks were
5:45
covered in ice, the normal weathering processes
5:47
that pull CO2 out of the air, they slowed down.
5:50
So the greenhouse gas just builds up. Right.
5:52
The climate rapidly heated, the glaciers melted,
5:54
and the oceans flooded back in. Freezing and
5:57
draining, followed by rapid heating and flooding,
5:59
it wiped out about 85 % of marine species. That
6:02
is... That's just wild. Which brings us to the
6:06
second event, the late Devonian, about 372 million
6:09
years ago. And what stood out to me here is that
6:12
this wasn't one single bad day, but a series
6:16
of drawn out pulses. Yes, very drawn out. And
6:19
the trigger seems incredibly ironic. It was actually
6:22
the success of life on land that murdered life
6:24
in the oceans. It's a profound biological paradox,
6:27
really. During the Devonian, plants on land began
6:29
developing deep, complex root systems for the
6:32
very first time. They started breaking up solid
6:35
rock, actively creating deep soil. Which, I mean,
6:38
sounds great for the land. It was revolutionary
6:40
for the land. But all those loose nutrients and
6:43
minerals from the new soil were eventually washed
6:45
by rain into the rivers and flushed straight
6:47
into the oceans. Oh, I see where this is going.
6:49
Yeah, this massive influx of fertilizer caused
6:52
unprecedented global algae blooms. in the shallow
6:55
seas. Wait, that sounds exactly like the agricultural
6:57
runoff dead zones we see today, like in the Gulf
7:00
of Mexico, just on a planetary scale. The mechanism
7:03
is identical. The algae bloom wildly, and when
7:07
they die and sink, bacteria consume them. And
7:10
those bacteria multiply and consume almost all
7:13
the dissolved oxygen in the water. Wow. So the
7:16
oceans just became totally oxygen star? Yes,
7:18
anoxic. The life that had survived the Ordovician
7:21
essentially suffocated because the plants on
7:23
land were doing too well. It just shows how deeply
7:26
interconnected the Earth's systems are. You change
7:29
the dirt, you suffocate the sea. Let's fast forward
7:32
a bit to the fourth extinction, around 201 million
7:35
years ago, just to keep things chronological
7:37
before we tackle the really bad one. Ah, the
7:39
N Triassic. Yeah. The N Triassic is a classic
7:42
case of geological forces tearing the world apart.
7:45
The supercontinent Pangaea began to rift, literally
7:49
unzipping to form what would eventually become
7:51
the Atlantic Ocean. Which sounds incredibly violent.
7:53
It was. This unzipping created volcanic rifts
7:57
that pumped colossal amounts of carbon dioxide
7:59
and sulfur dioxide into the ocean. to the atmosphere.
8:01
Another massive climate shock. And this is the
8:04
event that kind of cleared the ecological stage,
8:06
right? Because the sources point out that by
8:08
wiping out the dominant reptiles at the time,
8:10
this extinction opened up the space that allowed
8:13
the dinosaurs to rise to dominance. Exactly.
8:16
Dinosaurs were a relatively minor group before
8:19
that volcanic rifting. The enterassic essentially
8:22
handed them the keys to the biosphere. Okay,
8:24
so we covered the first, second, and fourth.
8:26
We skipped the third because it really requires
8:28
its own dedicated focus. About 252 million years
8:32
ago, at the end of the Permian period, the Earth
8:34
experienced what scientists grimly call the Great
8:37
Dying. The Great Dying. It is the closest complex
8:41
life has ever come to total annihilation. Roughly
8:44
90 % of all marine species and the majority of
8:47
land species were completely wiped out. Nine
8:50
in ten species in the ocean. The library wasn't
8:53
just burned, it was incinerated. What could possibly
8:56
drive the numbers that high? The root cause was
8:58
an almost incomprehensible volcanic event in
9:00
modern -day Siberia. Okay, let me ask for some
9:03
clarification here, because when people hear
9:05
volcanism or a volcanic event, they usually picture,
9:09
you know... One giant mountain blowing its top
9:12
like Mount St. Helens in a movie. Right. A big
9:15
explosion, ash, and then it's over. But flood
9:18
basalt volcanism is entirely different. Imagine
9:21
the Earth's crust splitting open and massive
9:24
fissures and rivers of highly fluid magma just
9:27
pouring out across the landscape. And this wasn't
9:29
a quick thing, right? No, this didn't last for
9:31
a few days. The eruptions pulsed on and off for
9:34
hundreds of thousands of years, eventually covering
9:37
an area the size of the continental United States.
9:41
It's hard to even fathom that scale. It makes
9:43
me think of an analogy. It wasn't an explosion
9:46
that blew up the garage. It was more like someone
9:48
parking a car in a closed garage, leaving the
9:51
engine running and letting the exhaust slowly
9:53
fill the space. The exhaust analogy is incredibly
9:56
apt, actually, especially when you look at the
9:59
specific geology of Siberia at that time. Why,
10:01
what was in Siberia? Well, the magma wasn't just
10:04
erupting onto the surface. It was intruding into
10:07
massive underground basins of coal and carbonate
10:11
rocks. The heat literally baked these ancient
10:15
fossil fuel deposits, releasing staggering volumes
10:18
of carbon dioxide and methane into the air. Oh,
10:21
wow. So it was essentially the ultimate natural
10:23
burning of fossil fuels. Exactly. And the cascade
10:26
of horrors that followed is what did the killing.
10:28
The atmosphere superheated, the carbon dioxide
10:31
mixed with water vapor to form acid rain, which
10:35
stripped the land of vegetation. And without
10:37
plant roots, the soil washes away again. Right,
10:39
into the oceans. The oceans absorbed the excess
10:42
CO2 and became violently acidic, which dissolves
10:45
the shells of marine life. And as the oceans
10:47
heated up, they lost their oxygen. The closed
10:49
garage. Acidic, suffocating, and boiling. It's
10:52
terrifying. It took millions of years for the
10:54
world to recover from that. But while the Great
10:56
Dying was the absolute worst in terms of numbers,
10:59
the Fifth Extinction is definitely the most famous.
11:01
The end Cretaceous, 66 million years ago. The
11:04
dinosaur killer. Yes, the end of the non -avian
11:07
dinosaurs. And unlike the drawn -out poisoning
11:10
of the Permian, this was a sudden, violent, external
11:13
shock. A city -sized asteroid slammed into modern
11:17
-day Mexico. The kinetic energy of that is just...
11:20
Mind -blowing. But reading the sources, it wasn't
11:23
just the explosion or the tsunamis that killed
11:25
the dinosaurs worldwide. It was exactly where
11:28
the asteroid hit. The location was uniquely catastrophic.
11:31
The bedrock there was rich in gypsum, which contains
11:33
high amounts of sulfur. The immense heat of the
11:36
impact vaporized that rock, injecting billions
11:39
of tons of sulfur aerosols high into the stratosphere.
11:42
And sulfur aerosols are highly reflective, right?
11:44
So they essentially formed a global mirror in
11:47
the sky. They blocked out the sun. The earth
11:49
plunged into a deep, freezing impact winter.
11:52
Without sunlight, the food chain collapsed from
11:54
the bottom up. Now, I do want to note a detail
11:56
from the sources. The biosphere wasn't exactly
11:59
in perfect health when that rock arrived, right?
12:01
True. There's evidence that massive volcanic
12:04
eruptions in what is now India were already stressing
12:07
global ecosystems. The asteroid was the final
12:10
knockout punch. But not everything died. And
12:13
this is where the story gets really fascinating
12:14
to me. The giant reptiles vanished, but a small
12:18
branch of avian dinosaurs survived. The ancestors
12:21
of modern birds. Right, which is amazing. And
12:24
alongside them, another group survived. Small,
12:27
furry mammals that had spent the last 100 million
12:30
years hiding in burrows. Their small size and
12:33
ability to burrow protected them from the worst
12:35
of it. They needed very little food. So when
12:37
the dust settled, those surviving mammals inherited
12:40
a newly empty world. Which leads to this massive
12:43
aha moment. Because those small, furry mammals
12:46
survived, we are literally here today because
12:48
of that specific asteroid hitting Mexico, 66...
12:51
million years ago. It is a humbling thought.
12:54
We are the beneficiaries of that specific apocalypse.
12:56
But how did we actually prove it? You mentioned
12:58
the stone diary earlier. What is the literal
13:01
text in the rock that proves a space rock killed
13:03
the dinosaurs? It's a beautiful piece of geology.
13:06
In rock layers all over the globe, right at the
13:09
exact geological moment the dinosaurs vanish,
13:11
scientists found a thin layer of clay. And this
13:14
clay contains unusually high concentrations of
13:16
a metal called iridium. And iridium is not something
13:19
you typically find in the Earth's crust. Right.
13:22
It's extremely rare on the surface, but highly
13:24
abundant in asteroids. Finding a worldwide dusting
13:28
of this cosmic metal right on top of the last
13:30
dinosaur fossils was the chemical smoking gun.
13:33
Okay, so we've looked at the Big Five. Ice ages,
13:36
ocean dead zones, continents unzipping, volcanic
13:39
coal fires, and an asteroid. Seems like a random
13:42
assortment of disasters. But the sources point
13:44
out a clear, common thread. The mechanism varies,
13:47
but the underlying killer is always the same.
13:50
The speed of the change. The speed. Let me try
13:53
an everyday analogy here. Think of an ecosystem
13:55
like a car's shock absorbers. They can handle
13:58
a bumpy, changing road if you drive slowly, right?
14:01
The background rate of evolution relies on that
14:03
slow change. That's a good way to picture it.
14:05
But if you're flying down the road and you hit
14:07
a massive speed bump at 100 miles an hour, the
14:10
suspension doesn't have time to compress. It
14:11
just snaps. Exactly. Earth's climate has always
14:15
fluctuated. But when those fluctuations happen
14:17
over tens of millions of years, life adapts.
14:20
When they're compressed into a few thousand years
14:23
or a few months, with the asteroid biology cannot
14:26
mutate fast enough to survive. The speed breaks
14:29
the system. Which brings us to a very serious
14:31
question about the world we're driving in right
14:33
now. Because many scientists argue we're in the
14:36
early stages of a sixth mass extinction. And
14:39
this time, the driver is human activity. Yes.
14:42
Habitat destruction, climate change, pollution,
14:45
overhunting, invasive species. We're applying
14:48
the pressures of multiple ancient mass extinctions
14:50
all at once. Now, to be fair to the scientific
14:52
debate, and we want to ensure we're looking at
14:54
the sources impartially here, the sources do
14:56
emphasize that we haven't lost anything close
14:59
to the 90 % seen in the ancient events, right?
15:01
And exact rates are genuinely difficult to measure.
15:05
That is a crucial distinction. The total percentage
15:07
of extinct species right now is relatively small
15:09
compared to the big five. But the alarm is not
15:12
based on the magnitude of what is already dead.
15:14
It's based on the rate of what is dying. The
15:17
speed bump. Exactly. While the exact rate is
15:20
debated, the direction and cause are undisputed.
15:23
Species are being lost significantly faster than
15:25
normal because of us. So we haven't hit the bottom
15:27
of the cliff yet, but we are changing the world
15:29
faster than life can keep up. That is the sober
15:32
lesson from the deep past, yes. It's a grim reality
15:34
to process. Looking at the stone diary, is there
15:38
any comfort to be found in there? Well, there
15:40
is the strange comfort of resilience. Life has
15:43
been struck down to a whisker of oblivion five
15:45
times, and every single time it recovered, reassembling
15:49
into new, wonderful forms. Life at the biosphere
15:53
level is astonishingly resilient. But there's
15:56
a huge catch to that resilience, isn't there?
15:58
Yes. The long run for recovery means millions
16:01
of years. Recovery offers absolutely no rescue
16:04
to the specific species caught in the fall, which
16:06
right now includes us. Yeah. Think about that
16:09
stone diary we talked about today. If life recovers
16:12
and a new intelligent species digs through the
16:15
rock layers 100 million years from now, What
16:17
exactly will our layer look like? It's a profound
16:19
question. Will they find a thin, strange band
16:22
of pollution, microplastics, and habitat loss
16:24
marking our time, just like we found the iridium
16:27
from the asteroid? And what wonderful, unimaginable
16:31
new forms of life will have taken our place?
16:33
Thank you for joining us on this deep dive. Keep
16:36
looking closely at the world around you and keep
16:38
asking questions.