Original subtitles

HUGH JACKMAN: Ice Age Australia.

Home to giant and bizarre animals known as megafauna.

Herds of wombat-like herbivores run vast grasslands.

Living alongside kangaroos, with strange short faces.

And ruling this land, the apex predators.

Heavily armoured giant lizards

and a marsupial-killing machine Heavily armoured giant lizards

and a marsupial-killing machine armed with bone-crushing jaws.

These majestic animals lived here for millions of years.

But in a blink of time...

..they were wiped from the face of the Earth.

So what happened?

Today, experts are gathering clues

from remote and inhospitable parts of Australia.

There's been 12 people on the moon and 10 people in this cave.

This place is like nowhere else in the world, really.

This is pretty bloody good. (CHUCKLES)

These scientists are testing the leading theories

behind this disappearance.

Is this the smoking gun that explains the megafaunal extinctions?

New evidence will rewrite Australia's Ice Age cold case.

What killed Australia's giants?

Deep within the Naracoorte Caves in South Australia

lies the ultimate megafauna crime scene.

WOMAN: This is one of the largest deposits of megafauna fossils

anywhere in the world.

In this tomb are the fossil remains of 26 species of extinct megafauna.

So we've got in front of us bones of a giant marsupial browsing animal

that was called Zygomaturus, weighed half a tonne. So we've got in front of us bones of a giant marsupial browsing animal

that was called Zygomaturus, weighed half a tonne.

that was called Zygomaturus, weighed half a tonne. We have Thylacoleo carnifex, the gigantic marsupial predator

We have Thylacoleo carnifex, the gigantic marsupial predator

which really did rule the jungle around here.

And we can see also skulls of gigantic kangaroos

that had short faces and ate leaves instead of grass.

For palaeontologist Liz Reed,

this crypt offers rare insights into the creatures' final moments.

These caves have literally acted as death traps

for hundreds of thousands of years, and we call them pitfall traps.

If you can imagine a small entrance on the surface,

If you can imagine a small entrance maybe concealed by vegetation,

maybe concealed by vegetation,

which leads down a relatively maybe concealed by vegetation,

which leads down a relatively narrow deep hole into a cave.

So animals are hopping around, moving around the surface.

They don't see this entrance until it's too late

and they'll just plummet down into these caves.

But their brutal deaths provide a treasure trove of information,

including a time line to their extinction.

What we're looking at here is what we call a stratigraphic section,

which is basically a series of sediment layers

that have washed into the cave over time.

At the same time that animal remains were accumulating.

At the same time that animal This is a classic fossil deposit,

This is a classic fossil deposit,

and we've dated this deposit from around 13,000 to 65,000 years ago.

If we look at a particular layer in time,

if we come down here to around 42,000,

if we come down here we see something quite remarkable.

we see something quite remarkable.

We see the last appearance we see something quite remarkable.

We see the last appearance of the megafauna.

And these were animals that had a long evolutionary history

And these were animals of millions of years,

of millions of years,

but that came to a halt of millions of years,

but that came to a halt for many of them

around this time when they became extinct.

Back then, the world was in the grip of an ice age.

While Australia stayed mostly free of glaciation,

the climate went through periods of rapid and extreme changes.

Dominating the country were huge species, weighing up to 2,500 kilos,

known today as megafauna.

So what could have caused almost 90% of these animals to disappear

by 40,000 to 50,000 years ago?

Many scientists around the continent

are trying to look at different lines of evidence

to understand what may have driven

these magnificent creatures to extinction.

One is whether climate had some kind of role to play.

So changes in the environment and the climate

did not favour those particular animals.

Everyone's aware that there is some dramatic changes happening

Everyone's aware that there on the Earth at the moment.

on the Earth at the moment.

One is biodiversity loss, on the Earth at the moment.

One is biodiversity loss, the other is climate change.

But we can't really get a long perspective on that

by just looking at what's happening now.

So studying megafauna helps us understand

So studying megafauna helps us what drives animals to extinction.

what drives animals to extinction.

So that the dead are actually helping us inform

So that the dead are actually about the future of our planet.

about the future of our planet.

PILOT: Traffic Arkaroola helicopter hotel alpha becoming airborne.

Tracking to the north-east for Lake Callabonna.

Tracking to the north-east Not above 2,500. Traffic Arkaroola.

Not above 2,500. Traffic Arkaroola.

Palaeontologist Aaron Camens Not above 2,500. Traffic Arkaroola.

Palaeontologist Aaron Camens is travelling deep into the outback

to an ancient lake bed that could contain important clues

about whether climate change caused the demise of the megafauna.

Lake Callabonna is like nowhere else in Australia or the world, really.

Lake Callabonna is like nowhere else It's a huge salt lake today,

It's a huge salt lake today,

It's a huge salt lake today, but 50,000 years ago,

but 50,000 years ago,

there was a lot more water around.

And literally thousands of megafaunal animals

And literally thousands got stuck in the mud.

got stuck in the mud.

And what this means is that we see, got stuck in the mud.

And what this means is that we see, rather than just one or two bones,

complete skeletons preserved where the animal got stuck.

And because they're stuck in the mud,

they're essentially partly buried as they die,

which means that the fossilisation detail is much higher.

And that is something that we do not see anywhere else in Australia

And that is something that we do not for any of these species.

for any of these species.

It's been four years since Aaron's last visit to the lake.

Ah, it's exciting to be back here.

These shifting sands continually reveal new megafauna remains,

while earlier discoveries disappear forever,

eroded by the harsh desert conditions.

Finding valuable evidence here is a race against the elements.

So here we can see that we've got to this one too late.

So here we can see that So the bone is reduced to powder.

So the bone is reduced to powder.

If it's in that weathering zone, it can be eroded or weathered

If it's in that weathering zone, very, very quickly.

very, very quickly.

But amidst the crumble remains very, very quickly.

But amidst the crumble remains is a precious discovery.

Wow, this looks great.

It's quite extensive too.

So what we've got here is the skeleton from Diprotodon optatum,

the thing that has made Callabonna famous.

Up at this end, we've got the fragmented remains of the skull.

This is so well preserved that we can see the body

still as it was when the animal died.

We've got the spine down the middle, the pelvis at the back,

and the legs are still in the position

where this giant marsupial fell over on its side.

This is the biggest marsupial that ever lived.

Stood about two metres tall at the shoulder,

3.5m long, weighed over two tonnes. at the shoulder,

3.5m long, weighed over two tonnes.

So it's about the size of your urban SUV.

Five lumbering giants have gathered by the water's edge.

There is a new member of the family.

At nine months, this joey is exploring his surroundings,

spending less time in his mother's protective pouch.

Diprotodons were widespread across much of Ice Age Australia.

Diprotodons were widespread Males were much larger than females,

Males were much larger than females,

suggesting they could have lived in small family groups

suggesting they could have lived characterised by one dominant male.

characterised by one dominant male.

(ROARS)

Their large nasal cavities might have been used

to make deep grunting calls.

Like koalas today, low frequencies

would have allowed them to maintain social communication

over long distances.

This is one of the most exciting parts of Callabonna for me.

The lake bed has safeguarded the most precious of details.

Nowhere else in Australia do we see the soft tissue

Nowhere else in Australia of megafaunal animals preserved.

of megafaunal animals preserved.

If we have a really close look here,

we can see this kind of dimpled surface.

Now that is actually recording the foot pads of Diprotodon.

So where Diprotodon got stuck in the mud here,

the mud has wrapped around its foot,

and that impression of the soft skin of the foot

has been preserved for over 50,000 years.

Staggeringly, the remains of nearly 900 Diprotodons

Staggeringly, the remains have been found at Lake Callabonna.

have been found at Lake Callabonna.

So why did so many take their last steps here?

If we go back 50,000 years,

Lake Callabonna is progressively getting drier.

We're talking about a time of extreme climatic variability,

and that change in climate

could be one of the driving factors

in what led to the megafaunal extinction.

As the lake contracted over time,

animals were forced to move through the mud

animals were forced to get to the shrinking water supply.

to get to the shrinking water supply.

And the heavier the animal, the more likely it was to get trapped

in a muddy grave.

Fortunately, for this family of giants,

they've safely navigated the perils of the lake.

But there may have been another deadly consequence to being so large

in a drying environment.

So one of the theories is that because these animals were so big,

they may have been very slow to breed.

They might have only had one joey

as opposed to two, three, four, or even ten,

like some of our carnivorous marsupials have.

There might have been a need for many good years in a row,

so maybe four or five years of good vegetation

to be able to raise a young to maturity.

As temperatures become more extreme and rainfall changes,

what was a fertile landscape dries out completely. As temperatures become more extreme and rainfall changes,

what was a fertile landscape dries out completely.

The lake and its vital water has disappeared.

Despite its mother's best efforts,

this baby Diprotodon cannot go on.

Now out at Lake Callabonna, we have a drought period,

but that doesn't mean that that was what drove the megafauna extinct.

At one area, we might see what's called extirpation,

which is the local extinction of a group of animals.

But that doesn't mean that they've gone extinct

But that doesn't mean on a continental scale.

on a continental scale.

So was drought driving extinctions in other parts of Australia?

Over 1,300 kilometres to the east of Lake Callabonna,

researchers are trying to determine

researchers are trying to determine if climate change could have caused megafauna to die out

if climate change could have caused megafauna to die out

if climate change could have caused in other parts of the continent.

in other parts of the continent.

The back yellow one should be loosened, John.

Haidee Cadd is a palaeoecologist who studies environments of the past.

Haidee Cadd is a palaeoecologist Flip it.

Flip it.

She's part of a team

aiming to reconstruct the environmental conditions

present here at Welsby Lagoon during the time of megafauna extinction.

Welsby Lagoon is a really interesting and important site.

It's one of the few sites that we've got in Australia

that hasn't dried up during the last 80,000 years.

So we've managed to get a continuous record.

It's disconnected from the regional water table

and it also has no inflowing or outflowing streams.

So it provides a really great natural case study

for us to look at things like changes in environment and climate.

That's Litoria cooloolensis, which is one of the acid frogs

That's Litoria cooloolensis, that only live in Wallum wetlands.

that only live in Wallum wetlands.

It's an endangered species.

They're very common in this wetland.

They're jumping everywhere off the reeds in front of me.

They're jumping everywhere It's quite amazing.

It's quite amazing.

That's all untangled, Pete? Yep.

Let's put them on.

Today, the team will drill down deep Let's put them on.

Today, the team will drill down deep enough to extract sediment layers

Today, the team will drill down deep dating back 40,000 to 50,000 years.

dating back 40,000 to 50,000 years.

Yep.

OK.

When we take our cores out of the sediment,

we see often just brown mud.

Ready?

OK. Slow.

But when we take those cores back OK. Slow.

But when we take those cores back to the laboratory

But when we take those cores back and we start to analyse them,

and we start to analyse them,

that's how we can get a precipitation record.

So that's how we can start to work out

whether the changes we're seeing in the environment So that's how we can start to work out

whether the changes we're seeing in the environment

are related to changes in climate as well.

are related to changes in climate as well. For the first time in tens of thousands of years,

For the first time in tens of thousands of years,

the lagoon's hidden history is revealed.

It's quite fibrous and peaty at the top here

and it looks like it changes gradually through here.

Much finer here. But look down there.

Yeah.

That last five centimetres or so is very different, isn't it?

Like you can feel an edge there. Yep.

This is very much lake sediment.

The sediment core tells a story

of significant environmental and climactic change.

The first part of our record, we see quite wet conditions.

There's rainforest growing around the site.

But as we move through between 60,000 and 50,000 years ago,

we start to see some really dry conditions

that shifts the vegetation into this open grassland kind of environment.

Shall we wrap it? Yep. Let's wrap her up.

We've looked at other records along the east coast of Australia,

and we've managed to find changes in the vegetation

in a lot of these other records that corresponds to the climate changes

in a lot of these other records that that we're seeing at Welsby Lagoon.

that we're seeing at Welsby Lagoon.

Forensic analysis of these sediment cores

has produced some of the most profound evidence

of the existence and disappearance of megafauna in the area

through, of all things, their poo.

What we have here are some photos from one of the cores

that we've analysed from Welsby Lagoon

and what we see at the start of the record,

so from 80,000 to 60,000 years ago,

is sporormiella spores.

These are the dung fungi that come from megafauna.

These are the dung fungi And we can see that

And we can see that

during the time period And we can see that

during the time period where it's dominated by rainforest,

we get quite a high abundance of these dung fungi.

And then as we move forward through time,

we start to see this drying of the climate just after 50,000 years ago.

And we also see a change in the vegetation.

And you can see we still see some spores

of our sporormiella in this period,

suggesting that megafauna were still around,

but the amount of the sporormiella is much lower.

And then again, at about 40,000 years ago,

we start to see really dramatic changes in rainfall.

we start to see really dramatic So quite dry conditions,

So quite dry conditions,

which is also changing the local environment.

And this also coincides with the time that we see

And this also coincides the disappearance of Sporormiella.

the disappearance of Sporormiella.

By 40,000 years ago,

it looks like there was no By 40,000 years ago,

it looks like there was no megafauna living around this site.

How did this changing environment

lead to the disappearance of megafauna here on Minjerribah? How did this changing environment

lead to the disappearance of megafauna here on Minjerribah?

The drying conditions that we see from Welsby Lagoon

The drying conditions that we see from Welsby Lagoon probably wouldn't have suited the megafauna particularly well,

probably wouldn't have suited the megafauna particularly well,

so they would have started to see changes

in the food sources that they've been eating

if the vegetation is starting to change.

But we'd also be seeing a lot of the fresh water sources

on Minjerribah and North Stradbroke Island start to dry up,

and the megafauna would have been struggling to find

and the megafauna would food to eat and water to drink.

food to eat and water to drink.

And perhaps this catastrophic drying that happened

during this 50 to 40,000 year period

might have been enough to push the megafauna over the edge.

But why is it that some species of megafauna still walk the Earth today

But why is it that some species of while others perished?

while others perished?

As the climate rapidly changed, only the hardy could survive.

It's possible that those animals

capable of travelling great distances

capable of travelling between diminishing water supplies

between diminishing water supplies

had a better chance of survival.

There is a group of megafauna who could do just that.

Today's kangaroos are the great survivors.

Kangaroos that we know today

have got a really beautiful, Kangaroos that we know today

have got a really beautiful, streamlined body shape.

have got a really beautiful, They've got a small head

They've got a small head

and what we'd call a gracile upper body

coming down to really big, powerful hip and thigh muscles

and long, thin legs and feet.

All of these are adaptations for really efficient bipedal hopping.

Comparative anatomist Natalie Warburton studies living animals

to learn more about their extinct relatives.

So, our big red and grey kangaroos, that get up to 70 kilos or more,

So, our big red and grey kangaroos, that get up to 70 kilos or more, were hopping around at the same time as Diprotodon

were hopping around at the same time as Diprotodon

and the giant extinct goanna megalania.

There was also a lot of diversity in kangaroos.

But one of the biggest of all of the extinct kangaroos

But one of the biggest of all was Procoptodon Goliah.

was Procoptodon Goliah.

Standing over two metres tall and weighing up to 240kg,

this was an enormous beast moving through the Australian landscape.

A pair of Procoptodon feast before the heat of the day.

Unlike today's kangaroos, who feed on grass,

they prefer bushes and leaves.

Their distinctive short face gives the jaw more strength

Their distinctive short face to chew the tougher plants.

to chew the tougher plants.

Procoptodon was the largest of the extinct short-faced marsupials

Procoptodon was the largest known as sthenurine kangaroos.

known as sthenurine kangaroos.

But could this giant do

what its modern-day relative takes for granted -

what its modern-day relative hop?

hop?

The hopping at high speeds is actually really efficient.

These really long tendons and muscles in the leg

stretch like elastic bands and then recoil.

That helps to transmit some of that stored elastic energy

That helps to transmit some into the hopping.

into the hopping.

So what about a 240-kilogram Procoptodon?

Were they able to hop or were they doing something different?

And was that the difference between life and death?

Today, a team of scientists

is putting an eastern grey's tendon to the test

to understand how its ancient relative got around.

MAN: So this is a sub alpha male. They get bigger than this.

But this guy has probably been in a fight.

He's been forced to leave his territory,

He's been forced he's had to cross the road,

he's had to cross the road,

and unfortunately he's had to cross the road,

and unfortunately he's been hit by a car.

And that's how he's ended up here today.

I'm just going to cut this skin away so we don't damage the tendon.

Christopher Clemente is a comparative biomechanist.

So when a kangaroo hops,

all that force has got to pass through the tendon.

And we think this might be one of the limitations all that force has got to pass through the tendon.

And we think this might be one of the limitations

to the maximum body size And we think this might be one of the limitations

to the maximum body size

or the maximum speed that these kangaroos are capable of moving at.

or the maximum speed that these kangaroos are capable of moving at. WOMAN: Yeah. Like cleaning up that cross and then it looks...

WOMAN: Yeah. Like cleaning up that cross and then it looks...

Yeah. Oh.

See all the individual muscles, Yeah. Oh.

See all the individual muscles, yeah.

It's nearly as thick as your finger, which is incredible.

For an animal that's a fourth... a quarter of the weight of a human,

these tendons are incredibly big.

Would a tendon like this be strong enough to support a hopping Goliath?

Would a tendon like this be strong Here you can feel it's quite tough.

Here you can feel it's quite tough.

It's basically this rubber band. You know.

They really have this unique tendinous structure.

Ah, yeah. There we go.

Once the tendon is removed,

it's secured in a specialised engineering rig,

where it will be stretched to the point of failure.

Can you feel it? Is it taut?

Yeah, it's very taut.

OK. We're going to do our failure test.

So we're going to pull it up all the way until it breaks.

Alright. And starting. Starting.

Alright. And starting. (BEEP!)

(BEEP!)

Perfect. That's...four.

Five. Now it's going to go.

Five. Six.

Six.

And now it's going. Now it's going.

300 newtons.

500 newtons. That's insane.

700 newtons. 500 newtons. That's insane.

700 newtons. 800 newtons.

700 newtons. Oh! Now it burst.

Oh! Now it burst.

You can see it at the top here it slipped.

So at 880 newtons...

There. It's about to go at the very top.

Sliding out.

The eastern grey's tendon withstood an immense 880 newtons

The eastern grey's tendon until it failed.

until it failed.

That's the equivalent of until it failed.

That's the equivalent of more than 80kg of force.

The force that this tendon can withstand

was about four times the body weight of this kangaroo,

in just one tendon of one leg.

We were able to get huge forces out of these tendons,

We were able to get huge forces and that's a little bit surprising.

and that's a little bit surprising.

So we think these tendons are and that's a little bit surprising.

So we think these tendons are actually able to withstand

So we think these tendons are higher forces than we thought.

higher forces than we thought.

I'm just going through today's simulation.

The team have used their knowledge of the eastern grey's anatomy

The team have used their knowledge to build a computer model.

to build a computer model.

So you've got the 50-kilogram model hopping at multiple speeds.

Is that right? I have.

This model can be scaled up

to test if kangaroos can become too big to hop.

So this one here is the 200.

By building these models

that are heavier and heavier than the animals we actually see,

we can make predictions about the ways in which they would have moved.

OK, so what happens when we get to 250 kilos?

This is about the mass of a Procoptodon.

This is about the mass Could it hop?

Could it hop?

(GROANS) It's an effort. Oh!

(GROANS) It's an effort. He's not doing very well.

He's not doing very well.

How much more the torso has to move.

So it really flings its body up.

And see how the ankles touch the ground?

Right. And what speeds is it capable of hopping at?

Right. And what speeds is it capable Just one.

Just one.

It can only hop at 2.6 metres per second.

It can only hop So it's now highly constrained...

So it's now highly constrained...

Yes. ..in the speeds that it can hop at.

Hopping at one speed is hardly a practical way of getting around.

So if hopping is off the table, how did Procoptodon move?

Deep in the vaults of the Melbourne Museum

are some of the best preserved Procoptodon fossils on the planet.

By comparing these ancient bones to those of modern kangaroos,

scientists may have found the answer.

MAN: Independent lines of evidence all seem to roughly

MAN: Independent lines of evidence be pointing in the same direction,

be pointing in the same direction,

that there were these giant striders in the Australian landscape.

NATALIE: This is an incredible specimen.

There's so many bits here. It's so amazing.

Today, Tim Ziegler and Natalie Warburton

have the bones of three kangaroo skeletons for comparison.

Laid out on the table is a box of eastern grey bones,

Laid out on the table is the giant Procoptodon fossils,

the giant Procoptodon fossils,

and an impressive, the giant Procoptodon fossils,

and an impressive, almost complete skeleton

of a 50,000-year-old Sthenurine kangaroo.

It's a typical-sized kangaroo.

It is, but it's very, very chunky.

Yeah. It's got amazing bones.

This short-faced kangaroo lived at the same time as Procoptodon.

It's a close relative, so can provide the best evidence

as to how Procoptodon might have moved around.

So when we look at these hips...

You know, in our grey kangaroo, we've got almost a straight line.

And when we look at this one...

You know, we've got this flexed angle probably corresponding

to a posture that's more upright.

There are several telltale signs within this skeleton

that prove these ancient kangaroos walked instead of hopped.

In terms of the shape, it's so different.

You can run through the entire skeleton of a short-faced kangaroo

to find features pointing towards striding.

The joint surface is just so much bigger,

The joint surface and big joint surfaces mean

and big joint surfaces mean

spreading of load to reduce the stress.

That's something a hopping kangaroo just doesn't have to worry about.

That's something a hopping kangaroo They're exceptionally adapted for

They're exceptionally adapted for

supporting their weight on one leg or another, They're exceptionally adapted for

supporting their weight on one leg or another,

rather than spreading their body mass across both feet at once.

rather than spreading their body mass across both feet at once. This is even more pronounced in the leg of the Procoptodon.

This is even more pronounced in the leg of the Procoptodon.

The knee joint at the end of the bone as well is interesting.

The knee joint at the end It's very broad.

It's very broad.

It's supporting It's very broad.

It's supporting spreading that weight

and spreading stress from movement, side-to-side movement,

and spreading stress from movement, really effectively.

really effectively.

This is a knee for an animal

that is moving its body side to side This is a knee for an animal

that is moving its body side to side as it walks.

One of the strongest signs a short-faced kangaroo didn't hop

One of the strongest signs was the length of its tail.

was the length of its tail.

In living kangaroos,

their long tails act as a counterbalance when they take off.

This means their body doesn't tip forward or sideways.

There's fewer vertebrae.

You know, it looks like it's probably going to get to about here,

which is much shorter than we would expect in a hopping kangaroo.

That's really suggesting that this is moving in a really different way.

That's really suggesting that this Sure.

Sure.

These bones paint a picture of a very different kangaroo...

..one with a short tail that walked upright like a T-rex,

rather than hopping like a contemporary kangaroo.

Could this difference in locomotion have played a part in its extinction?

In terms of climate,

unpredictability has become an advantage in some ways

for living kangaroos.

Because they can move such vast distances in a short amount of time

with their incredibly efficient hopping locomotion.

That's a response that just wouldn't have been available to Procoptodon.

Once those distances are too great for this animal to move,

Once those distances are too great well, then, it's kind of stuck

well, then, it's kind of stuck

and therefore at more risk to local spontaneous events

like the decline in regular seasonal water or disease.

The ultimate proof to whether these prehistoric kangaroos strode

The ultimate proof to whether these prehistoric kangaroos strode may lie in the sands of Lake Callabonna in South Australia.

may lie in the sands of Lake Callabonna in South Australia.

MAN: To unpick what happened over 40 to 50,000 years

MAN: To unpick what happened in the past is a major challenge.

in the past is a major challenge.

What we have, though, in Australia

is that opportunity What we have, though, in Australia

is that opportunity to do more discovery

to identify fossil deposits is that opportunity to do more discovery

to identify fossil deposits

no-one has seen before

no-one has seen before to look for that unexpected piece of evidence.

to look for that unexpected piece of evidence.

This is pretty bloody good. (LAUGHS)

Completely unexpected.

Aaron Camens has discovered Completely unexpected.

Aaron Camens has discovered exciting new evidence.

I think that's about it.

These footprints could change scientific theory into fact.

Yep. I think that's what we've got.

Excellent.

We have just stumbled across this trackway.

If we have a look, we can see that

there are short oval prints. If we have a look, we can see that

there are short oval prints.

there are short oval prints. They're fairly wide set,

They're fairly wide set,

and the animal is taking fairly short strides.

So this is nothing like we would expect to see

if we were looking at an emu.

The trackways are much narrower,

and the shape of this foot doesn't really fit with anything

except one of our extinct short-faced kangaroos.

So, this is the first time I have come across a trackway like this

that potentially does demonstrate we've got a striding kangaroo.

Now, as someone who works on fossil footprints,

Now, as someone who works this is a pretty exciting moment.

this is a pretty exciting moment.

One of the great things about trace fossils is that

they're an actual record of behaviour.

And so if an animal is moving in a specific way, in a track way,

it's not a theory anymore.

We've got actual evidence that that's how that animal moved.

To learn that there are preserved striding trackways

of short-faced kangaroos in the Lake Eyre basin in South Australia,

of short-faced kangaroos in the Lake Eyre basin in South Australia, that's incredible.

that's incredible.

This is the...the smoking gun

that proves these giant kangaroos weren't hoppers, they were striders.

People wait lifetimes to see evidence like this.

We see proof with our own eyes

because we can walk We see proof with our own eyes

because we can walk in their footsteps.

The shores of Lake Callabonna provide a wealth of knowledge

on the megafauna that lived here more than 40,000 years ago.

But there's one species in particular that reveals

But there's one species in particular that reveals the fatal consequence of a changing climate.

the fatal consequence of a changing climate.

the fatal consequence (THUNDER RUMBLES)

(THUNDER RUMBLES)

Aaron's colleagues, Phoebe McInerney and Trevor Worthy,

are investigating what led to the extinction of Genyornis, Aaron's colleagues, Phoebe McInerney and Trevor Worthy,

are investigating what led to the extinction of Genyornis,

a large flightless bird that disappeared during the last ice age.

MAN: Somewhere down here.

Yes. That's a... Drumstick?

Yeah. Ankle. Foot going down there.

Gizzard. Yeah. Ankle. Foot going down there.

Gizzard.

Yeah, that's really nice with the gizzards.

The breastbone. Yeah.

Across there the femur. You can basically just see the bird.

Another femur here. You can basically just see the bird.

Another femur here.

Another femur here. These fossilised remains vividly display the position of the bird

These fossilised remains vividly display the position of the bird

at the time of death.

So we have the chest plate, or sternum, poking up through here,

and coming down either side of it, or sternum, poking up through here,

and coming down either side of it,

we have the two legs.

we have the two legs. So they would start about here, and then head down

So they would start about here, and then head down

and going down into the mud.

And it's gotten its feet quite deep and it hasn't been able to move.

And it's gotten its feet quite deep And that's how it stayed since then.

And that's how it stayed since then.

Genyornis newtoni comes from a group of birds that are all extinct today.

And this species is associated with a family

called the Dromornithidae.

And they all managed to grow to these really large, gigantic sizes.

So Genyornis newtoni was kind of mid range

and was stood about two metres to 2.5m tall at the head,

and weighed probably about 250 kilos on average.

Two of these giant birds are in search of food

Two of these giant birds along the lake shoreline.

along the lake shoreline.

Genyornis, also known as along the lake shoreline.

Genyornis, also known as the Thunderbird,

Genyornis, also known as was heavily built.

was heavily built.

It resembled more a giant goose than today's emu.

Like some contemporary birds, it's believed they mated for life.

It was a herbivore and had adapted to live on the margins of wetlands,

It was a herbivore and had adapted feeding on fruits and reeds.

feeding on fruits and reeds.

But 50,000 years ago, as the climate dried and the lake diminished,

so too did Genyornis's sources of food.

In this barren landscape,

this couple will have to venture further out onto the lake,

this couple will have to venture where an uncertain fate awaits.

where an uncertain fate awaits.

We found a number of these individuals

We found a number across Lake Callabonna...

across Lake Callabonna...

..but when we take them back to the lab, we can clean them up

..but when we take them back and piece them all back together.

and piece them all back together.

And then that's when you start to see these little features

that indicate that something wasn't quite right with this bird.

Forensic analysis of these Genyornis bones

Forensic analysis could yield important information

could yield important information

about the condition of the big bird at the time it died. could yield important information

about the condition of the big bird at the time it died.

Interestingly, while we were looking at these bones,

we noticed that a few of them didn't quite have

the clean, beautiful surface that we were expecting from the bones.

So, for example, this one over here,

you can see that this region here is really nice and smooth,

you can see that this region here beautiful, healthy bone.

beautiful, healthy bone.

But then once you come up beautiful, healthy bone.

But then once you come up to the top here,

But then once you come up all of a sudden there's this lump.

all of a sudden there's this lump.

It's a bit rugose. There's lots of holes in it.

These clues point to one cause - disease.

So that means that the individual itself is probably struggling

with other things as well in its environment.

Something else that we see, though, is that

several individuals actually have these symptoms.

So this bone that we have here is the sternum or the chest plate

So this bone that we have here on Genyornis newtoni.

on Genyornis newtoni.

And it's also an individual that shows signs of pathology.

So you can see at the front here that we have this extra lumpy bone,

and when we looked at this in a scan data -

so we saw the structure of the bone using a CT scanner -

we could actually see that all of this is extra bone growth

we could actually see that on the surface of the original bone.

on the surface of the original bone.

So we also see this on the other side of this bone,

So we also see this where we have these lumpy regions

where we have these lumpy regions

with a different texture where we have these lumpy regions

with a different texture to the rest of the healthier bone.

And there's this really big cavity here as well,

which is associated with a compromised immune system.

So we have multiple Genyornis newtoni individuals

walking around at Lake Callabonna,

getting stuck in the mud,

and having these pathologies,

which indicates to us that it's not an individual problem,

but it's actually a population problem.

These birds were suffering from a compromised immune system,

likely due to poor nutrition.

If we correlate that

with what was happening in the environment at the time,

we start to see that

there were these major fluctuations in the environment,

with often really severe periods of drought,

with often really severe periods which went for quite a long time.

which went for quite a long time.

So these birds were likely having these environmental changes

So these birds were likely having causing significant stress levels.

causing significant stress levels.

Those types of factors potentially contributed

to the extinction of Genyornis at Lake Callabonna.

Megafauna, like Genyornis, lived in Australia for millions of years,

surviving repeated climate changes throughout the fluctuating ice ages.

So what could have made the environmental changes

So what could have made 40 to 50,000 years ago so different

40 to 50,000 years ago so different

that many species were 40 to 50,000 years ago so different

that many species were wiped off the face of the Earth?

There's an extraordinary geological phenomenon

There's an extraordinary that some scientists now believe

that some scientists now believe

may have been the final nail that some scientists now believe

may have been the final nail in the coffin.

MAN: So it was only about three or four years ago,

I was having a really detailed conversation

with a great colleague of mine, I was having a really detailed conversation

with a great colleague of mine,

complaining that I could see these major changes

in the animals and megafauna and Australia

in the animals and megafauna and Australia all around 42,000 years.

all around 42,000 years.

What was happening?

Why was it all happening then?

And he said, "Well, the only thing that happens 42,000 years ago

"in the geological record

"is the Laschamp excursion."

And I was like, "What's that?"

The Laschamp excursion was an incredibly rare geological event

where the Earth's poles switched places.

Normally, our Earth's core generates a magnetic field

that protects our planet from harmful radiation

that protects our planet from the cosmos and our sun.

from the cosmos and our sun.

But 42,000 years ago, as strange as this may sound,

Earth's magnetic north and south swapped.

The change lasted just 1,000 or 2,000 years,

The change lasted but during that time

but during that time

our protective magnetic shield was dramatically weakened,

resulting in a massive bombardment of radiation from space.

Until recently, the precise timing of this event was hard to pin down.

But in 2019, an extraordinary fossil was discovered in New Zealand,

But in 2019, an extraordinary fossil which held the answer,

which held the answer,

and with it perhaps the ultimate clue to megafauna extinction.

MAN: These pieces of kauri that we have in the archive

represent unique windows that we can peer into for Earth's history.

This cross section of a kauri tree is over 42,000 years old,

preserved in a bog until a few years ago,

when it was discovered during excavations

when it was discovered for a geothermal power plant.

for a geothermal power plant.

Kauri tree is a unique type of conifer

that grows in the northern part of New Zealand,

and it's an incredibly important tree for climate science

and it's an incredibly important because we have preserved kauri

because we have preserved kauri

that date back tens of thousands of years.

About 41,000 years ago is somewhere in this range of the tree...

For climate scientist Drew Lorrey, it's not just a tree,

For climate scientist Drew Lorrey, it's a gold mine.

it's a gold mine.

This ancient kauri represents

the truest reflection of atmospheric radiocarbon This ancient kauri represents

the truest reflection of atmospheric radiocarbon

the truest reflection that we have on the planet.

that we have on the planet.

The rings of the kauri tree

The rings of the kauri tree reveal significant change in the Earth's atmosphere 42,000 years ago.

reveal significant change in the Earth's atmosphere 42,000 years ago.

We looked at the atmospheric radiocarbon signature

through the time period that this tree lived,

and what we can see is an enrichment of radiocarbon,

that spikes quite significantly to nearly 800 per mil.

That's the strongest radiocarbon signature that we have at any time

That's the strongest radiocarbon in the last 50,000 years.

in the last 50,000 years.

The increase in radiocarbon was the clue scientists were looking for,

indicating more radiation from space was penetrating Earth's atmosphere.

The radiocarbon signatures in the naphtha kauri tell us that

the Earth's magnetic field weakened greatly

during the time of the Laschamp excursion,

to the point where

we probably barely had a protective magnetic field.

But what would a weakened magnetic field have meant for life on Earth

during this time?

So one question is, OK,

is this the smoking gun that explains the megafaunal extinction?

It's a controversial hypothesis,

but for evolutionary biologist Alan Cooper,

the theory offers an intriguing picture

of how devastating the effects could have been.

Suddenly, with the magnetic field turned off,

that ionising radiation is penetrating right into the earth.

It's doing enormous damage to the ozone layer.

We think it probably removes it completely.

Like the ozone layer is completely gone.

At that point, ultraviolet light is coming...charging in,

causing major climactic shifts.

One theory, which I think might be right,

is that ionisation in the atmosphere causes clouds as well.

You actually get cloud cover building up.

And clouds are a really good way of cooling something down.

And clouds are a really good way They reflect sunlight.

They reflect sunlight.

As a result, the surface underneath

is not getting that heat As a result, the surface underneath

is not getting that heat down to surface level.

is not getting that heat Cooling and drying.

Cooling and drying.

Why would this cause megafaunal extinctions?

Why in Australia would the impacts be so bad?

Well, one of the things about Australia is the aridity.

Is the difficulty of water as a resource.

Australia is already starting to dry out,

Australia is already the lakes are starting to dry up.

the lakes are starting to dry up.

And one of the things we think has happened

is that the magnetic excursion causes sudden cooling.

Cooling is nearly always associated with dryness.

You get less water coming off the ocean to form rain,

You get less water coming so the continent starts drying out.

so the continent starts drying out.

Suddenly you've got this major impact

Suddenly you've got of cold, dusty, sandy...

of cold, dusty, sandy...

Think of Mad Max type environment.

(CALLS)

Now, in other parts of the world, the megafauna can move.

They can migrate to try and deal with these changing conditions.

The problem in Australia, you're stuck there.

The environment's shifting,

the plants changing from trees to more grasses,

the plants changing fires going off

fires going off

from the lightning coming down fires going off

from the lightning coming down from the ionised atmosphere,

and the megafauna are suddenly being confronted with all these changes.

But there's another major change during this time

But there's another major change that some academics think

that some academics think

could have played a part in megafauna extinction.

So in the next episode...

(INHALES, EXHALES)

MAN: All good?

More people have actually been on the moon.

There's been 12 people on the moon and 10 people in this cave.

..scientists search for clues in an underwater lair,

once home to a fierce Ice Age predator.

Now, this is an animal that is like no other that is ever existed.

We discover the largest lizard to have ever walked the Earth.

MAN: Megalania must have been venomous,

which is a really mind-blowing discovery.

which is a really mind-blowing And the ultimate question...

And the ultimate question...

Well, that's the giant kangaroo that we're looking at now.

Well, that's the giant kangaroo That's where the head is.

That's where the head is.

..was megafauna extinction caused That's where the head is.

..was megafauna extinction caused by Earth's most successful predator -

..was megafauna extinction caused humans?

humans?

Our people used to live right through here

with the... (SPEAKS INDIGENOUS LANGUAGE)

Captions by Red Bee Media

Copyright Australian Broadcasting Corporation