Since the emergence of life more than three billion years ago,
life on our planet has suffered
a series of devastating mass extinction events.
These have killed off uncountable species,
and almost threatened to end life on Earth.
I'm Professor Richard Fortey of London's Natural History Museum.
I've spent all my working life
studying the remains of animals long extinct.
But now I'm setting off to discover
why some animals and plants have survived.
Hello, Snaky.
I'm going in search of living fossils. Old timers.
Here's the little face.
That somehow managed to survive when so many others perished.
Mm, excellent.
In the process, I hope to find an answer to a profound question.
Is being a survivor a question of having some very special features?
Or nothing more than pure chance?
From living fossils that are our most ancient relations
to gigantic relics from the age of the dinosaurs.
Life as we see it today is not just the product of evolution.
It's also the consequence of mass extinction.
We are all the sons and daughters of catastrophe.
Oh, my goodness.
2.8 million years ago,
triggered by changes in the Earth's orbit around the sun
and shifts in its ocean currents, our world began to cool.
Within a few thousand years,
much of our planet was shrouded in a dense cloak of ice
that would come and go until only 10,000 years ago.
We call this age of ice the Pleistocene,
and it transformed the hierarchy of nature.
When ice sheets grow, animals and plants have a stark choice.
They can adapt to the cold,
they can move to stay with their own comfort zone, or they can die.
And die many of them did.
Even in the Arctic today,
temperatures still regularly sink below minus 50 centigrade,
and wind speeds have been known to reach 180 kilometres an hour.
You might think nothing could live in such harsh conditions.
But you'd be wrong.
Strangely enough, when Pleistocene ice sheets grew,
many animals saw it as an opportunity.
New species evolved specially adapted to cold conditions.
This is the story of how a few specialist species
that evolved to live in the biting cold survived to the present day.
It's summer in northern Norway.
Here on the edge of the Arctic Circle,
we can peer through a window back in time to a habitat
that would not have been out of place in the Pleistocene.
This is tundra.
The word tundra comes from the Russian,
meaning "treeless mountains".
And it once covered huge tracts of the northern hemisphere.
Summer here is short - a few months at most.
Even in June, the temperature can drop below freezing.
Daylight lasts more than 20 hours a day.
With the ground below the surface still frozen solid in permafrost,
no plant can sink deep roots.
It's extraordinary how life can adapt to extremes.
This little arctic birch I'm looking at crouches down away from the cold.
A tiny little tree which is not much bigger than my hand,
and it doesn't get much bigger.
Yet it's related to trees that, in our garden,
grow to 50 feet high or more.
This is one strategy for coping with extreme conditions.
Become miniature.
Dwarf birch carry both male and female flowers,
and its microscopic seeds are the result of wind pollination.
And one thing you can always be certain of on these cold slopes
is constant wind.
But if trees shrink to adapt to the cold,
the solution for many animals is the opposite.
One way to cope with the onset of freezing conditions is to go large.
That's because, as you grow bigger,
volume increases disproportionately to surface area.
In other words, if you're very big, you can hold in more heat.
So, with the onset of icy conditions,
several mammal groups in particular grew large.
And for this reason, they're sometimes known as the mega-fauna.
Great herds of mammoth, as well as woolly rhinoceroses,
would once have lived in similar habitats.
They have long gone, but one species of ice age mega-fauna
that grazed alongside them still clings on in this special place.
In this bleak and barren landscape,
a landscape where the snow still lies
every year for months at a time,
it takes a very special creature to survive.
There's little to eat except grass, lichens and a few herbs.
And it's called the musk ox.
Musk ox live in the Dovrefjell Sunndalsfjella National Park
on land formerly used by NATO for war games and testing weapons.
They are famously bad-tempered,
and have been known to kill tourists and hikers, even charge trains.
So to keep safe, I need the expertise
of local Norwegian guide Johan Schonheyder.
Ah, so we're off. Yeah. Now we're going.
We're getting quite high here, and I can see the trees are thinning out.
Is this the sort of place that musk ox might like to live?
Yeah, it's not so often down here.
But it comes in the springtime.
It comes down to the valley here to give birth to their calves.
So they live together, I guess, in small herds, with a dominant bull?
Yeah, you can say that. There is one ruler of the whole herd.
Well, of course, the obvious question.
The name musk ox refers to musk, which is an odour, a scent.
Is that to mark territory, or is that to establish dominance?
What's the story with the musk?
The only time we really can smell it is in the fall
when they have this mating period.
OK, so it's to do with sex? Yeah, I think so. Yes.
Oh, I can see them now!
A fully-grown musk ox bull can weigh half a ton
and charge at 50 kilometres an hour over rough terrain.
Here, in one of the last great cold wildernesses
in the northern hemisphere, we creep closer,
trying to peer back in time at a rare survivor
our Stone Age ancestors might once have hunted for food and clothing.
I think that the one we saw and the herd is behind this hill,
so we have to be very careful, because they're walking against us,
and they're watching us, and they know we are coming now.
There are newly born calves, so we need to be extremely cautious.
Now, we are very close.
And the three mums are on the other side of the river.
It's over there.
And they're waiting for each other on each side.
Musk ox travel in small herds of about 20.
But fording a fast flowing river, this group have become separated,
putting them on edge.
Now we have to be careful, because we are now a little too close.
He's really watching us now.
Apparently indifferent, but they're aware of everything we're doing.
Oh yes.
Such defensive behaviour dates back to the ice age,
when giant bears still hunted them.
They remain fiercely protective of their young.
So is that a yearling? How old is the little calf?
That's a yearling.
When threatened, herds have sometimes been known
to form into defensive circles,
facing out at any potential foe.
Yes, there we are.
See how easily it goes in the water.
Now it's going to be exciting to see what it does with the calf.
I don't think the calf could make it across there, could it?
Now it's crossing with a calf.
They obviously go in for a lot of parental care, these animals.
Yes, it's actually shading the baby from crossing the river.
So it's making an easy crossing for the calf. Yes.
So really, they are quite social animals,
and operate very effectively together as a group.
Normally always together, some more than one.
Rarely you find a single one.
So now, they will regroup. That's great.
So that's a family.
Being loyal, nurturing parents,
it's a good evolutionary strategy
for ensuring the survival of the next generation.
But musk ox have other tricks
that help them survive in this bleak tundra.
See, they are grazing all the time, 24-hours a day.
They have this digestive system,
they have two hours eating grass,
and some flowers they really love,
and then after two hours,
they have a quiet period of two hours,
where they're digesting the food they've had.
This they do 24-hours, all the time.
Once winter comes, they will conserve their energy
by standing almost motionless, for days on end.
Not only can the musk ox digestive system adjust itself
to consume extremely tough food stuffs in times of scarcity,
but their liver and kidneys can slow down,
and shrink to half normal size to improve fat conservation.
And there's yet another strategy for surviving the cold.
If one way of coping with the ice age climate was to grow large,
then another was to get woolly.
This is musk ox wool.
It's very soft to the touch,
but I can see little globules of rain on it, which aren't absorbed.
It's extremely water-repellent,
and against my skin it's extremely warm and comfortable.
This allows the musk oxen to cope with temperatures
which can be as low as minus 50 degrees.
It's incredible stuff.
In short, if you want to survive a cold snap,
get hairy.
Although all attempts to domesticate them have failed,
musk ox steaks are a popular Norwegian delicacy,
best enjoyed with a glass of red wine.
For me, their taste is a taxonomic revelation.
Well, in taste it's not really like beef at all,
it's more like mutton.
Slightly older sheep.
Well, perhaps that's not a surprise,
because the musk ox is not really an ox,
it's more closely related to the sheep and goats.
It doesn't have the slightly rancid flavour you sometimes get with goat.
Well, we've heard of a wolf dressed up in sheep's clothing,
this is more like a sheep dressed up in cow's clothing.
7,000 kilometres west,
in Yellowstone National Park,
another giant ice age survivor has evolved a very different method
of sustaining itself through the harsh winter months.
Here in the United States, it's frequently called the buffalo,
but its correct scientific name is the bison.
Yellowstone's bison live at an average height above sea level
of 2,400 metres,
where the temperature only briefly
warms up into double figures.
The lowest temperature ever recorded here was a staggering minus 54C.
To help fuel the bison through this bone-numbing cold,
it grazes constantly,
even when the ground is covered in snow and ice.
But getting to the shrubs and grasses entombed beneath the snow
can be a formidable challenge.
One that bison have waged with the elements since the ice age.
There's something extraordinarily primeval
about these wonderful animals
finding something to eat in this land.
You can see them pushing through snow,
if they need to find food in the winter.
They're really adapted to survive under the most harsh
conditions you can imagine.
Unlike musk ox,
bison don't store large reserves of fat for the cruel winter months.
They feed all year round.
But to dig in snow, you need traction,
and to grip in the ice, bison have evolved a feature
they share with deer, sheep and goats, but not horses.
Their cloven hooves are split into two toes,
and help the bison grip,
when using their hugely muscular upper bodies.
Snow ploughs, to uncover the grass concealed beneath.
For a closer peek at how bison are built for such feats of strength,
I visit vet Don Warner, known locally as just Doc.
Well, Doc.
So let's just have a quick look at the front of the head, rather flat.
That's right. Relatively small horns,
compared with say, a bovid, like a cow.
That's right. The male bison has a very large bonnet.
A very thick hide in this area,
and the horns kind of adorn the edge of that bonnet,
and they use that for head-butting.
Fighting is a lot of head pushing, a lot of pushing each other around.
One pushes one way, then the other pushes him back.
It's a test of strength.
It is, and they're pushing hard,
and then they try to come past the head,
and hook them in the side or the abdomen.
But this is a female,
and the base of the horn is quite a bit smaller.
If I was to look at this straight off,
I might think that's a cow - and it's bison.
So how does this differ from other bovines?
Well, the most obvious thing are these dorsal spinous processes.
They're very long.
A cow skeleton, they'd be about half that length.
So this is what gives the sort of hump-like appearance. Correct.
Another differentiation is the scapular.
That's the shoulder. The shoulder blade, right.
That's quite a bit longer in a bison.
This connection gives the buffalo more leverage to move his skeleton,
and to move his whole body quickly.
What we're talking about here
is a very heftily muscular area.
Yes. There's muscle on each side, connected to a large ligament,
that goes up and connects to the back of the skull.
So I guess there must be a nice piece of meat in there.
There is. That's the hump. It's very accessible.
Peel back the skin. Peel back the skin and you've got...
Cut it off, and you have a big steak.
Take a big slab of the hump.
If we put all this together, what we have is an animal
designed for survival through hard winters,
because all this muscle adds up to allowing the head
to swish down through the snow and find buried vegetation.
That's right. It's just a big pendulum
on a very strong anchoring system up at their back,
and it allows them to forage in the winter time, through snow.
So they can get through the really quite long winters
you can have up on the prairie land. Yes.
When it's cold, there's another advantage to being big.
Big stomachs.
The moose is also a giant ice age survivor.
It doesn't store fat like the musk ox,
and it lacks the muscular shoulders, as in the bison.
The moose browses for food on the move.
What it does have in common with both of them is its stomach.
The moose is a ruminant,
a herbivore whose stomach possesses four chambers.
Ruminants use these extra stomachs to store bacteria
that help to break down and ferment coarse, otherwise inedible plants.
Their stomach contents can then be regurgitated,
chewed again and redigested.
A process called ruminating,
or more simply, chewing the cud.
But it wasn't just herbivores that found ways to adapt
to a diet of frozen food.
Scavenging in the ice is the speciality of the wolverine,
and this Pleistocene survivor
also possesses an ice-resistant pelt
that is one of the most highly prized in the animal kingdom.
I'm with Debbie Harris,
who looks after one of the few wolverines in captivity
in the western United States.
The wolverine.
Sometimes known as "the glutton".
Gulo gulo.
What kind of adaptations does the wolverine have
that might help it to become such a survivor?
Well, one of the things is his fur does not absorb moisture.
He has these very large paws,
with a lot of fur of them,
and they're like snow shoes.
He lives very high in the mountains,
where there's a lot of snow.
So he can run very fast on his little snow shoes.
A fully grown wolverine may weigh around 20 kilograms.
But it's an ice age giant - of sorts.
The wolverine is the largest mustelid in North America,
and that family includes the skunk, the otter,
as well as the North American mink.
Wolverines are weasels.
The giants of the weasel family.
Now, the wolverine has something of a reputation
as a bit of a fierce creature.
Is that reputation justified?
That little wolverine at 30, 35 pounds, he can go out there,
and he can spook a grizzly bear as well as pack of wolves
if he's really willing to get out there
and he's hungry enough to get that food that they've got.
Believe me, he could do it.
Ferocity is a useful survival strategy,
even if this little chap seems unreasonably playful.
But it helps to have the hardware to back threats up.
In the case of the wolverine,
it's the teeth that are its special secret.
Of use not just against competitors and prey,
but to help it feed in the freezing cold.
Its rear molars are angled backwards,
so that the jaws can rip and tear deep frozen meat,
making it a perfect ice age scavenger.
He also has a very powerful jaw,
capable of snapping bones,
so that he can eat the marrow from the bones.
So they are animals adapted to cold conditions.
Absolutely. They need that cold weather.
Wolverines, bison, musk ox and moose.
All ice age survivors that still live in the cold higher latitudes.
But not all the Pleistocene mega-fauna
lived in the northern hemisphere.
Nor did they all live in ice.
My next survivors live more than 16,000 kilometres away,
on the other side of the world.
Back during the Pleistocene Age,
while the north was covered in glaciers,
conditions were very different, but no less harsh,
in the southern hemisphere.
To find out more,
I join genetic scientist and caving enthusiast Alan Cooper
in the Naracoorte Caves of South Australia.
Originally formed 200 million years ago
from coral and marine fossils,
the limestone has eroded,
creating underground caves
and holes on the surface that have entombed unwary animals.
They're a World Heritage Site, a popular venue for caving adventures,
and a mine of valuable environmental and ecological data.
Despite my claustrophobia,
Alan is eager to show me a palaeontological site
that contains evidence for what happened here
while the northern hemisphere was covered in ice.
So here we are in a bone pit.
Layer after layer,
fossils have been preserved in the floor of the cave,
giving us a narrative of history of hundreds of thousands of years.
Layers of death.
You're basically looking back through time,
potentially 40-50,000 years of ecological dandruff, I suppose.
How did they get there?
Well, there was a hole in the surface
through which animals tumbled.
In other cases, predators brought bones in here to devour.
But what kind of animals are there?
You have a huge diversity of Australian mega-fauna here.
We have Tasmanian devils,
which are still around today.
Tasmanian tigers, which of course aren't.
Short-faced kangaroos. We can see a skull there,
with the eye socket.
We're used to thinking, in a selfish way, us Europeans,
of the ice age as if it's our ice age.
But the fact is the world is one great connected system.
The problem with Australia is we don't have the same sort of quality
of fossil record that we do in the northern hemisphere,
so trying to work out exactly what's happened is much more difficult.
We do know it's a very cold, dry period during the glacial maximum,
with sand dunes rolling across the interior of Australia,
and life during that stage would be almost impossible.
Life on the plains was especially harsh.
So it's perhaps ironic my next survivor began life,
just as we probably did,
in the trees.
Some of the oldest kangaroo species are also the smallest,
and still live in forests.
One of the secrets of their success was reproduction.
Something kangaroo foster mum Tania Melville knows all about.
This is a very charming one.
What's the species here?
This one is a swamp wallaby.
So one of the important things about these in general,
is people tend to think of them as primitive, but actually they're not.
They're beautifully advanced,
beautifully adapted. Definitely.
Kangaroos reproduce like a well-timed production line.
Females can control their fertility,
so while one joey is in the pouch,
another fertilised egg can be waiting to develop.
They can hold an embryo en reserve, as it were.
And when conditions improve
it can complete the rest of the development process.
That's it. What that means is when conditions fall bad,
so the water dries up or not much food around,
she can drop off the joey on foot,
she can drop off the joey in the pouch,
but she'll have that embryo waiting.
So as soon as the conditions fall good again,
it's bang, give birth straight away,
and start the whole breeding process off again.
And, of course, they always try and scramble back in, don't they?
Yeah, the mother will usually call if there's any sign of danger
and as soon as the joey hears that call,
it's a mad scramble back in the pouch again.
She's actually got a drawstring-like muscle
so she can close it or she can open it, depending on what she feels like.
It's like slamming the door, really. Definitely. Yup, "you stay in."
"Don't. You just stay in here." Yup, for sure.
Many of Australia's marsupials
had also become giants during the good times.
As climate became more arid, they needed to toughen up to survive.
Well, we know kangaroos' system of reproduction
is extremely efficient, but what else helped them to survive,
do you think, when climate changed in this drastic way?
One of the big advantages of the marsupials
is their metabolic rate is very slow.
They can survive on much less food and much harsher conditions
than, for example, the placental mammals.
And, quite often, people tend to think of the marsupials
as being more primitive
and that's why they could only survive in Australia
because there was no competition.
And, perhaps, in many ways this very harsh, arid environment
meant that something like the kangaroos,
with their unique skills, were ideally suited.
Whereas placental mammals might have actually had
a much harder time surviving here.
Like its cold-adapted mammalian counterparts,
kangaroos also chew the cud.
But while the bison and the musk ox build muscles or store fat,
the kangaroo can extract water from the meanest shrub.
So long as they could find even the driest plants,
they could go for months on end without a drop.
Interred in the Naracoorte Caves
are fossils of some of the giant marsupials
that were contemporaries of the ice age mammals
in the northern hemisphere.
Hippopotamus-sized herbivores like zygomaturus.
The wolf-like thylacine, sometimes called the Tasmanian tiger.
And the meter-and-a-half-long thylacoleo,
largest of all known marsupial predators,
superficially resembling a cross between a lion and a bear.
And what about thylacoleo, the so-called marsupial lion?
Yeah, a remarkable beast, we've got one here.
Here's the skull.
And as you can tell very quickly by looking at the teeth,
this is no placental mammal.
The two incisors at the front, quite unique
and this modified slicing molar down the side, here.
And look at the strength, the thickness in the skull.
Looking at the feet,
some new research is showing that it appears to be climbing,
spending a lot of time, or, certainly, capable of climbing trees.
You can imagine this, you know,
100 kilos coming down with those teeth on top of you.
Except that it would be hunting, presumably,
possums and things like that.
We're talking possums, small wallabies,
the, sort of, medium-size animal.
So, they drag them into the cave for leisurely...? Presumably, yes.
We really, even despite this, the evolutionary adaptations,
which are so remarkable,
it's still quite difficult to work out what this thing is doing.
But these exotic, marsupial giants lacked the survival trait
that would make the kangaroo king of the continent.
It was by hopping out of the forests, 15 million years ago,
that kangaroos came to dominate dry age Australia.
Hopping is one of the most effective means of locomotion
in the animal kingdom.
And the largest kangaroo species alive today
can reach speeds in excess of 65 kilometres per hour.
Such speed is useful for escaping from predators
but recent studies suggest
the real survival bonus hopping gives kangaroos is range.
This supremely efficient mode of locomotion
allows mobs of kangaroos to cover vast ranges
in pursuit of scarce, grazing food,
perhaps the best way to cope with conditions of drought.
Plants too began to suffer from the tough, dry conditions.
Yet, the extreme aridity provided the perfect opportunity
for another of Australia's most iconic dry-adapted species
to spread across vast stretches of the continent.
Eucalyptus trees.
Their secret survival weapon is a tolerance
to one of the bush's most regular and feared natural phenomena.
Fire.
Eucalyptus trees have adapted to cope with periodic fires.
Some species like these stringybark Eucalyptus
even have highly flammable, loose bark
that will erupt in flames with the smallest spark.
The reason for this adaptation is as simple as it is efficient.
Any plants that cannot cope with fire are incinerated.
After the fire has passed,
Eucalyptus trees put out new shoots and re-grow once more.
The competition goes up in smoke.
An even more special example of a fire-adapted plant
is this wonderful bush.
Named after an illustrious former president of the Royal Society,
Banksia is the phoenix of the plant world.
In fact, it can't live without fire because its seed pods burst
when fire passes through, releasing the seeds to germinate
in the newly enriched ground.
The last extreme period of aridity started drawing to an end
around 30,000 years ago.
By then, Australia's landscape had been completely transformed
and many of the continent's large animals
and plant species were beginning to disappear.
Great transformations were also beginning elsewhere.
Around 20,000 years ago, the great ice sheets
that had covered much of the northern hemisphere
for more than two million years began to melt.
The end of the ice age would bring mass extinction.
So, what happened when the ice sheet finally retreated?
The change in conditions
made many of those specially-adapted species go extinct.
It was not the cold that killed them, it was the thaw.
Some of the animals that did not survive
can be found for sale in a fashionable street
in Soho, New York City.
They're carnivores that disappeared at the end of the Pleistocene Age.
They were found preserved in tar pits in California.
Extinct mega-fauna. These are the animals that didn't quite make it.
Hunters all, these fell into the tar
while in pursuit of the prey which are also there.
The sabre-tooth tiger, well, I used to call it that, it's no tiger.
But it does have these huge incisors,
these great fangs for killing its prey.
A lot of debate about how they worked.
The dire wolf.
The dire wolf, of course, hunted in packs, just like the living wolf,
but they were larger. Now, extinct.
The American lion.
It may come as a surprise to find lions in America
but there they were.
Characteristic teeth, of course, on both jaws, here.
They didn't survive.
These were all victims of a changing world.
Just how dramatically the landscape changed can be seen in Yellowstone.
This would once have been tundra
like the frozen north of Norway today.
But as the ice began to retreat, so many species,
long-exiled further south, seized the opportunity
to expand their dominion northwards.
Yellowstone's tundra was conquered by an empire of conifers
that took root across the northern hemisphere
as the ice sheets receded.
As the ice sheets waxed and waned,
these cold-loving conifers could adapt their ranges in harmony.
Warm times, they spread.
Cold times, they contracted.
Tough, enduring trees.
And they're still with us today, of course,
cladding every high mountainside.
Modern conifer trees are descended
from some of the oldest types of tree in the world.
And conifers date back more than 100 million years.
To colonise new territory,
the conifer uses cones to house its seeds.
But, in spite of their woody appearance,
the cones are not impenetrable.
So, here's one of the cones that gives these trees their name.
These have seeds tucked within them that form a food source
for animals today as they have done for millions of years.
But the spread of conifer trees led to the decline of habitat
for some tundra-dwelling animals.
A woolly mammoth in the Ipswich Museum.
A wonderful, gigantic relative of the elephant
but with differently-shaped tasks curved into this elegant spiral.
And these mammoths once roamed in their millions
all the way from East Anglia, here, to Siberia.
They fed on a special kind of tundra vegetation, rich sedges.
But they became extinct.
A few diminutive mammoths lingered on
in Wrangel Island in Arctic Siberia until almost historical times.
The rest of the population became extinct
as their special habitat declined.
Their numbers went from millions to a few and then they died out.
Small numbers of musk ox survived by retreating with the tundra
towards the inhospitable edge of the Arctic Circle.
Whilst, in North America,
climate change was driving other animals towards extinction.
With the thaw, tundra gave way, eventually, to prairie.
Grasses quickly came to dominate the open landscape.
Grasses are also a relative newcomer to the plant world,
evolving around 23 million years ago, long after flowers.
Grasses are special plants,
continually regenerating their leaves from the base,
so giving a continuous feed to grazing animals like the bison.
The bison survived, but only just.
They were saved by chewing the cud.
Their ruminant stomachs gave them the ability to extract nutrition
from grasses while mammoths, who were not ruminants, could not.
Today in Yellowstone, the local bison have even adapted
to eating the poisonous, sulphur-rich grass
found around geysers.
They can't endure it for long but when conditions are really icy,
it temporarily keeps them from starvation.
A more temperate world also signalled
the return of flowers that had been pushed far to the south.
Warmth encouraged species that needed long summers
and suited their pollinators.
These sunflowers almost seem to appreciate the light of the sun,
following the course of it as it tracks across the sky.
Flowers need their own, particular pollinators to help them set seed.
Nowhere could escape the effects of the Pleistocene cold periods.
Even in the tropics, rainforests contracted.
That wasn't a problem for these butterflies,
which could wing their way into refuges to see out the hard times.
But, when the ice sheets began to contract,
the rainforests began to expand to their present proportions.
The very first ancestors of the butterflies
were very inconspicuous at the time of the dinosaurs.
Subsequent collaborative evolution between pollinators
and flowering plants stimulated
an unparalleled burst of invention in both insects and flowers.
As a result, today, both pollinating insects and flowering plants
are among the most varied and widespread of all species.
Other animals were also released to a new freedom by changing climates.
One young mammal had been confined to eastern Africa
for most of its existence.
But as temperatures warmed, it swiftly spread outwards.
And this species is, perhaps, the ultimate Pleistocene survivor.
13,000 years ago, another species moved into North America.
Man.
The same species arrived in Australia
more than 40,000 years ago.
And, in both cases, the mega-fauna,
the large animals, seem to become extinct in a short time.
Surely these events must be connected.
Could it be that this new top predator,
armed with a brilliant mind and a capacity to make tools
is implicated in the demise
of some of the most glamorous, large animals that have existed on Earth?
The idea that as man arrived on new continents
he hunted its large animals to extinction
is called the overkill or sometimes blitzkrieg hypothesis.
As the name suggests, it proposes that man advanced around the world
like a division of Panzer tanks, annihilating everything in his path.
After swiftly spreading to Europe and Asia,
this new hunter reached Australia around 40,000 years ago,
armed with fire.
A weapon that would bring about the downfall
of the last surviving giant marsupials.
We know that these new interlopers had fire, don't we?
Absolutely, you can see that in the archaeological record.
And modern Aborigines used fire as a very effective hunting tool
and, of course,
that habitat alteration is going to have a huge impact on mega-fauna.
So, I suppose there will be the perennial question of,
was it all down to us humans?
Or did us humans merely administer the final coup de grace?
Certainly, in my opinion, from the genetic data that we've got,
climate change is playing a major role
and humans are, as you say, applying the final blow.
And I think, in many cases,
these species wouldn't have gone extinct, or might not have done,
if humans hadn't been there to disrupt the environment
and prevent populations from dispersing
and reinforcing one another.
I think it's the combined double blow that is the extinction.
The only large marsupial that survived
the duel ravages of climate change
and the arrival of a new apex predator was the kangaroo.
Its hopping, water efficient
and ingenious reproductive ability gave it the tools to thrive.
Today, there are almost three times as many kangaroos in Australia
as there are people.
By some estimates, nearly 60 million of them.
The musk ox survived the appearance of man,
but not here in Norway.
In fact, these animals came from Greenland
after the Second World War,
reintroduced after the native population was hunted to extinction.
Back in North America,
the bison had evolved to dominate the Great Plains.
And, for more than 10,000 years, successfully coexisted with man.
Then, 200 years ago, a new arrival threatened to finally exterminate
these great, ice age survivors.
If I'd been sitting here 200 years ago, I would have seen behind me,
not a few dozen, but tens of thousands, even millions of bison.
It was the arrival of this object, the rifle,
that changed its fortunes and almost drove it to extinction.
Particularly after the American Civil War,
when these rifles came into commission.
It is a survivor but only by the skin of its teeth.
Today, under the broad skies of Montana,
250 kilometres north of Yellowstone National Park,
ranchers like Tana Blackmore
dedicate their lives to conserving the remaining herds of bison.
Part Native American,
she keeps more than 200 bison on her land in the Crow Reservation.
And with Tana at my side, I can get far closer
to these magnificent survivors
than I would ever dare in Yellowstone.
As a native woman, and doing the work that I was doing with the land
and so forth, they literally, different people, literally,
bought these baby buffalo and gifted them to me.
And it's like, you should ask before you start giving people buffalo.
It's, it's kind of a responsibility, isn't it?
It's a huge responsibility.
You know, I wouldn't want to trust myself out there, somehow.
They just look too massive and powerful.
See the big bull? Look at the massive head on him.
There he is. That is some animal.
I mean, that bull really does look like he's in charge.
Do you want some of that? You want some that?
They look like they're being independent, here,
but buffalo will stick together.
They're never far from each other unless, you know,
their particular band's... And once they hit about 100 head,
they start to break into a new band.
They have another survival trait that would have helped protect them
from Pleistocene predators.
Though, sadly, not hunters armed with long-range rifles.
They've got incredible circular vision.
Eyes in the sides of their heads.
All they need to do is slightly turn their head
and they can see what's going on back here, like rear-view mirrors.
Yeah. Yeah. You want some of that? You want some of that?
Today's bison are all descendants of a handful
that survived the human cull of the 1800s.
Hello, babies.
They were brought back from the brink
by breeding the remaining wild animals
with some kept in the Bronx Zoo.
After we finish our safari,
I join Tana for what she insists is a truly free-range bison burger.
So, it's my chance for a taste
without having to pay the buffalo bill.
Mmm.
Mmm. Excellent. I'm glad you enjoy it. It has a very different flavour.
It's a wholesome flavour, very robust and full, I believe.
Of course, its history, the history of this animal, is not a happy one.
I mean, it was a mass exploitation.
Well, there's also another unhappy side to that too.
And that has to do with the Indian Wars.
The primary reason that the masses of buffalo were eliminated
was to eliminate the food source for the native
because it was the life force for the native people.
And if they eliminated the food source,
then they would also eliminate the threat from the Native American.
Yes, I mean, it was a political, partly a political motivation,
no doubt. Yes. Hmm.
The flavour's, well, it's like beef, in a way,
But, I would say, overall, sweeter.
The burgers are certainly delicious.
I wonder if a mammoth burger would have been half as nice.
Hunting simply for food can't explain
why so many of the other ice age giants died out.
It was climate change and the disappearance of their habitat
that caused their extinction.
The link between habitat and survival
has been one of the enduring themes of our series.
Every survivor we have seen is just one small part
of a vast and interconnected tree of life.
Just one of the millions upon millions of species
that have ever lived on our planet.
But have we, in our small selection,
discovered the secret to being a survivor?
Luck alone may have helped propel some species
over life's great hurdles.
Like mammals inheriting the Earth
when an asteroid killed off the dinosaurs.
But there is more to long-term survival
than just one lucky throw of the dice.
Like adaptation.
We've seen how exquisitely adapted to their way of life,
everything from a musk ox
to a duck-billed platypus can be.
But plenty of animals that went extinct,
from dinosaurs to trilobites,
were probably just as finely adapted and that didn't save them.
Some survivors, like the echidna or the turtle,
seem to live a long time
or invest a lot of effort in producing offspring.
Stacking the odds in the survival of their own progeny.
Could I see where the head is right at the moment? Right beside you.
Others, like snakes and emus, can go without food for long periods.
Or even go into suspended animation. Like nuts and seeds.
But all living things have one need in common.
Habitat.
From the ancient time havens of the intertidal zone in Delaware
and Hong Kong,
the green refuge of Daintree,
and the ice-covered mountains of the wolverine,
persistence of habitat is the fundamental basis
of persistence of a species.
Today, Homo sapiens is master of all he surveys.
We have transformed the natural world
and are changing ancient and enduring habitats
like never before,
triggering a new era of man-made mass extinction.
But by threatening habitats around the world
we, ultimately, threaten the survival of our own kind.
A big city is so full of energy, so full of excitement,
so full of consumption.
It seems that we humans have come a very long way
in a very short time.
And it might be the best place to ask
whether mankind will burn himself out in an extravagant splash
or is he, possibly, going to be one of the survivors?
MUSIC: "Stayin' Alive" by Bee Gees
Subtitles by Red Bee Media Ltd
E-mail subtitling@bbc.co.uk