The Great Rift: Africa's Wild Heart

The Ethiopian highlands.

The roof of Africa.

30 million years ago,

this was a vast high-altitude plateau the size of Spain.

Since then, the elements have carved the landscape

into Africa's equivalent of the Grand Canyon.

Millions of years of erosion

have created isolated islands of alpine terrain

supporting strange and unique creatures found nowhere else on Earth.

The gelada baboon needs a head for heights,

where a single slip would mean certain death.

Geladas are exclusive to this chilly plateau,

kept warm by a long fur coat.

They are the highest-dwelling primates on the planet,

ranging up to 4, 500 metres.

But what's really surprising is their sheer numbers.

Geladas are the most sociable monkeys on Earth.

Like the game herds of the Serengeti, they are vegetarians.

In these high pastures, they take the place of grazing antelopes.

But geladas aren't the only peculiar creatures up here on the roof of Africa.

This is a giant mole rat...

...a specialised grass-eater that's unique to the Ethiopian highlands.

The mole rat lives underground,

emerging at dawn and dusk to snatch mouthfuls of grass,

which it drags back to its burrow to munch at leisure.

There's every reason to be cautious.

This foxy-looking Ethiopian wolf

preys almost exclusively on giant mole rats.

Despite the wolf's best efforts, mole rats are incredibly common.

It's estimated there may be more than 15, 000 of them

in each square mile of grassland.

Yet despite their high numbers, mole rats and geladas never meet.

So, how can that be?

The answer is this.

Dividing geladas on one side from mole rats on the other,

the Great Rift Valley runs through Ethiopia,

slicing the roof of Africa in two.

But it doesn't stop there.

Extending 3,000 miles further south,

the Great Rift shapes and defines the entire landscape of East Africa,

creating intense hot spots of evolution.

All along the Rift Valley, soaring above the hot, dusty plains,

isolated mountain peaks harbour unique wildlife communities.

A kaleidoscope of strange and remarkable creatures.

As well as extraordinary wildlife,

there's something else that's special about these mountains.

They all share a common origin

that is key to understanding how the rift was formed...

...and why it has had such a far-reaching impact on this part of Africa.

The first clue lies here,

at the very northern margin of the Great Rift Valley.

Ethiopia's Danakil Depression.

This is Erta Ale,

Africa's most active volcano.

It contains the world's only permanent lava lake.

Like a window into the fiery heart of the earth,

Erta Ale provides a terrifying glimpse of the inner workings of our planet.

A cauldron of molten rock beneath our feet.

In geological terms, Erta Ale's volcano is still in its infancy.

But 1, 000 miles to the south,

another Rift Valley volcano has been growing for at least a million years.

Rising to the east of the Great Rift Valley,

snow-capped Kilimanjaro towers nearly four miles high.

It's Africa's loftiest peak

and the tallest free-standing volcano on Earth.

Kilimanjaro and Erta Ale are just two links in a long chain of volcanoes

that runs the length of the Great Rift.

In fact, all the mountains along the rift are volcanic,

born deep beneath the Earth's surface.

Scientists believe that a plume of superhot lava

has been rising up beneath the crust of East Africa

for millions of years.

The thick crust above the plume has been lifted more than a mile high,

causing cracks to appear around its margins.

The Eastern and Western Rift Valleys mark the fractured edges

of the uplifted East African Plateau,

with further branches extending outwards.

And wherever molten rock has seeped through from below,

huge volcanoes have grown up.

It's estimated that over the last 30 million years,

the rift's volcanoes have poured out enough molten rock

to bury an area the size of Wales to a depth of 15 miles.

Mount Kenya's 5,200-metre-high summit is an eroded lava plug,

suggesting it was once even taller.

Below the summit,

a belt of strange and unique alpine plants clings to the slopes.

Even on the equator, mountain weather is unpredictable.

Just a few miles away,

lions and zebra are sweltering under a blazing African sun,

yet here, on Mount Kenya, a sudden squall can bring snow,

creating a winter wonderland.

This extreme climate poses a real challenge

to the mountain's wild inhabitants.

Highland rock hyraxes are found only on Mount Kenya.

High mountain hyraxes have evolved to cope with the cold.

They are much larger than lowland hyraxes

and have exceptionally dense fur,

which allows them to maintain their core temperature

in freezing mountain conditions.

When the sun reappears, the hyraxes save energy by sunbathing.

Giant alpine plants insulate their delicate buds from frost damage

by wrapping them in a protective duvet of leaves,

which unfurl one by one as the temperature rises.

The giant ostrich plume lobelia has evolved a peculiar life cycle.

After several years of growth, it sprouts a huge flower spike,

containing hundreds of tiny flowers swathed in a mass of furry insulation.

Up here, over two and a half miles above sea level,

insect pollinators are scarce,

so the lobelia has developed an alternative partnership.

The plant attracts high-flying sunbirds with a rich supply of nectar.

The sunbirds use the lobelia as a fuelling depot,

a social meeting point

and a sunbathing perch.

In return, the lobelias get pollinated and set seed.

Further down the mountain, at around 3, 000 metres,

lives perhaps Mount Kenya's strangest resident -

the side-striped chameleon.

This cold-blooded reptile is uniquely adapted for mountain life,

far above the altitude where reptiles normally survive.

As dawn breaks, it creeps onto an exposed perch,

where it lines up its body at right angles to the rising sun.

Then, flushing its skin with dark pigment,

it soaks up the rays like a solar panel.

It takes only minutes to raise its temperature 30 degrees Celsius.

Once warmed, it changes to normal daytime colour.

The male chameleon is fiercely territorial.

If another male has the nerve to set foot on his bush,

there's bound to be a fight.

A sneaky bite clinches the bout...

...and the loser slinks off to nurse his bruises.

Further up the mountain,

the hyraxes have also completed their morning sunbathe.

Though it looks like a short-eared rabbit,

the hyrax has some peculiar features

which suggest a much stranger kinship.

The layout of its toes is practically identical to those of an elephant.

Its teeth and bones are also similar, though scaled down a little.

The life of a baby hyrax looks cosy.

But sunbathing out in the open makes them a tempting target for predators.

An augur buzzard.

40 miles west of Mount Kenya,

the Aberdares Mountains are the legacy of an older volcano

that's been severely eroded but still towers over two and a half miles high.

In the frosty alpine grassland, a rare predator is on the prowl.

At lower altitudes, servals have a spotted coat,

but up here, many of them are black.

Some scientists believe

that a dark coat may help the cats to absorb heat on cold, bright days.

In the cool mountain conditions, giant heathers form a dense carpet -

food for Africa's highest resident elephants...

...including one of the biggest tuskers in the world.

Unlike most herbivores, elephants can process almost any plant they find,

including tough heather bushes.

A century ago, elephants migrated freely between the Aberdares and Mount Kenya.

But as the human population has increased locally,

the herd has become marooned in its upland enclave with nowhere to go.

In these mountain forests,

one of Africa's rarest antelopes, the mountain bongo,

is making a last stand.

Bongos are extremely shy and secretive,

staying in dense, shady forest during daylight hours.

Human pressures have now reduced their numbers to around 100

within the Aberdares National Park.

And these have been bred for release into the wild.

The Aberdares lie along the eastern escarpment of the Kenyan Rift Valley,

which here is up to 100 miles across.

Right in the centre of this landscape sits a remarkable volcano,

virtually untouched by the outside world.

Mount Suswa is home to Masai pastoralists,

who graze their herds on the flat crater floor.

But Suswa is no ordinary volcano.

At its centre, steep cliffs plunge 500 metres

into a ring-shaped Rift Valley in miniature.

Around 100 families from the Red Trees Masai community

share the resources of Suswa's grassy crater.

Cattle are at the centre of Masai life - the measure of a family's wealth.

With a six-month dry season

and no permanent streams or lakes anywhere inside the crater,

keeping a supply of drinking water is a constant challenge.

But the Masai have found an ingenious solution to the problem.

A system of metal pipes condenses steam from volcanic vents.

At the centre of the crater,

Mount Suswa's unique circular moat

encloses a lava plateau two miles across -

a miniature lost world protected by a ring of cliffs.

It's thought that Suswa's volcano last erupted

just a couple of hundred years ago,

and parts of the "lost world" are still smouldering.

The Masai consider this wild, tangled jungle a sacred place,

where no hunting or grazing is allowed.

When Suswa's volcano last erupted...

...this moat was a cauldron of molten rock.

The liquid lava has since cooled and solidified,

but along the moat's rim, the volcano still hides a dark secret.

This sunlit boulder pile was formed

when the rock collapsed above an underground cavern,

opening a window into the volcano's main artery,

where molten lava once flowed unseen beneath the skin of the mountain.

Once the volcano stopped erupting and the lava drained away,

it left behind a vast network of empty passages,