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.
MOLE RAT SQUEAKS
There's every reason to be cautious.
This foxy-looking Ethiopian wolf
preys almost exclusively on giant mole rats.
MOLE RAT SQUEAKS, WOLF GROWLS
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.
高达 5,200 米的肯尼亚山就是岩浆堆积经多年风化的产物
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.