A hundred years ago there were one and a half billion people on Earth.
Now, over six billion crowd our fragile planet.
But even so, there are still places barely touched by humanity.
This series will take to the last wildernesses and show you the planet and its wildlife as you have never seen them before.
Imagine our world without sun.
Male Emperor Penguins are facing the nearest that exists on planet Earth - winter in Antarctica.
It's continuously dark and temperatures drop to minus seventy degrees centigrade.
The penguins stay when all other creatures have fled because each guards a treasure: A single egg rested on the top of its feet and kept warm beneath the downy bulge of its stomach.
There is no food and no water for them, and they will not see the sun again for four months.
Surely no greater ordeal is faced by any animal.
As the sun departs from the Antarctic it lightens the skies in the far north.
It's March and light returns to the high Arctic, sweeping away four months of darkness.
A polar bear stirs.
She has been in her den the whole winter.
Her emergence marks the beginning of spring.
After months of confinement underground she toboggans down the slope.
Perhaps to clean her fur, perhaps for sheer joy.
Her cubs gaze out of their bright new world for the very first time.
The female calls them, but this steep slope is not the easiest place to take your first steps.
But they are hungry and eager to reach their mother, who's delayed feeding them on this special day.
Now she lures them with the promise of milk, the only food the cubs have known since they were born deaf and blind beneath the snow some two months ago.
Their mother has not eaten for five months and has lost half her body weight.
Now she converts the last of her fat reserves into milk for her cubs.
The spring sun brings warmth but also a problem for the mother.
It starts to melt the sea ice.
That is where she hunts for the seal she needs to feed her cubs.
And she must get there before the ice breaks up.
For now though it's still minus thirty degrees and the cubs must have the shelter of the den.
It's six days since the bears emerged and spring is advancing rapidly.
But even now blizzards can strike without warning.
Being so small, the cubs are easily chilled and they will be more comfortable resting in the den.
But their mother must keep them out and active.
She's becoming weak from hunger and there's no food on these nursery slopes.
The sea ice still holds firm, but it won't last much longer.
Day 10, and the mother has led her cubs a mile from the den.
It's time to put them to the test.
They've grown enormously in confidence, but they don't have their mother's sense of urgency.
At last it seems that they're ready for their journey and they're only just in time, for a few miles from the coast the ice is already splitting.
Now the mother can start hunting for the seals they must have, but she's leading her cubs into a dangerous new world.
Nearly half of all cubs die in their first year out on the ice.
Summer brings 24 hours of sunlight and the thawing shifting landscape.
Further south the winter snows have almost cleared from the Arctic tundra.
Northern Canada's wild frontier.
Here nature stages one of her greatest dramas - Every year three million caribou migrate across the Arctic tundra.
The immensity of the herd can only be properly appreciated from the air.
Some herds travel over 2,000 miles a year in search of fresh pastures.
This is the longest overland migration made by any animal.
They're constantly on the move.
Newborn calves have to be up and running the day they are born.
But the vast herds do not travel alone.
Wolves.
Packs of them, eight to ten strong, shadow the migration.
And they are hungry.
It's the newly born calves that they are after.
Running directly at the herd is a ploy to generate panic.
The herd breaks up and now it's easier to target an individual.
In the chaos a calf is separated from its mother.
The calf is young, but it can outrun the wolf if only it manages to keep its footing.
At this stage the odds are even - either the caribou will make a mistake or after a mile the wolf will give up.
Midsummer on the tundra and the sun does not set.
At these latitudes the sun's rays are glancing and not enough of their energy reaches the ground to enable trees to grow.
You'll need to travel 500 miles south from here before that is possible.
These stunted shrubs mark the tree line - the beginning of the boreal forest - the taiga.
The needle-shaped leaves of the conifers are virtually inedible so this forest supports very little animal life.
It's a silent place where the snow is unmarked by footprints.
In the Arctic winter snow forms a continuous blanket across the land.
But as spring creeps up from the south the taiga is unveiled.
This vast forest circling the globe contains a third of all the trees on Earth and produces so much oxygen it changes the composition of the atmosphere.
As we travel south so the sun's influence grows stronger and at 50 degrees of latitude a radical transformation begins.
Summers here are long enough for broadleaf trees to replace conifers.
Broadleaves are much easier to eat and digest so now animals can collect their share of the energy that has come from the sun.
It's summer and these forests are bustling with life.
But the good times will not last.
Broad leaves must be shed in winter for their damage by frost.
As they disappear, so the land becomes barren with little for animals to eat.
The inhabitants must migrate, hibernate, or face months of near starvation.
The Amur leopard - the rarest cat in the world.
Here, in the deciduous forests of eastern Russia the winter makes hunting very difficult.
Pray animals are scarce, and there's no concealing vegetation.
The cub is a year old and still dependent on its mother.
Deer are frequent casualties of the harsh winter and these leopards are not above scavenging from a corpse.
African leopards could never survive here, but the Russian cats have thick fur to shield them from the cold.
There are only forty Amur leopards left in the wild and that number is falling.
Like so many creatures, the cats have been pushed to the very edge of extinction by hunting and the destruction of their habitat.
The Amur leopard symbolizes the fragility of our natural heritage.
The future of an entire species hangs on survival of a tiny number of mothers like this one.
All animals, rare or common, ultimately depend for their energy on the sun.
In Japan the arrival of the cherry blossom announces the beginning of spring.
The sun's energy brings color to the landscape.
The earth, as it makes its annual journey around the sun, spins on a tilted axis.
And it's this tilt that creates the seasons.
The advance of the seasons brings constant change.
As the sun's influence diminishes in the north, so the deciduous forests of America begin to shut down losing their leaves in preparation for the dark cold months ahead.
One season hands over to another.
Some organisms thrive on decay, but most must make special preparations for winter and a life with little sun.
Whole populations of animals are now forced to travel great distances in pursuit of food and warmth.
300,000 Baikal teal gather to escape from the Siberian winter by migrating south to Korea - the world's entire population in a single flock.
But there are parts of the world that have no seasons.
In the tropics the sun's rays strike the earth head on and their strength is more or less constant all year round.
That is why the jungle grows so vigorously and supports so much life.
This forest covers only 3 percent of the planet's surface, but it contains more than 50 percent of all its plants and animals.
The canopy is particularly rich.
There are monkeys, birds and millions of species of insects, exactly how many we have no idea.
The character of the forest changes as we descend, becoming ever darker and damper, favoring different kinds of animals and plants.
Less than 2 percent of the sunlight reaches the floor, but even here there is extraordinary variety.
In the great island of New Guinea there are 42 different species of birds of paradise, each more bizarre than the last.
This forest is so rich that nourishing food can be gathered very quickly.
That leaves the male six-plumed bird of paradise with time to concentrate on other matters like tidying up his display area.
Everything must be spick and span.
All is ready.
Very impressive, but no one is watching.
The superb bird of paradise calls to attract a female.
And he has more luck.
But what does he have to do to really impress her?
She retires to consider her verdict.
It's hard not to feel deflated when even your best isn't good enough.
The sun influences life in the oceans just as it does on land.
Its richest parts are those where waves and currents bring fertilizing nutrients to surface waters that are bathed in sunlight.
The seas off the Cape in South Africa have this magic recipe and are hugely productive.
Summer is the time of plenty and it's now that the seals start to breed.
The strike of a great white shark lasts a mere second.
Slowing it down forty times reveals the technique and immense strength of this massive predator.
If surprise fails, there will be a chase.
The shark is faster on a straight course but it can't turn as sharply as the seal, its agility versus power.
Once the seals have finished breeding the giant sharks will move on.
It's now becoming clear that great whites migrate thousands of miles across the oceans to harvest seasonal abundances in different seas.
The sun, b*ating down on tropical waters, powers the weather systems of the globe.
Moisture evaporates from the warming ocean and rises to create great storms.
The winds generated out at sea sweep inland across the continents.
As they travel across the Sahara they create the biggest of all sand storms blowing sand halfway round the world to fertilize the Amazon jungle.
Winds blowing across the Indian Ocean collect moisture and sweep northwards towards the Himalayas.
As the air rises, so it cools.
The water it carries condenses into clouds and then falls as the life giving rains of the monsoon.
So air currents powered by the sun carry wet air to the middle of continents.
Without water there can be no life, but its distribution over the land is far from even.
Deserts cover one third of the land's surface and they're growing bigger every year.
This is the Kalahari Desert in Southern Africa.
It's the dry season and thousands of elephants have started to travel in desperate search for water.
All across Southern Africa animals are journeying for the same reason.
Buffalo join the great trek.
Nowhere else on Earth are so many animals on the move with the same urgent purpose.
They're all heading for the swamps of the Okavango, a vast inland delta.
At the moment it is dry, but water is coming.
The travelers are hampered by dangerous dust storms.
Females and calves can easily get separated from the main herd.
For this pair sanctuary lies in the patch of woodland a few miles ahead.
They can't rest until they reach it.
The main has already got there safely.
Finally, the stragglers emerge from the dust.
The exhausted calf is still blinded by sand.
Its mother does everything possible to help it.
The storm is now subsiding, but not all the elephants have been so lucky.
One youngster has got lost.
Thirsty and exhausted, it follows the tracks of its mother, but sadly in the wrong direction.
At the peak of the dry season in the Kalahari water arrives in the Okavango.
It fell as rain a thousand miles away in the highlands of Angola and has taken nearly five months to reach here.
The water drives out insects from the parched ground, which are snapped up by plovers.
Catfish, traveling with the flood, collect any drowning creatures the birds have missed.
It's a seasonal feast for animals of all kinds.
Birds are the first to arrive in any numbers - wattled cranes, then black storks.
Behind the birds come buffalo.
After weeks of marching their trek is coming to an end.
As the water sweeps into the Okavango a vast area of the Kalahari is transformed into a fertile paradise.
Nowhere on our planet is the life giving power of water so clearly demonstrated.
The Okavango becomes criss-crossed with trails as animals move into its heart.
The new arrivals open up paths like arteries along which water flows, extending the reach of the flood.
This is an Africa rarely seen - a lush water world.
Some creatures are completely at home here.
These are lechwe - antelope with hooves that splay widely, enabling them to move its speed through the water.
For others the change is far less welcome.
Baboons are somewhat apprehensive bathers.
The water brings a season of plenty for all animals.
Hunting dogs.
These are now among the rarest of Africa's mammals, but then nonetheless the continent's most efficient predators.
Their secret is teamwork.
Impala are their favorite prey.
They start to hunt and the pack splits up.
An aerial viewpoint gives a new insight into their strategy.
As the dogs approach their prey they peel off to take up separate positions around their target.
They seem to form a cordon around the impala.
Moving in total silence they take up their positions.
Those ears can detect the slightest rustle.
The hunt is on.
Three dogs close in on one impala.
Missed.
The lead dog drives the impala towards the hidden flankers.
Anticipating their line the leader cuts the corner and joins a flanker for the final as*ault.
It's all or nothing.
One on one.
The dog has stamina, the impala has speed.
Leaping into the lake is an act of desperation - impala can barely swim.
The dogs know their prey must come out or drown - now it's a waiting game.
The rest of the pack are calling.
They've made a k*ll in the forest and this is an invitation to join in the meal.
The impala is in luck.
A pack this size kills once a day and everything is shared.
And this impala is reprieved.
The elephants are nearing the end of their long journey.
After weeks of marching they're desperately tired.
The matriarch can smell water and encourages the herd to make one last effort.
The youngsters are exhausted but their mothers have made this journey before and they know that they're close to water.
After many hundreds of miles they've arrived.
The lives of these elephants are dominated by the annual rhythm of wet and dry, a seasonal cycle created by the sun.
At the southern end of the earth, after four months of total darkness, the sun once more rises over Antarctica.
Now at last the Emperor penguins abandon their huddle.
The males are still carrying the precious eggs that they've cherished throughout the Antarctic winter.
With the returning sun the eggs hatch.
Other birds have not even arrived.
But the Emperors by enduring the long black winter have given their chicks a head start.
These youngsters are now ready and eager to make the most of the brief Antarctic summer.