(dramatic music) [Narrator] Cut off from continents and cast away on vast oceans, the remote islands of the world have become floating arks... (birds chirping) attracting migrating species and forging new ones of their own.

Existing in splendid isolation, the evolutionary pressures of these islands are greatly amplified, with adaptation often proving the key to survival.

They are home to dragons, places where fish walk, and where lizards swim... and fly!

They are crucibles of life, where the incredible is commonplace... and where time has been left behind.

By studying life upon them, we've come to a greater understanding of the world around us and in the process, have found our own place within the grander scheme of things.

These are the world's greatest Island Arks.

(waves crashing) (inspirational music) (sea lion barking) Of all the world's islands, there is one with a reputation inseparable from the strange creatures that inhabit its shores.

The Galapagos.

This isolated chain of islands straddles the equator in the eastern Pacific Ocean around a thousand kilometers off the coast of South America.

Adrift in a vast sea of blue, this archipelago is more than an island group.

It's a flotilla of island arks, each profoundly influencing the life it carries.

The Galapagos is home to one of the highest concentrations of endemic species anywhere on Earth.

In fact, the vast majority of land mammals, reptiles, and birds that call these islands home can be found nowhere else.

It was here, on his voyage of the "Beagle" in the mid-18th century, that Charles Darwin first developed his theory of evolution.

By witnessing variances between species, both subtle and pronounced, Darwin began to draw correlations between the environment and the wildlife that dwelled within.

He mused that it was as if the species here had somehow adapted to the conditions around them.

These islands had opened Darwin's eyes to the process of natural selection, a scientific revelation that would forever change the way we view the world around us.

But how and why did this lost world become such a crucible of life?

(dramatic music) In geological terms, the Galapagos are relatively young.

Its 13 main islands and more than 100 islets form an archipelago that's scattered across 60,000 square kilometers of ocean.

Some of these islands are densely forested, brimming with life, while others are smoldering wastelands, a clue to their origins.

The Galapagos are situated in one of the most volcanic regions on Earth.

The oldest of its islands were born of fire and ash around four million years ago, but are still undergoing change today, forever moving southeast upon the tectonic plate on which they sit.

(upbeat music) As the older islands drift away from the volcanic hotspot, new ones are continually born, thrust from the seafloor in their wake, the youngest, little more than half a million years ago.

But it's not so much how these islands were formed, it's where, that transformed the Galapagos into such a unique bastion of life.

The archipelago sits at the intersection of four ocean currents that drift in from the farthest reaches of the Pacific.

While some deliver nutrient-rich waters, laden with coral larvae from the warm tropics, others arrive from the frigid Antarctic, bringing with them passengers of their own.

This diversity of water temperature nurtures a staggering array of marine life.

As Galapagos sea lions feed on an abundance of tropical reef fish, they're joined on the hunt by Galapagos penguins, the only species of penguins to be found in the northern hemisphere, but they too must be wary, as larger predators also prowl these waters.

On dry land, the absence of predators has been a key factor in the evolution of many species.

Amongst the more well-known of the island's residents is the Galapagos tortoise.

(suspenseful music) Often weighing more than 400 kilograms, and living for more than a century, they are the largest and longest-living tortoises on Earth.

Their ancestors are thought to have arrived to these shores by floating on their backs from South America over a thousand years ago.

Galapagos tortoises are able to survive on even the most inhospitable of the islands here, as they can go without food and water for months at a time.

But many of the other creatures that inhabit these islands need to feed more often, and for that, most rely on the surrounding ocean.

(serene music) Frigate birds spend much of the day in flight, hunting for fish, while blue-footed boobies feed the voracious appetites of their chicks with fresh bounty from the same life-giving waters.

As dawn breaks over the seabirds' nesting site, the rocks appear to come to life.

(upbeat music) Marine iguanas.

Their dark skin blends in seamlessly with the craggy volcanic formations of the coastline.

Rather than camouflage, it's thought that this dark coloration better absorbs the sun's rays, allowing these cold-blooded reptiles to store enough energy for the day's work ahead.

Smaller iguanas compete for algae growing on rocks in the shallows, while the larger amongst them take a more daunting path.

(adventurous music) Galapagos marine iguanas are the world's only seafaring lizards, their long, flattened tails an adaptation to help propel them through the strong currents.

The risks are greater in open water.

But so are the rewards.

Algae is more plentiful out here, and there's less competition.

But the cold water soon takes its toll on the iguana's body temperature, and they must return to shore.

These strange creatures have adapted to an aquatic lifestyle in the most remarkable ways.

Marine iguanas forcefully expel any excess sea salt consumed in their seafaring diet via specialized nasal glands.

While this strategy is primarily used to detoxify the lizard's system, it also helps deter burrowing insects, as the salty residue forms a hard crust on their skin.

Charles Darwin first described these marine iguanas as disgusting, clumsy lizards, but he quickly came to realize that there was method in the madness of their hideous biology, that they were, in fact, perfectly adapted to the environment which they inhabit.

(lighthearted music) Darwin's unique lens of perception was magnified on the Galapagos, bringing into focus a scientific rationale that would not only influence the course of human history, but change our understanding of life itself.

(dramatic music) (ethereal music) Tasmania is an island ark that was cast from the Australian mainland around 10,000 years ago.

Situated in Bass Strait, around 240 kilometers from the continent's southern shores, today, Australia's only island state hosts around half a million residents.

(waves lapping) (helicopter droning) Human populations are largely centered around Hobart, Tasmania's largest city, but more than 40 percent of the island's 65,000 square kilometer landmass has been set aside for national parks and World Heritage sites.

Tasmania is best known for its diverse range of ecosystems and the bizarre creatures that inhabit them.

(birds chirping) (idyllic music) While craggy coastlines shaped by strong oceanic winds define the island's rugged outline, Tasmania's interior is dominated by ancient snowcapped volcanic mountain peaks and lush valleys that conceal some of the last remaining temperate rainforests in the southern hemisphere.

The towering eucalyptus trees, known as swamp gums, found in the island's southwest are amongst the tallest flowering trees on the planet, second only to the American central redwood.

The tallest swamp gums here measure around 90 meters tall, and are thought to be up to 400 years old.

Those that remain today are under the protection of World Heritage status, but most have already been lost to the ravages of logging and wildfire.

(suspenseful music) Like the flora here, much of Tasmania's fauna can trace its genetic roots back to Gondwanaland, a prehistoric supercontinent that once connected Australia to South America via an Antarctic land bridge around 50 million years ago, but Tasmania has been geographically and genetically isolated for millennia, giving rise to endemic species that can be found nowhere else.

(vicious howling) (dramatic music) (rhythmic percussion music) The Tasmanian devil is the world's largest marsupial carnivore.

The size of a small dog, devils once roamed the Australian mainland, but can now only be found in the wild in Tasmania.

They are nocturnal hunters.

This female is on the prowl.

She uses her keen sense of smell to track down prey... (ominous music) but she's just as likely to scavenge, or, when the opportunity arises, steal a meal from a close relative.

This spotted quoll has taken down a small pademelon.

It's also a marsupial carnivore, but no match for a devil.

Before consuming her prize, the devil drags the carcass into the undergrowth.

Tasmanian devils possess the strongest bite force of all mammals.

Her powerful jaws can cut through flesh, tendon, and even bone.

Given the chance, she could consume the equivalent of her own body weight in just a few hours, but she doesn't have that long.

The scent of blood in the air will soon draw a crowd.

(devils growling) As other devils muscle in on the action, the weaker amongst them are forced from the carcass, through intimidation... or a brutal show of force.

(birds chirping) (rhythmic percussion music) As fierce as they are, devils were, until quite recently, overshadowed by a much larger and more ferocious marsupial carnivore.

Until the early 20th century, Tasmania was home to the thylacine, a beast more commonly known as the Tasmanian tiger.

Before the last of its kind died in a Hobart zoo in 1936, the Tasmanian tiger, which was more than twice the size of a devil, was the undisputed apex predator on the island.

The tiger's common name was derived from the striking markings that adorned its rust-colored coat, and its imposing jaw, which could be opened to nearly 180 degrees.

It was the tiger's formidable appearance that led to it being unduly branded as a sheep killer by early settlers, who ultimately hunted it to extinction.

(majestic music) Many believe that the Tasmanian tiger still exists deep within Tasmania's remote and rugged wilderness, taking refuge in the same splendid isolation that gave rise to this unique species many eons ago.

(dramatic music) (waves lapping) Borneo is the third largest island on the planet, covering an area nearly twice the size of California.

(peaceful music) Located in the warm tropical waters of southeast Asia, it's collectively governed by the nations of Indonesia, Malaysia, and Brunei.

The island supports a wide range of habitats, from the towering alpine regions of Mount Kinabalu, to vast swathes of tropical rainforest, to sprawling floodplains and coastal mangroves, all receiving up to half a meter of rain each year.

These wet conditions have given rise to one of the most diverse collections of flora on Earth.

In an area little bigger than a football field, over 700 species of trees have been recorded.

By comparison, the entire east coast of North America can boast less than one-third of that number.

The rainforests here are amongst the oldest on Earth.

These steamy bastions of life support a staggering array of flora and fauna, many of which are unique to this island.

And new species are discovered almost daily.

Towering emergent trees, known as dipterocarps, dominate the jungle landscape.

They can measure over 80 meters tall, making them the tallest of all rainforest hardwoods.

Incredibly, it's said that up to a thousand species of insect can live on a single tree... a predator's paradise.

To survive in the lofty canopy here, one reptile has adapted in a way that even Darwin would have been surprised by.

The Draco is the world's only flying lizard.

(lighthearted music) While it's not capable of true flight, the Draco is an adept glider.

When faced with a threat, this unique creature can launch from the treetops and glide up to eight meters to safety.

The secret to this gravity-defying trick is the extendable flap of skin between the lizard's limbs.

These thin membranes can be stretched out to create wing-like structures, while the brightly colored flag on its neck is used to help stabilize the lizard as it glides through the air.

This adaptation has served the Draco well.

Their numbers on the island are stable, but the same can't be said for all of Borneo's unique creatures.

(suspenseful music) In the local Malay language, orangutan means people of the forest.

And as these great apes share around 97 percent of the same genetic material as us, this is no small coincidence.

Today, wild orangutan can only be found in Borneo and the neighboring island of Sumatra.

They were once thought to be a single species, but have recently been classed as distinct subspecies, divided by geography and genetics, reflecting subtle differences in evolutionary pressures on nearby but separate island arks.

Orangutans are the most arboreal of the great apes, spending the majority of their lives in the canopy.

By day, they feast on an abundance of food provided by the rainforest-- fruits, honey, and the occasional bird's egg.

(lighthearted music) (birds chirping) But as twilight approaches, they return to the sanctuary of their nests high above the forest floor.

Orangutans are also amongst the most intelligent of primates.

They construct these sturdy nests by bending and weaving together supple branches before lining them with leaves for insulation and comfort.

The females of the species rear their young for several years, teaching them the ways of the forest before eventually sending them off to live a largely solitary life.

Dominant males can be identified by their distinctive cheek pads, known as flanges, which are developed over time to intimidate other males.

This phenomenon is unique to orangutan and not yet fully understood.

Males can weigh up to 100 kilograms, and their enormous arm span can stretch over two meters, allowing them to adeptly navigate the forest canopy.

(suspenseful music) These great apes were once prolific on the island of Borneo, but human encroachment into wilderness areas has led to a rapid decline in their numbers.

In recent years, the indiscriminate clearing of rainforest to cater for the booming palm oil industry has wiped out over 20 percent of the orangutan's natural environment, and 80 percent of their population.

Today, these majestic creatures are classed as critically endangered.

(ominous music) Orangutan, along with other large mammals, such as the Bornean elephant and rhinoceros, are considered keystone species here.

Any significant reduction in their numbers is symptomatic of a much bigger problem, a catastrophic collapse of the island's biodiversity.

In 2014, the last of Borneo's free-roaming rhinos unexpectedly died, condemning the wild population of this 30-million-year-old species to extinction.

If greater care is not taken to protect Borneo's natural assets, many other species, including the orangutan, will face a similar fate.

(dramatic music) (mysterious music) Adrift in the Indian Ocean, Christmas Island is a remote territory of Australia, around 1500 kilometers northwest of the mainland.

In the mid-1600s, passing European mariners named it for the holiest day of the Christian calendar, but the island wasn't settled until the late 19th century.

(upbeat music) Today, it supports a modest human population of just under 2,000, but its rugged 135 square kilometer landmass, and the warm tropical waters that surround it, play host to a staggering array of terrestrial and aquatic species, many of which are native to the region.

At its highest point, Christmas Island sits at just 300 meters above sea level, but this unassuming peak is just the tip of the iceberg, or more accurately, volcano.

(melancholy music) Lying beneath the surface is an enormous underwater volcanic mountain, measuring nearly 4,500 meters tall, or half the height of Mount Everest.

Today, much of the island's surface is covered with coral-based limestone, rather than igneous rock, bearing witness to its previous life beneath the sea.

Perched on the edge of the Indian Ocean's deepest trench, Christmas Island has some of the steepest and longest drop-offs in the world.

Along its fringes, coral reefs support almost 600 species of tropical fish, but just a little further offshore, marine giants frequent much deeper waters.

Whale sharks are the world's largest fish.

They can measure more than 12 meters long and weigh as much as 20 tons.

Despite their ominous name, this species feeds on a diet comprised largely of microscopic plankton, and pose little threat to humans.

(ethereal music) As Christmas Island was formed in the middle of an ocean, the ancestors of those species that live here have come from elsewhere, swimming, drifting, or flying here over the eons.

Today, the island plays host to a diverse range of seabirds, including populations of rare and indigenous species.

(upbeat music) Amongst the most prolific is the red-footed booby, which nests in trees along the shoreline.

They're an anomaly amongst seabirds, as many of their web-footed relatives prefer to roost and mate on the ground.

During the breeding season, the bright coloration of the male's feet helps to attract a suitor.

Once paired, the couple will raise a single chick every year or so, nurturing it to maturity.

But when it comes to reproducing, not all of the island's creatures take a slow and steady approach.

On the sandy floor below, another of the island's residents waits out spring's mating frenzy, but by November, they are on the move, and one of the greatest mass migrations in the natural world gets underway.

(suspenseful music) As the wet season arrives, red crab populations of up to 100 million make their annual journey from the island's interior to the sea where they'll spawn.

(waves lapping) The ancestors of the red crab crawled from the sea over 10 million years ago to seek refuge on land, where there were far fewer predators.

They eventually traded gills for lungs and are, today, well-suited to a terrestrial lifestyle.

But to reproduce, these crabs are still reliant on the seawater that surrounds the island.

They must make a five-kilometer journey from their rainforest home to the rugged coastline... which is no small task, given that they're used to moving just a few feet each day.

(agitated music) The full moon draws them from their moist burrows.

The coming of the rains has paved their way, soaking the ground to protect them from dehydration, the crab's greatest foe.

To reach their destination, this red army must run the gauntlet, overcoming obstacles both natural... and manmade.

10 days after setting out on their mass migration, it's the males that arrive to the coastline first.

They dig shallow burrows to attract a female, and competition for prime real estate is fierce.

As females arrive, they select males with the most appealing burrows, and begin to mate.

Remaining within the burrow, females produce in excess of 100,000 eggs.

After 12 days or so, under the light of the waning moon, they make their way to the water to deposit their eggs.

(idyllic music) Upon contact, the gently lapping water stimulates the eggs to hatch, releasing millions of tiny larvae to the sea, where they will quickly develop into juveniles before eventually returning to shore to restart the cycle.

(mysterious music) Born of a collision of fire and water, and rising from beneath the ocean as a beacon to distant life, since its emergence, Christmas Island has nurtured a truly unique blend of endemic and migrating species, and hosts one of nature's most spectacular shows.

(dramatic music) (suspenseful music) Existing on the edge of civilization, the Chatham Islands lie in one of the most remote regions on Earth.

This small island chain is located in the south Pacific Ocean, around 800 kilometers east of New Zealand, of which it's an external territory.

(upbeat music) The Chatham Islands are known for their rugged beauty, and for the rare and native species that have either evolved here, or found refuge upon them.

Like New Zealand, this island chain rides on Zealandia, a submerged continental fragment that was set adrift from the Australian mainland more than 60 million years ago.

These islands have traveled so far east that they now sit in their own time zone, some 45 minutes ahead of New Zealand alongside the international date line.

(sheep bleating) (peaceful music) The archipelago is made up of 10 islands within a 40-kilometer radius.

Over the eons, their rugged features have been shaped by the Roaring Forties, a powerful and unrelenting wind that circles the southern reaches of the globe.

The Chatham Islands were once covered in dense forest, but today, thanks to both the forces of nature and of man, they're defined by windswept pasturelands, fern-lined peat bogs, and wooded areas dominated by endemic plant species that have acclimatized to the harsh environmental conditions here.

(waves crashing) The islands were first inhabited by Polynesians around 500 years ago, and later by New Zealand Maoris, but it was European settlers who have had the most impact here, as they cleared land for pasture from the late 1700s.

(suspenseful music) Despite the many changes that the Chatham Islands have undergone, they still play a vital role as breeding grounds for indigenous and migrating seabirds, and today, two of the islands, Mangere and Rangatira, have been set aside as wildlife sanctuaries.

If it were not for these conservation efforts, species such as the local black robin would have long ago fallen to extinction.

Like much of the wildlife here, evolving in splendid isolation in the absence of large predators, the black robin, with its limited capacity for flight, was vulnerable to the introduction of European species, like cats and foxes.

In the 1980s, just five black robins remained here, but the removal of introduced species and a dedicated breeding program has since allowed their numbers to recover.

Today the island plays host to a population of around 250, and the black robin has been brought back from the edge of oblivion.

(upbeat music) Offshore, the rich sea life attracts an abundance of marine mammals... including leopard seals, southern elephant seals, and the New Zealand sea lion, the world's rarest sea lion, which inhabits the island's coastline.

Known as whakahau' in the local Maori language, these sea lions are amongst the region's largest animals.

Males can measure up to three and a half meters long, and can weigh as much as a grand piano.

They spend much of the day sunning themselves on the shoreline before entering the water to prey on fish, squid, and marine birds.

They are formidable hunters, but they're not alone in these frigid waters.

(ominous music) Sea lions are, in turn, hunted by great white sharks, the world's fiercest oceanic predator.

(dramatic music) Those that survive the savage onslaught display scarring from previous attacks as testament to their resilience.

(idyllic music) For the many species that continue to rely on these islands for food, safe haven, and breeding the Chatham Islands remain an ark in perpetual motion... drifting ever eastwards into the vast expanse of the wild south Pacific.

(dramatic music) The Coral Triangle is recognized as the epicenter of Earth's marine biology.

(inspirational music) It spans sections of the Pacific Ocean and South China Sea, covering a region half the size of Europe.

While this only accounts for around two percent of the planet's oceanic area, these life-giving waters support over 75 percent of the world's coral species... which in turn give rise to an unparalleled abundance of marine life.

(ominous music) Within the heart of the Coral Triangle resides the famed isle of Komodo.

In the 16th century, this small Southeast Asian island gained notoriety as the haunt of mythological creatures.

Early nautical maps of the region were inscribed with the warning "Here be dragons."

More than the myths and legends of ancient sailors, this reputation was well-earned.

Komodo Island is home to a creature found nowhere else on Earth... the Komodo dragon.

Komodo dragons are the world's largest lizards.

They can measure up to three meters long and weigh as much as 70 kilograms.

(lighthearted music) Their unusually large stature is generally thought to be the result of island gigantism, a phenomenon of natural selection in which isolated island species grow much larger than their mainland relatives in the absence of competing carnivores, but some believe these so-called dragons are relics of prehistoric times, simply outliving other megafauna.

In either case, it's easy to see why ancient mariners may have mistaken them for mythological creatures.

Westerners first documented Komodo dragons in 1910, describing them as land crocodiles.

Like other lizards, Komodo dragons use their long forked tongue to smell and taste potential prey on the air.

By swinging their heads from side to side, they can pinpoint the source of a scent from up to 10 kilometers away.

Komodos are not above scavenging, but are also active hunters, and while they maintain a relatively modest pace as they forage, they're capable of surprising bursts of speed.

Komodos are ambush predators and can be very patient.

It was once thought that this giant lizard's mouth contained concentrated deposits of bacteria that would infect a bite wound, eventually incapacitating a victim, but it's now known that the Komodo's bite is venomous.

Venom glands within the dragon's cheeks produce toxins, including anticoagulants, which ensure its prey leaves a bloody trail to follow.

All it takes is one bite... (dramatic music) and the victim's fate is sealed.

Despite its size advantage, this water buffalo is doomed.

The solitary predator follows patiently, waiting for his venom to take its toll.

But when it comes to devouring his prize, he won't be alone.

Komodo dragons are an anomaly amongst reptiles.

They're one of the few species to feed communally.

Others gather to enjoy the spoils.

The dragons dismember the carcass with long, serrated teeth that can measure up to two and a half centimeters in length.

Their saliva is continually stained with blood, as their teeth are sheathed in a thin membrane which is naturally torn open during feeding.

This added fluid is thought to aid in the digestion of the lizard's meal.

Attacks on humans are rare, but several documented deaths can be attributed to this formidable apex predator... a giant that has evolved in splendid isolation outliving its prehistoric forebears and bringing myth and legend to life on this ancient isle of dragons.

(waves lapping) (rhythmic percussion music) It's no surprise that the seeds of Darwin's theory of evolution first sprouted on the Galapagos.

Evidence of natural selection is most pronounced on islands.

Existing in isolation, in often harsh and unforgiving environments, life continually finds a way.

These island arks are inextricably linked to the often rare and bizarre creatures that have been shaped by their excesses and limitations, and offer a lifeline to migrating species which would otherwise be lost to the vast oceanic expanse.

On our Blue Planet, they are the cornerstone of life, but they are also incredibly fragile.

As humans, we owe a great debt to these island arks, and as their custodians, we must take great care in their preservation.

(dramatic music)