Living on the peaks of the world's highest mountains is a tough challenge. At high altitudes the sunlight is intense, cold winds buffet you from every direction, and it's difficult to even breathe because there is so little oxygen in the air.
But despite these extreme conditions, life endures. If you climb to the summit of Mount Everest, the world's tallest mountain, you will find life at every level. Jungles give way to alpine meadows and finally to snow-covered rock, but there will always be animals and plants if you know where to look.
These organisms have made their homes in one of the most extreme environments on Earth. How do they do it? To find out, we must take a walk into the sky.
Between the vertiginous plateau of Tibet and the fertile plains of India rises the most majestic mountain range in the world. The Himalayas are famous for their high peaks, with more than 110 jutting into the sky at 24,000 ft (7315m) above sea level. Mount Everest soars to elevations of 29,035 ft (8848m).
We begin our journey in the foothills of the Himalayas, about 2000 ft (610m) above sea level. The foothills extend eastward for more than 1000 miles (1609km) from northern Pakistan across north-west India and through Nepal, until they meet the Teesta river in north-east India.
Tigers also roam the foothills of Bhutan
They were once draped in forests, but many of the trees have been cut down to make way for farms. The remaining forests are a sanctuary for Asian elephants and rhinoceroses, which still roam parts of the Tarai region in southern Nepal. The forests are also home to Asiatic black bears, clouded leopards, the goat-like Himalayan tahr and more than 340 species of birds.
Tigers also roam the foothills of Bhutan, and in 2010 it emerged that they venture far higher than anyone expected. Wildlife expert Alan Rabinowitz became suspicious after hearing reports by panicked villagers, and visited the area with a BBC film crew. After trekking to a height of 4000m (13,000 feet), he set up camera traps.
Rabinowitz recorded a wealth of wildlife, including red foxes, jungle cats, monkeys, leopards, a Himalayan black bear, tarkin, serow, musk deer and even a red panda. The cameras also captured two tigers, a male and a female. The spot, which is a closely guarded secret to prevent poaching, is the only place where tigers, snow leopards and leopards share the same territory.
As we climb higher we leave the deciduous forests and farms of the foothills, and enter coniferous forests with waterfalls cascading through them. These gatherings of pine, hemlock, spruce and fir receive almost 80 inches of rainfall each year.
The snub-nosed monkey's odd features make it look like a victim of bad plastic surgery
A huge range of wildlife can be found, such as the golden langur monkey. Thick fur protects the monkey from the cold. Like many animals in the region, it spends its winter in the lower valleys and follows the snowline up the mountains in the spring.
However, langurs aren't the highest living monkey in the world. That title goes to the Yunnan snub-nosed monkey, whose thick fur allows it to live higher than any primate, except for humans. The snub-nosed monkey's odd features make it look like a victim of bad plastic surgery.
Takins and musk deer, famous for secreting a scent found in almost all perfumes today, can also be found among the trees.
The deciduous hardwood forests on high mountain slopes provide the perfect hideout for one of nature's cutest animals, the red panda.
An incredibly shy creature, the red panda avoids predators by living high up in forests at altitudes of 5900 to 13,000 feet. Its dense fur protects it from the cold, and it has hairy feet to keep its paws warm and stop it from sliding about in the snow.
Red pandas can spend as much as 13 hours a day looking for and eating bamboo
Unlike other mountain animals, which cope with cold winters by migrating down the mountains, the red panda is trapped. It relies on bamboo as a food source, so it can only live where bamboo grows.
Red pandas can spend as much as 13 hours a day looking for and eating bamboo, even though it is full of fibre that is incredibly difficult to digest, and their guts are not specialized to handle plant matter.
In the summer they will also eat fruit and insects, and even steal birds' eggs, but in the winter they may lose as much as 15% of their body weight. They cope with the lack of food and cold temperatures by slowing their metabolism, which is almost as low as that of sloths.
We trek upwards, and the forests give way to alpine dry scrubland. Above that come meadows of flowering plants.
Mountain carp have powerfully muscular, cylindrical bodies
The plants here grow short and strong, and are experts at conserving water to protect themselves from the dry winds. Although winters are cold, summers are still mild. Shrubs, rhododendrons, mosses and lichens cover the lower shrublands, while wildflowers such as blue poppies and edelweiss cover the alpine meadows.
Highland tribes graze animals on these slopes in the summer, and wild goats, sheep and wolves can be found. What's more, some of the glacial lakes in Kashmir contain fish, such as brown trout and mountain carp.
These have a number of adaptations for surviving in torrential mountain streams and plateau lakes. Mountain carp have powerfully muscular, cylindrical bodies to help them power through fast currents, whilst other fish shelter amongst pebbles and stones to ward off the strong current. To protect themselves from the near-zero temperatures that strike in winter, mountain carp migrate down the hillsides.
Climbing higher, the scrublands and meadows give way to windswept rocks. Snow leopards live here, at elevations of between 9800 and 17,000 feet (2980-5180m) above sea level.
Markhors lock horns and fight for their right to breed on the sheer cliffs
They have thick fur to protect them from the cold, and massive paws to help them grip the rocky terrain. They use the rocky outcrops, ravines, and steep cliffs to camouflage themselves, allowing them to stalk their prey.
The snow leopard's prey includes ibex and a wild mountain goat called the markhor, which has spiralling horns that grow to over 1.5m long.
The steep ravines and loose stones make it difficult to do anything. Markhors lock horns and fight for their right to breed on the sheer cliffs. Any fall would be deadly, and the males often try to throw their rivals off the cliffs. If one does fall, snow leopards give chase, even though they must haul their kill back up the mountain to their lairs.
On the other side of the jagged mountain range, on the flat plateau of Tibet 14,800 feet above sea level, live hot-spring snakes. Being cold-blooded and thus unable to keep heat in their bodies, they ought to be at great risk from the cold. They survive by hiding out in hot springs, a kind of natural sauna fuelled by underground volcanic activity.
People have also made their home here
Hot-spring snakes are one of only a few snake species that live so high.Their nearest cousins live across the world in America.
The Tibetan plains are also home to square-jawed Tibetan sand foxes. They live in barren slopes and stream beds at altitudes of 9840-13,120 ft (3000-4000m). The foxes make their dens in the nooks and crannies of the boulders.
People have also made their home here, and just like the animals they have learned to cope with the conditions. But it's not just a matter of altering their lifestyle: native Tibetans have evolved.
Perhaps the biggest problem people face when living at high altitudes is the low air pressure. This makes it more difficult for oxygen to enter our lungs, causing a host of problems.
It is harder to conceive a child high up in the mountains
Humans tend to suffer from altitude sickness at elevations higher than 8000 feet. The symptoms include a lack of appetite, vomiting, headaches, and difficulty thinking and sleeping. In more serious cases fluid can accumulate in the heart and brain, lungs can begin to haemorrhage, and the heart can fail.
Some of the effects of high-altitude living are subtler. For instance, it is harder to conceive a child high up in the mountains.
However Tibetans don't seem to suffer from these problems, thanks to mutations in their genes.
About 8000 years ago, a gene called EGLN1 changed at one point. Nowadays, up to 88% of Tibetans have this new variation, which is virtually absent from closely-related people living in the Asian lowlands.
The change in EGLN1 protects Tibetans by stopping their bodies from overreacting to the low oxygen content of the air. In people without the adaptation, the thin air causes their oxygen-carrying red blood cells to swell up, which can lead to heart failure.
Tibetans' altitude adaptation has also been linked to a gene called EPAS1, which is involved in the body's reaction to low oxygen levels. Rasmus Nielsen of the University of California, Berkeley and his colleagues found in 2014 that no other group of modern humans has the Tibetan variant of EPAS1.
Our red blood cells carry oxygen using a molecule called haemoglobin. When people with the common variants of EPAS1 travel to high altitudes, they boost their haemoglobin levels too much, says Nielsen. This thickens the blood, "leading to hypertension and heart attacks as well as low birth weight babies and increased infant mortality."
The Tibetan-specific version of EPAS1 came from Denisovans
The Tibetan variant avoids this. It "raises haemoglobin and red blood cell levels only slightly at high elevations, avoiding the side effects seen in most people," says Nielsen.
Tibetans inherited their form of EPAS1 from the ancestors of the Nepalese Sherpa people about 30,000 years ago. But Nielsen believes that the variant does not originally come from humans at all.
"The Tibetan-specific version of EPAS1 came from Denisovans, a mysterious human relative that went extinct 40,000-50,000 years ago," says Nielsen. Denisovans are only known from a few teeth and fragments of bone, and no one even knows what they looked like.
Other animals have evolved to cope with altitude by changing similar genes. Tibetan Mastiff dogs live alongside Tibetans at high altitudes, and were originally domesticated from the Chinese native dogs of the plains. A 2014 study decoded their DNA and found that they also have different variants of EPAS1.
Above 4800 meters, the rocky tundra begins to give way to snow fields. Not many animals can survive these heights, but one that can is the mighty yak, which can climb up to 6100m (20,000 feet).
Several genes have rapidly evolved since yaks split from cattle 4.9 million years ago
The yak has two thick coats of fur that help it survive the cold. Enlarged hearts and lungs help it get the oxygen it needs.
Yaks are so well adapted to the heights that they cannot normally live below 10,000 feet. They suffer from heat exhaustion in temperatures above 15°C and are very susceptible to disease.
In 2012, researchers read the yak genome and found that several genes have rapidly evolved since yaks split from cattle 4.9 million years ago. They found three genes that are involved in regulating the body's response to oxygen deprivation. Another five genes help the yak optimise the energy it gets from its food, essential in the winter when food is scarce.
Big animals like yaks do have one advantage: their size means they retain heat well. But even tiny creatures have found their way to these heights.
In 2008 a colony of bumblebees was discovered on Mount Everest. At more than 18,000 feet up, they are the highest known insects. In subsequent tests, some of the bees managed to fly in a flight chamber that recreated the even thinner air of 29,500 feet.
Tardigrades have been found 20,000 feet up a mountain in the Himalayas
Spiders can also be found. Jumping spiders are more often seen in tropical forests and low-lying scrubland, but there are species that live way up high. The small Himalayan jumping spider is the highest living animal in the world, having been found living up to 22,000 feet up Mount Everest. They feed on insects blown up from lower down.
Strange creatures called tardigrades, which are famous for being able to survive almost anywhere, have also been found 20,000 feet up a mountain in the Himalayas.
Close to the summit, only the most hardy kinds of plants, such as lichens and mosses can grow. One particular type of moss grows at 21,260 ft on Mount Everest, and is said to be the highest growing plant on earth.
The only animals that pass higher than this level are birds.
Flying at extremely high altitudes is challenging for birds. The forces produced when birds flap their wings are directly proportional to air density, which reduces the higher up they go. This means that unless they flap much harder, birds flying at high altitudes will experience much weaker lift forces.
Despite this, birds do fly high in the Himalayas. Snow partridges are found at elevations of 18,600 feet, but the bar-headed goose goes much higher than that.
Above this height, no animal or plant can survive
This goose spends the breeding season on the high alpine lakes of the Tibetan Plateau, and the winter months in the wetland of India. This requires an annual roundtrip migratory flight over the crest of the Himalayas, at altitudes of over 23,000 feet.
At these heights, oxygen levels are one-third of what they are at sea level. The geese cope because mutations in their DNA allow their haemoglobin to take on more oxygen. They also have large lungs that allow them to breath faster and deeper in low-oxygen conditions, massively increasing the rate at which their hearts pump blood. Their large wingspan gives them greater lift and reduces the power needed for flight.
Above this height, no animal or plant can survive. Yet microorganisms can.
Bacteria have been found in the atmosphere 5-10 miles above the Earth's surface. Researchers have even found bacteria in storm clouds.
Despite the hardships and struggle that altitude brings, many animals choose the high life
In 2013, life was found even higher. British researchers sent a balloon into the stratosphere over England, where it collected samples between 14 and 17 miles (22-27km) up. They found a single microscopic alga called a diatom. Those involved claimed it came from outer space, but it seems more likely that it was simply blown very high.
These experiments suggest that we haven't yet found the absolute limit to life at high altitudes.
It's clear that despite the hardships and struggle that altitude brings, many animals choose the high life. For some, like the red panda, their favourite food is up there. For others, altitude offers an escape from the intense competition and threatening predators on the lowlands. Either way, the extreme environment seems to be a price worth paying.