In 1963, when comic book legends Stan Lee and Jack Kirby first created the X-Men, they could have had no idea what they had spawned. At the time, Spiderman, the Hulk and the Fantastic Four were hogging the limelight, and the motley team of mutants struggled to attract a readership.
Fast-forward four decades, and six X-Men films have grossed more than US$2 billion ($2.3 billion) worldwide. This week the seventh in the franchise, Days of Future Past, was released. It is expected to bring in US$125 million on its opening weekend alone.
Clearly, as the age of science has advanced, something about the concept of genetically mutated superhumans has captured the popular imagination. In the Marvel universe, mutation allows the X-Men to create ice formations, generate flames or emit "concussion beams".
Disappointingly for comic book fans, there is no equivalent in the real world. But there are a surprising number of people who -- by way of genetic mutation -- have acquired abilities that could comfortably be classed as superhuman.
In 2006, scientists at Cambridge University discovered a Pakistani street performer who was able to cut himself with knives without experiencing pain. Upon investigation, they found that he was one of a handful of local people who had a defect in a gene called SCN9A, meaning that pain did not flow from the nerves to the brain. Researchers are studying whether this may provide the key to treating chronic pain conditions.
Similarly, a biotech company called Chiroscience is developing an osteoporosis drug based on the study of a community in South Africa, which has exceptionally strong and dense bones on account of a "bone mass gene".
This year UCB, a Belgian pharmaceutical company, launched a competition to find more such superhumans, in the belief that their extraordinary genetic make-up could form the basis of new medicines.
"Individuals or groups who exhibited exceptional wound healing warrant further investigation," explained Dr Duncan McHale, UCB's vice-president of global exploratory development. He is also interested in "those who have consistently displayed exceptional resistance or immunity to infections, or who, after a robust clinical diagnosis, displayed unusually fast or spontaneous disease remission". This is only scratching the surface. In all corners of the world there are ordinary people with powers that leave mere mortals astonished. Some can swim like fish; some are able to withstand extreme temperatures; others have extraordinary mental powers, and are able to recall every word they have ever read. Perhaps it is only a partial exaggeration to suggest that these are the real-life X-Men.
The little Hulk
Young Liam Hoekstra.
In the late nineties, Dana and Neil Hoekstra from Michigan adopted a baby boy called Liam. They understood that he had been born five weeks prematurely, and that this might result in poor health as he grew up.
If anything, the opposite was true. As Liam grew older, he was able to eat constantly without gaining weight. By the time he was five months old, it was apparent that the boy was developing superhuman strength. Not only was he able to walk, but he could support his entire body weight on his arms. He did not have the protruding belly common to toddlers; instead, he had well-defined abdominal muscles. Before long, he was lifting huge weights, climbing ropes without difficulty, and spending evenings at the gym in order to burn off his excess energy.
Liam was soon diagnosed with a rare condition known as myostatin-related muscle hypertrophy, which is characterised by the absence of proteins that regulate muscle development. It has been suggested that the stories of Hercules may have been based on individuals with this condition.
Thankfully, there are no detrimental effects associated with muscle hypertrophy. This leaves 9-year-old Liam free to -- as his Facebook page points out -- "have fun being a kid".
Iceman Wim Hof.
One of the original X-Men characters was Robert "Bobby" Drake, other-wise known as Iceman. His superpower is cryokinesis, which allows him to turn things around him -- or even himself -- to ice.
As far-fetched as it sounds, in the real word, the Iceman lives. He may not be able to turn everyday objects to ice, but he holds 20 world records related to withstanding the cold, including the longest time in a bath of ice (one hour, 13 minutes and 48 seconds). His name is Wim Hof.
Among Hof's achievements are standing submerged in ice for one hour and 44 minutes; and running a marathon above the polar circle in Finland, wearing only shorts. He does a lot of things wearing only shorts. In 2007 he attempted to climb Mt Everest wearing only shorts, but aborted the mission because of a foot injury.
In 2012, an American television programme called Fact or Faked confirmed Hof's abilities. Over a period of 20 minutes in an ice bath, his temperature and heart rate remained normal.
Scientists have found that Hof is indeed able to regulate his body functions, raising cortisol levels, repressing the production of cells relating to immune responses and making automatic processes faster and more efficient.
He believes that his skill is the result of meditative practices, and that these can be learned by anybody. He has written a book about it.
The modern-day samurai
Science is baffled by Isao Machii, a 40-year-old Japanese practitioner of Iaido, the art of the samurai sword. Having trained in swordsmanship since the age of 5, he is the holder of several Guinness world records, including "most sword cuts to straw mats in three minutes"; "fastest 1000 martial arts sword cuts"; and "fastest tennis ball cut by a sword".
To win the latter title, Machii cut through a tennis ball that was travelling at 700km/h.
Most impressive of all, however, is a stunt that he performed in Los Angeles in 2011. An airgun fired a 5mm plastic pellet at Machii, at a speed of 320km/h. At this speed, it is impossible for the human eye to track an object of this size and impossible for the human reflexes to respond. Yet amazingly, he was able to draw his sword and slice it in two while it was in mid-air.
In order for the stunt to be recorded, a specialist video camera was set up, and slowed down by 250 times. A psychologist, Dr Ramani Durvasula, observed the experiment. "He was engaging in anticipatory processing, sort of like Spiderman," she says. "His form of mindfulness gave him a unique form of anticipatory awareness, which he could bring to a practised task such as slicing the pellet in half."
It is thought that Machii is using a novel type of consciousness to respond with such speed, but the true nature of this power remains unknown.
Marathon man Dean Karnazes
Dean Karnazes' muscles have extraordinary properties. When the 52-year-old exercises, he never reaches a "lactate threshold" -- the experience of intense fatigue as the muscles seize up -- because his body clears the lactic acid with ultra efficiency. This means that he can remain hydrated and functional for remarkably long periods when engaging in feats of endurance.
Lactate is a byproduct of glucose, which can be converted back into energy. But once the "lactate threshold" occurs, a saturation of lactate is created and acid builds up in the muscles. At this point, most people are forced to stop exercising. Not so for Karnazes.
"At a certain level of intensity, I do feel like I can go a long way without tiring," he has said. "No matter how hard I push, my muscles never seize up."
His 22-year career includes running 563km in 80 hours and 44 minutes, without sleep; completing a 217km ultra-marathon in Death Valley, California -- the lowest and driest region in North America -- in temperatures of 49C; and, in 2006, running a marathon in each of America's 50 states on 50 consecutive days. Some believe that so long as he is properly fed and hydrated, Karnazes could run continuously at a speed of 16km/h until he "dies of old age".
In addition to extreme running, Karnazes spends his time giving motivational speeches and doing charity work. His remarkable abilities even earned him a place on Time magazine's list of 100 most influential people.
Strangely, Karnazes did not embark on his journey until 1992. "I wasn't even aware of my 'gift' until a TV show did a scientific study on me," he says. "My belief is that there are people like me walking around all over the place, they just don't know they have such powers. Until you test yourself you may be just as naive as I was."
The human fish
The Bajau Laut people, from Southeast Asia, are nomads who spend all their lives on the ocean. They live in "lepa lepa" houseboats or in stilt villages built on top of coral reefs almost a mile out to sea. Known as "sea gypsies", the Bajau Laut spend 60 per cent of their time submerged in the water, which is equivalent to a sea otter. They are so deeply acclimatised to an aquatic life that when they occasionally spend the night on terra firma, they complain of feelings of "land sickness".
The Bajau Laut are known for their free-diving fishermen. One particular member, a man called Solvin, is an expert. On a single breath, he is able to remain submerged for five minutes, descending more than 20m to the ocean floor to hunt fish.
Research cited in the BBC's Human Planet series has shown that when diving, Solvin's heartbeat slows to 30 beats a minute. His lungs can cope with the air inside them being compressed to a third of its usual volume, and even without weights he is negatively buoyant enough to stride across the ocean floor as if he were hunting on land.
Studies have shown that some Bajau Laut children have eyes that have adapted to the ocean, allowing their underwater vision to be twice as clear as that of other humans. Many intentionally rupture their eardrums in order to assist with equalising. But despite all these adaptations, many members of the tribe have been crippled or killed by the bends.
The man with no fear
Eskil Ronningsbakken is blessed with an extraordinary sense of balance, which he demonstrates at lethal heights.
Among other stunts, the Norwegian has walked a tightrope between two hot air balloons, balanced on a bicycle on a wire 1km above a fjord, and done a handstand on a pile of chairs balanced on a rock wedged between two rocks over a 1.6km drop.
Scientists have established that Ronningsbakken does not feel fear in the normal way. "When I'm concentrating, I do not produce excess adrenalin and my heartbeat remains the same," he says. "It is only after the act that the adrenalin becomes activated and I feel a rush."
Astonishingly, readings of Ronningsbakken's heart rate were the same whether he was on solid ground or balancing upside down on the edge of a 30m building.