A young woman swept away in an avalanche, a Scottish skipper captured by pirates, a woman imprisoned in her own home by her own troubled mind, an undercover cop who survived a real life episode of The Sweeney, the oil rig worker who plunged from the sky in a deadly helicopter crash, the soldier who lost his legs in Afghanistan.

For the last few months I have been anatomising fear for a BBC radio series. The people I met - the survivors - were all remarkable, but every one of them had been changed by fear. Each person taught me what fear can do to a human being - what it robs us of, what damage it does to the mind, spirit, body. But I also learned that we can steal something from fear too. It doesn't just take, it can be taken from. We can learn resilience, perseverance, truths about our own frailty and mortality. Fear teaches us to treasure being alive because death really is too close for comfort; to love our family a little bit more because they are just as transient as us. Fear isn't always about loss, we can salvage something when we come face to face with fear.

Hannah McAleese is the young woman who went up a mountain and fell back down again - that's how she refers to herself. But she doesn't want to be that person any more. She wants to shake off the identity fear dealt her. When she was a student, an avalanche on Ben Nevis tumbled her 1000 feet down the mountainside. It took a long time for medics to put her body back together again, and McAleese is the first to admit the other wounds left - the ones on her soul and in her mind - are not yet healed.

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Her emotions are raw - at skin level. "My biggest focus is trying to get back to normal," she tells me. "My body is OK. Psychologically I've good days and bad days." She talks of "losing her way" in life after being discharged from hospital, and is overcome by the fact that only through the help of others did she finally "find her way back". It's not just fear that's affected McAleese so deeply, it's also the unasked for kindness of others: family, friends, medics, rescuers.

"I wasn't doing very well on my own," she says. Her sobs overtake her. "I can't stop the emotions from coming sometimes. I'm overwhelmed by the support of people around me - if I hadn't had that, I wouldn't be here. Being the recipient of all that love is truly amazing. I can never pay that back - unconditional love from complete strangers ..."

She trails off, but says one day she plans to "finish the journey" and beat Ben Nevis. "But it will have to be when I'm ready," she adds. "I still don't think I'm there yet."

Fear is the primary emotion of every creature on earth, if evolutionary psychology is to be believed. From it, all other emotions flow. Without fear there's no joy, for what is happiness but the absence of threat; without the flood of adrenalin fear releases into our blood there would be no panic or anger, no fight or flight; anxiety and dread are the mother and father of fear; relief and stress its two markedly different children.

In evolutionary terms, fear's supremacy makes perfect scientific sense: the primordial soup was a permanent lunch in which every one-celled creature ate every other one-celled creature. As we evolved from protozoans into more complex life forms with early brains capable of understanding the world around us, the eating continued. Fear became the innate reflex of every crawling, swimming thing.

Joe Westland is from Arbroath: a smiling, 64-year-old tattooed, earringed sailor who's been round the world and back again. But in May last year, his life wasn't just changed by fear, it was ruined. Twenty pirates boarded his oil vessel off the Nigerian coast, sledgehammered their way into his cabin and abducted him at gun-point. They ferried him back to shore and took him to their hide-out. "It was all jungle and creeks and I thought, 'No-one will ever find where I am'," he says.

The ransom negotiations were fraught. His company wouldn't offer what the pirates wanted. Westland says he thought he was a dead man. "There was a young lad about 14, he held a gun to my face and said: 'We hope your company has plenty of money. If they don't pay, we'll burn you alive'."

Fear overcame Westland then. Realising he faced a terrible death, Westland fell back on the greatest human resource - cunning. He knew he was a "golden goose". If he died the pirates would get nothing. So Westland faked an Oscar-winning heart attack - rolling on the ground, eyes swivelling, tongue lolling. The pirates, fearing pay day was about to die, settled for a smaller amount of ransom cash and handed Westland over. The joy of being reunited with his family was tempered, though. "It's ruined my life," says Westland, who can no longer face going to sea. "I get flashbacks every day. I've been driving and I see that young guy pointing his gun at me saying, 'I'm going to set you afire' and I have to stop the car. I start sweating."

Fear evolved as we evolved. All mammals have a fear of heights - a fear evolutionary psychologists believe developed in the Mesozoic era, between 252 and 66 million years ago. Our shrew-like ancestors learned falling from trees was dangerous, and that knowledge remains deep in the ancient part of our brain. The fear of snakes seems to have developed at the start of the Cenozoic era, about 66 million years ago. Back then we were early simians and snakes our predators. A fear was born, and the dread of snakes still burns in modern humans. The fear of mice and insects stems from the stone age when the last thing a hunter-gatherer wanted was a mouse scampering over food, or a weevil in their water. Fear walks hand in hand with humankind. Where we go, it goes too.

Trevis Craig sits in a park near her East Kilbride home. Getting there was a remarkable feat for a woman who, just a few years ago was imprisoned in her own home through agoraphobia. A troubled childhood left her an anxious young woman, and soon the anxiety grew into devastating panic attacks. She was gripped by paralysing nameless terror; a fear of fear. Something full of dread stirred up inside her for no understandable reason, took hold of her and wouldn't let go. It was an

almost existential dread of being alive. Craig speculates that perhaps she was too sensitive to life - that she saw more clearly how dark life is: the war, pain, poverty, cancer, suffering, cruelty. Soon her fear of life left her crippled and unable to leave her home. Forget any idea that agoraphobia is a fear of crowds - agoraphobia is a fear of life, an all-encompassing terror of the world. Craig retreated to her bedroom and sat in the corner weeping - for two unrelenting years. "I called my fear the red blob," she says. "It was a monster. It controlled me." Slowly, she rebuilt herself - through years of therapy and baby-steps to recovery. She is now a respected therapist herself - and off on a trip to New York very soon.

We know where fear lives - in a reptilian part of the brain called the amygdala. Some patients found with lesions on their amygdala do not exhibit fear. Take that little almond-shaped lump out of our heads and we'd think we were superhuman, although, of course, we'd also be dead within minutes, as fear keeps us alive just as much as our heart pumping blood around our bodies.

Duncan MacLaughlin did his first parachute jump for fun aged 16. He was still a teenager when he left Scotland and joined the Met, desperate to bust bad guys. "I didn't want to be an accountant," he says. After he made it to Scotland Yard's elite drug squad, his day-to-day life became one long episode of The Sweeney. His greatest collar came when he cooked up a James Bond-style sting on an international drugs cartel. Posing as an army deserter, along with another cop pretending to be a pilot, MacLaughlin set up an operation to fly heroin into the UK. He'd strap the drugs to his body, parachute to earth, get in a sports car, drive the drugs to Mr Big and nick the cartel.

On his way to set up the sting, MacLaughlin was meant to meet one of the gang - instead, three turned up and got into his car. "They were hard-looking, serious gangsters," he said. Something rattled him for the first time in his life: fear. A man who seemingly had no sense of fear was hit with a wave of terror. If he lost control, he could lose his life.

He was sure the three men had rumbled him and planned to execute him. "I got in the car and I remember not being able to control the clutch as my left foot was shaking so much. I turned the ignition off, got out, made an excuse there was something wrong with the fuel, went to the petrol cap and that gave me enough time to calm down and say to myself, 'Get a grip'."

He got a grip, finished the operation and busted the gang. "It was fun," he says, smiling.

The father of behaviourism, psychologist John Broadus Watson, spent his life studying fear. Watson however became something of a modern figure of fear himself: the "mad" scientist. As part of his investigations he conducted the infamous "Little Albert" experiment - a twisted version of Pavlov's classic dog conditioning. If you remember, Russian doctor Ivan Pavlov fed dogs when a bell rang. Eventually, just by ringing the bell he made the dogs salivate.

Watson, however, turned his attention to humans - in fact, human children, and he taught them to be terrified not salivate. Watson took his subject, nine-month-old Albert, and showed him a white rat. When Little Albert didn't react, Watson introduced the rat again, but this time banged a hammer against metal, causing Albert to cry. Eventually, Albert was so traumatised that anything white, soft and fluffy - a puppy, a seal skin - scared him. One photograph of the experiment shows Albert with the caption: "Now he fears even Santa Claus." Watson would be in prison today.

Paul Sharp was coming back from the North Sea rigs in August, 2013 when the Super Puma helicopter he was travelling in crashed, killing four fellow passengers. "There was a whooshing sound like a spin drier," he recalls, "and then a crack like someone cracking their knuckles - we were still 600ft in the air - and the helicopter turned on its side and we started falling to the sea. As soon as we impacted someone screamed. The helicopter turned and filled with water instantly. I thought, 'At least I am insured - my wife and daughter will be OK'."

Sharp was upside-down in pitch black and underwater. He started struggling with an escape window, but it wouldn't open. "I thought, this isn't a good day to die. I need to get out," says Sharp. He punched out the window and rose to the surface, in a pool of aviation fuel. Sharp pulled the inflate tab on his life jacket - nothing happened. He inflated it manually, looked around and saw another man floating nearby. "I pulled him towards me; as he turned over his eyes were wide open and I knew he was dead."

Sharp's survival suit was ripped and filling with freezing water. Five survivors were now in the sea, desperately trying to stay together, but then a three-metre-high wave swept them apart. "That was when fear really kicked in," says Sharp. "I thought, 'I don't want to be by myself'. Before then, I'd just thought about survival. I hadn't thought about dying."

The rescue team arrived, and Sharp got back into a chopper just hours after crashing in one. He wants to go back to the rigs but fears he can't. Sometimes he breaks down. He has nightmares, wakes up soaked in sweat, screaming, his wife comforting him. If he returned to the rigs, he worries his night-time screams would upset colleagues.

He's trying hard to get better but has a long way to go. "I know deep down," he says, "that in a week, two weeks, six months, a year I might be in the supermarket and bang - I'll see something and it'll hit me and I may break down."

Fear has a smell, and that makes it contagious. In extremis, our sweat contains the pheromone androstadienone. Interestingly, it is also linked to dominance and aggression. In blind tests, subjects are able to distinguish between ordinary sweat and sweat containing androstadienone - that's the smell of fear. Further experiments showed that once a subject had smelled the fear pheromone, they were themselves more prone to anxiety. It takes a lot therefore for any ordinary human to rise above fear - we are evolutionarily, psychologically and chemically hard-wired by it.

Micky Yule wouldn't bend or budge if fear took human form and walked up to him with a scythe. Yule was a sergeant in the British army who happily hunted for IEDs - booby traps - in Afghanistan. "Everyone was trying to avoid IEDs, but it was our job to find them," he says. One day his luck ran out and his legs were blown off in a dusty back street. "It felt like I'd fallen down a hole," says Yule, "but that was just my legs going." He remembers a "red mist" - one of his legs being vaporised.

In the aftermath of the bombing even the birds stopped singing. Yule struggled to sit up to see his injuries. "I was dreading the sight because I knew my injuries would determine whether I lived or died," he said.

Yule had seen this before. He was the man who planned how to evacuate injured soldiers from the frontline. He knew the clock was ticking. He was bleeding to death. Tourniqueted and carried by his mates to a helicopter, he was flown to an army base where medics placed him in an induced coma.

When he came round, he focused on recovery, not self-pity. "I wasn't going to be defined by my injuries," he says. 'When you've sustained injuries as severe as mine, you're left with two choices: sit back and feel sorry for yourself, or do everything in your power to get your life back to some sort of normality."

Yule chose sport to give him direction - competitive weight-lifting. It suits his tough, no-bullshit character and driven personality.

Now, he's competing in the Commonwealth Games and tipped for a medal. "It'll be one of the biggest days of my life," he says - and then he's off, wise-cracking and taking no prisoners in his wheelchair.

Linguistically, fear is as much about awe as it is about terror - an etymological hint that fear can cow us, bring us to our knees. The root of the word lies in the proto-Germanic "fera" meaning danger. This in turn became the Old English faer, which means to frighten but also to revere. It makes sense, if you think of the phrase "the fear of God".