I spent my childhood needling Paul, but he was my constant companion. And when I needed a kidney transplant this year, he was the first to volunteer

Late in the morning of 23 August, my brother and I were wheeled into adjoining operating theatres at the Royal Free hospital in London for a surgical procedure to splice a living part of his body – a healthy, shining kidney – with mine, making literal something I’ve always felt. He is part of me, as I am part of him. Our lives are entwined like vine and tree.

Not that I’ve always been thrilled about it. I remember feeling murderous when Paul first appeared – pretty unexpectedly, in my book – in my parents’ bedroom in Balham, south London, in June 1967. I was almost three. Who was this swaddled intruder, rattling our tiny house with his distress? How could my mummy be his mummy, too? And Daddy? Et tu, Brute? So, when the midwife gave me a red Dinky toy in consolation, the sense of offence stayed with me until after I graduated from college.

On hearing that Paul would be my donor, a stream of images went through my mind – a well-worn flicker book of memory and dreams. We are kids, playing in the walled garden in Maltravers Street, Arundel, West Sussex. It’s 1972, probably, and Paul is wearing a plastic Roman helmet. Or was that me? Anyway, he’s crying. Why? Because I’ve kicked him in the goolies. Now, we’re 50km away in the garden of Grove Cottages, Binsted. It’s 1978 and he’s crying again – for the same reason. What’s the matter with me? And what’s this? A food fight in Southfield Road, Oxford? I think it’s 1984. I’ve poured a bag of flour over his head and locked him in the larder. He still talks about it. This is mutual roughhousing, I like to think, and they’re certainly not my only memories of our relationship – he was the constant companion of my childhood. But I’m sensing echoes of a primal struggle here.

I could have been kinder, of course. Bigger, stronger and older, I was giddy with seniority – the first to ride a bike, the last to go to bed. I lorded over him. Unfortunately, it would take leaving home (and wrestling some demons, on my part) before I could really appreciate my wise, funny and honourable little brother – or anyone else, for that matter.

Facebook Twitter Pinterest ‘His life was my life and he was always with me, through good times and bad and everything in between’ ... Mark Tunnicliffe, right, with his brother, Paul. Photograph: Sarah Lee/The Guardian

But let’s rewind – to Balham, 1969. There may be an early exhibit for the defence. One day, Paul stopped breathing. I knew this was bad, so I ran to tell Mummy. Frantic, she pummeled him back to life and cried. It seemed my ambivalence about my brother had briefly given way to something else. And I had no words for that “something”. Was it love?

My young self wouldn’t have understood the question. Paul and I were peas in a pod. His life was my life and he was always with me, through good times and bad and everything in between.

Observe any relationship over time and you will see how it has been made, as rings of growth make a tree. Our early lives were eventful. The rings are dense. After our parents split up in 1970, we moved to West Sussex with Mum. We were marked out from the start. Our hair was long, our clothes were patched and our London hippy values assigned us to a different tribe. Paul and I now had more in common than blood alone.

We weathered the stares and sought our people. Mum, a freewheeling spirit, got a job at the UK campus of an American college. Our flat filled quickly with like-minded students, bringing the energy and optimism of Woodstock straight into our living room. As kids, most of our mates looked like David Crosby, making the club of our childhood more exclusive than ever.

Eventually, through Mum’s work, we moved to the US during the Watergate-era and for six months lived communally in a scruffy clapboard house in rural New Hampshire. Inside our private bubble, Paul and I played, and fought, as ever: hanging out, day in, day out – exploring the streams and snow, the woods and endless space – and going to school. Much of the time, we were so close and niggly that we transcended friendship.

Facebook Twitter Pinterest ‘Our hair was long, our clothes were patched and our London hippy values assigned us to a different tribe’ ... the Tunnicliffe boys with their mum. Photograph: Mark Tunnicliife

Back in England, my brother and I carried on as before, keeping one another company, winding each other up and enjoying the companionable silence only siblings know. Usually in Mum’s domain, we treasured our long school holidays with Dad in Cornwall.

In 1979, my childhood fading, our family came horribly close to losing Paul. He developed a blood clot on his brain after an accident and needed surgery at Great Ormond Street hospital. He recovered, but not before I had pondered life without him. The idea was unprocessible.

With leaving home came an inevitable fork in our road and, in time, a poignant turning of tables. Although he is younger, Paul always seemed the older soul; having a steadiness I lacked, he soon found direction. I had strong convictions and some creative talent, but I struggled to define myself; ever anxious, I did what I could to make a life.

After doing a diploma at art college, I went to Nicaragua to work on a coffee farm, then took a degree in anthropology and geography and worked at Oxfam’s HQ as a marketing writer for nearly a decade. But something was wrong. The pressures and responsibilities of working for a big organisation stressed and depressed me. A relationship collapsed and I spiralled into breakdown. I was hospitalised twice.

After getting better, I pulled things from the fire and went freelance, but my physical health began to unravel – arthritis led to vasculitis and the clobbered kidneys I now nurse. Depression, mercifully, I learned to control.

Meanwhile, my little brother became the “grownup”. Always better at holding down relationships and full‑time work, Paul married his long‑term partner, Beth, had two beautiful children, bought a house and co‑ran a respected design agency, which he still does.

Big brother had to swallow a bucketful of pride – and no small amount of envy. Wasn’t I supposed to blaze the trail? Wasn’t it my job to set an example, stay strong and do things for him? But it didn’t happen. Paul modelled “successful” adulthood instead: juggling his commitments, guiding, supporting and giving counsel. Grateful, I’ve had to get used to it. It has been a challenge to my instincts.

Am I canonising him? I hope not – he was an annoying little blighter sometimes. The oldest often carries the can – crimes in the garden and larder aside. And he’s only a man – he has had his problems. But this doesn’t reduce my sense of debt, of having let him down somehow. Now, with him giving me a kidney, I know I can never repay him. Is this competitive kindness, Paul? (He volunteered, by the way – I stopped bullying him when he reached 6ft 3in.)

But I survive. And better. I moved from Oxford to an old boat on the Deben estuary in Suffolk last summer, a wide-eyed leap of faith bringing tides and birdsong to my days – and a degree of peace I cannot remember having felt before. Meditation and philosophy keep the horizon steady and in view.

However, as Jim Morrison observed, no one gets out of here alive. While transplants are an everyday success, my ambitions are … realistic. The future hinges on a gift from Paul that will hopefully keep on giving – so far, so good. I honour him. His sense of rightness has been made brutally graphic.

“Sorry, Paul,” got old long ago, as did “thank you”. I’ll try to express my feelings somehow.

I love you, little brother, as I did, incoherently, back then. These days, I can put it better than hoofing you in the tackle.

• Mark Tunnicliffe blogs at River Rat Diaries