(CNN) A potent mix of intense sibling rivalry and brotherly love has proved the catalyst for one of the most compelling sporting head-to-heads of our time.
Triathlon has not been the same since the Brownlee brothers, Alistair and Jonathan, began dominating the professional circuit in the immediate years leading up to the London 2012 Olympics.
Their supremacy in the three-discipline sport -- swimming, cycling and running -- culminated in a gold for older brother Alistair and a battling bronze for Jonny, which might have been silver but for a 15-second time penalty.
On home soil in London's Hyde Park, their every step was cheered as they went on to share an Olympic podium -- the first time British brothers had done so in an individual sport since 1908.
Born two years apart in the proud northern England county of Yorkshire, their quest to be "best" at everything from sport to household chores began as young children.
"I think everything turned into a competition, games of crazy golf ended in massive rows so that's an overwhelming memory," Alistair tells CNN's Human to Hero Series.
"Even doing the tidying up we both wanted to do it as quickly as we could," the 27-year-old adds.
Their long-suffering parents Cathy and Keith, who are both doctors, had always encouraged the hyperactive boys to burn off plenty of energy with an outdoor lifestyle, bracing walks, bike rides and runs on the rugged Yorkshire moors, as well as taking part in conventional sports like football.
Both also developed a talent for swimming, so the building blocks for success were already in place when Cathy's brother -- Simon Hearnshaw -- decided to take part in a local triathlon.
Jonny was immediately hooked. "I was running a little bit at school and then my uncle did a triathlon, and I thought it's a cool, exciting thing to do," he says.
"Alistair being the older brother actually started a few months before me."
The siblings have been dedicated to the sport ever since, establishing themselves as the best at the triathlon Olympic distance of 1500 meters swimming, 40 kilometers cycling, with a closing 10 km run.
They regularly train for more than 30 hours per week, pushing each other to their limits.
Childhood spats apart, their rivalry has rarely spilled over into animosity, although Jonny is quick to highlight one his scariest moments in the sport -- almost being drowned by Alistair in the opening moments of a swim in France as the super-fit competitors jostled for position.
"He just pushed me under straight away -- obviously that's not the best way to start a race because after 10 meters the next person pushes you under," recalls the younger Brownlee.
"You keep on going down and it can be pretty scary, but you want to try and avoid that if you can."
Such rivalry, however, has proved incredibly helpful for the pair.
"We do definitely both motivate each other because on days if your brother is going out training and you don't fancy it, you obviously aren't just going to let him go out," Alistair says.
"What I love most about doing triathlon is the training, the long easy training rides for four hours on a beautiful day," Jonny adds.
"What I enjoy least is the opposite side of that, so the really cold training days when you've got to ride your bike for three or four hours, it's snowing outside and you can't feel your hands after an hour -- that's not much fun."
There have been other sacrifices too: Alistair quickly decided to abandon prestigious medical studies at Cambridge University to concentrate on his triathlon career.
Aged 18, he was already world junior champion and opted to do a less demanding degree in his home city of Leeds, where he could be nearer to his coach and elite training facilities.
Jonny went down the same route, studying in Leeds to enjoy the same advantages, and of course to train full time with his brother.
Alistair first sampled the Olympic arena at the 2008 Beijing Games, becoming the highest British finisher in 12th place -- an invaluable experience.
The following year he was crowned world champion, winning five of the International Triathlon Union's (ITU) World Series races.
By 2010, with Jonny fully maturing as an athlete, the brothers were consistently battling it out at the front of World Series and major championship races, often accompanied by arch-rival Javier Gomez of Spain, a triathlete with formidable running speed.
They went into 2012 with Alistair the reigning world champion at Olympic distance and Jonny as the two-time world champion in the sprint distance (750-meter swim, 20 km bike, 5 km run).
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Rivals muttered darkly about "team tactics" as the two brothers moved to the front of the pack on the swim and cycle legs to give themselves the best chance on the concluding run where the medals are decided, but Alistair puts it into context.
"Once we're on the race course we are very competitive and we both want to win, but we know that if we help each other for the swim and the bike and even parts of the run, that's our best chance of getting a good result," he says.
"But once it comes down to the last part of the run, all those bets are off and we are just racing to win."
And so it proved in London, but with just one gold medal on offer, there was even talk of the brothers manufacturing a dead heat for first place.
In reality, that was never going to be the case, particularly with the ever-present Gomez a constant threat.
Alistair finally shrugged off the dangerous Spaniard in the final stages of the run to take gold.
He had covered the closing 10 km in a remarkable 28 minutes 50 seconds, not much more than a minute slower than compatriot Mo Farah's winning time in London over the same distance in the track-and-field competition.
After finishing in splendid isolation among a sea of union jacks, Alistair then looked and waited for his brother to cross the line.
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Jonny, who had incurred a 15-second penalty for mounting his bike too early after the swim, took his time forfeit ahead of the final lap of the run, but still came in just over 30 seconds down for the bronze.
It was one of the most memorable moments of a memorable Games for the host country, and the brothers became national heroes.
"Obviously it was a fantastic experience both being on the podium at the same time, it was what we'd worked for," Alistair says.
"It would have been better if he's come second, but we just stood there really just having a bit of a joke saying, 'You better not burst into tears on me' -- yeah it was good."
The double medal success captured the imagination of the British public, and celebrity beckoned.
"The reaction was mad, I don't think we ever quite expected our lives to get so busy and hectic for a little bit, to be so recognized to the point we couldn't walk down the street in London," Alistair says.
But with races still to come that year, it was soon back to reality and the daily grind of training and early starts.
"It very quickly came around again that you were back in the pool at seven o'clock in the morning with the same coach, the same training partners, doing the exact same thing so I very quickly got back in to my routine again," Jonny recalls.
It paid off, as he claimed the world championship crown at the end of 2012 in Auckland, New Zealand.
The intervening years have seen the redoubtable brothers battle injury as well as perennial rivals such as Gomez, who claimed a historic fifth world title in Chicago in 2015.
Highlights for Alistair have been gold at the Commonwealth Games in Glasgow in 2014, with Jonny taking silver. They then paired up to win team gold in the mixed relay event.
The 2014 European championship title also went to the elder Brownlee to complete a rare triple of Olympic, European and Commonwealth gold.
Now they are focused on August's Rio Olympics, where Jonny hopes to avenge his defeat in London, "to win to stand on that podium."
He hopes the tough nature of the bike course in the Brazilian city will draw the sting from his brother's running speed.
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"Rio is a different course to London, it's a harder bike, the swim is not a lake swim it's a sea swim which makes that harder," Jonny says.
"Whereas in London Alistair ran 28.50 to win, in Rio it will hopefully be a little bit slower."
Like all great rivalries, that sense of competition is real, but like other sporting siblings such as the Williams sisters in tennis, the natural instinct is to look out for each other.
"The hardest part about competing against Alistair is if he's had a bad race and I've had a good race, then you are torn emotionally because you want to celebrate your race, but at the same time he's had a bad race so I'm upset for him," Jonny says.
Alistair, playing the role of older brother and more used to coming out on top, cannot resist adding a bit of edge.
"Obviously it is hard when you've had a good race and Jonny's had a bad race.
"The worst part, of course, would be if he beat me -- which is what he wanted to say but didn't!"
After Rio, Alistair says he might turn his attention to longer distance racing such as Ironman (4 km swim, 180 km cycle and 42 km marathon run).
Jonny has penciled in the 2020 Tokyo Games for a further crack at Olympic gold, but has also firmly set his sights on Ironman.
"It's the pure side of the sport in that it's man versus man," he says.
Either way, sporting fans can look forward to more excitement from the sibling superstars, who have come a long way since their back garden battles.
"I think we are both aware it's special we can do it and compete to such a high level together," Alistair says.
"We are both very grateful that we train together, push each other on, travel together and have those experiences together."
But he can't resist having the last word.
"As long as I win it's a good thing!"
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