Picture the scene: it is the late 1950s, and in an English field a young David Attenborough and 16-year-old Richard Dawkins are hunting for tadpoles. "We had wellington boots on, and we had little fishing nets," says Dawkins, "and we spent the whole day tramping around through ponds and ditches." He had met Attenborough through his uncle and aunt, who encountered the presenter when he was making a programme in Sierra Leone.

It is easy to imagine the pair of them trawling through the mud, because that is how we have seen Attenborough on our screens for nearly 60 years: exploring and explaining the behaviour of animals in their natural habitat. He has become part of our collective imagination, his voice a soundtrack to British television. Yet Attenborough, when we meet, is wary of his public image. "If you appear on the box, people think you know what you are talking about, and it's patently not so," he says.

This is modesty; he reads all the latest zoological literature, and his documentaries, according to Dawkins, don't just show "things the world hasn't seen but that scientists haven't seen either. You can think of it as a very fertile part of scientific work, of scientific research." Attenborough, 84, won't have it, describing himself as simply "a chap from the television".

When I ask him if he ever regrets not becoming a scientist, he shrugs. "I am not a specialist, and I can't pretend to be a specialist." He is keen not to be cast as a figurehead, or a champion of a cause. Environmentalists often try to recruit him, and in films such as State of the Planet he has spoken out on the subject, yet he "fights against being put in that situation when people say, 'So, what's happening to the climate?' I say, 'I don't know. I'm just looking at the scientific world, and this is what the majority of scientists say. It's no use attacking me. I'm a reporter.'"

Perhaps it is understandable that a figure so widely celebrated should wish to underplay his achievements. He does this to an almost comical degree, improbably describing himself as someone who is "by nature rather idle, sitting in a bath chair watching it all going on around me". He avoids aligning with any political party (he votes, but "secretly"), saying carefully that he appreciates the progress all the parties have made in taking the environment seriously.

If he has a "cause", it is overpopulation, which he believes is "at the root of almost every affliction that the world faces today". He advocates female emancipation - where there is good education and free medical care for women, they choose to have fewer children. The Catholic Church, and its opposition to contraception, must anger him, I suggest. "You're telling me!" he says. "Yes, absolutely."

Religion carries no weight in Attenborough's life. Many believers write to him, criticising his failure in his films to acknowledge the role of a divine creator. "You're never going to silence them because the fundamental problem is accepting what evidence exists," he says. "They say, 'It's written down on this page and what is there is beyond argument and it was put there by God.' If you believe that, well, I'm awfully sorry, but there's no point in us discussing it."

This is the pragmatic and tough-minded side of Attenborough. He makes programmes because he thinks "the way a spider weaves its web is breathtaking" - but he has no desire to preach. As he says, "I have a certain compulsion to tell people stories, but I don't have a compulsion to persuade them. I'm not one of them."

Fossil fuelled

Attenborough's first love was the fossil. He was born in 1926 and brought up, together with his brothers Richard (the film director) and John on campus at the then University College, Leicester, where his father was principal. As a boy, he would spend hours searching Charnwood Forest for specimens. "They are just gorgeous, and so you become intoxicated by them," he says now. "You have to be pretty stolid and phlegmatic not to be thrilled by the perfection of the fossil."

That intoxication led him to Cambridge, where he studied natural sciences, specialising in zoology and geology. When he left, he joined the navy for his national service, hoping for adventure but ending up on a Reserve Fleet aircraft carrier in the Firth of Forth. The disappointment continued as he joined a London publishing house that produced science textbooks, a job he found so dull that he thought the clock on St Paul's Cathedral had stopped, because he checked it so often.

Attenborough wanted to be out in the world, bringing the facts to life, not limiting them to paper. So, in 1952, he applied for a training course at the BBC and joined the talks department, where he made his first natural history series, The Pattern of Animals. He was eager to show the animals - trapped and frozen in the bright studio lights used on the show - in their natural environment. That brought about Zoo Quest (1954), a colonial-style adventure with Attenborough, dressed in a safari suit, accompanying the curator Jack Lester on a mission to capture wild animals for London Zoo's collection.

Their first quest, in Sierra Leone, was for Picathartes gymnocephalus, the white-necked rockfowl. The programme was supposed to be presented by Lester, but he fell ill after the first transmission and Attenborough was drafted in to replace him. Zoo Quest was exceptional for the time: filming in the wild, the team travelled to seldom-visited parts of the world. Attenborough later led missions to South America searching for anteaters and anacondas, remote Indonesian islands for the Komodo dragon, New Guinea and Paraguay for birds of paradise and armadillos. Some were never caught. Others - parrots, monkeys, pythons and bushbabies - he brought back to live in his house in Richmond, Surrey (where he still lives), cared for by his wife, Jane, and studied in wonder by their two children, Robert and Susan.

To boldly go

Attenborough's success pushed him through the ranks of the BBC. In 1965, he became the first controller of BBC2 and in 1969 director of programmes across BBC TV. But he was trapped behind a desk again. As he says: "It was very nice for me running a network for a few years, in the sense that it was very flattering for one's ego. But it's not much fun." So he resigned in 1973 and took up programme-making again, starting with a series in south-east Asia and research for Life on Earth, the first of nine Life series for the BBC that would shape the next 30 years of his career.

Alastair Fothergill, the former head of the BBC's natural history unit, was a teenager when Life on Earth was broadcast in 1979. "It was like the most gripping drama; I just had to watch next week's episode. I absolutely remember deciding that was what I wanted to do." Fothergill went on to make Trials of Life and Life in the Freezer with Attenborough, as well as The Blue Planet and Planet Earth (both of which Attenborough narrated).

After nearly six decades inside and outside the BBC, Attenborough has a better sense of the organisation's trajectory than most. "I think the BBC has strayed from the straight and narrow on a number of courses at the moment," he says. "The sails need to be trimmed and [it] needs to be refocused." And, in a rare flash of indignation about the politics: "But it is crucially important in our society and [represents] the highest aspirations of our society. I'm appalled anybody thinks otherwise." His warning to the government is clear. "If you remove the licence fee, it would be gone in a decade, finished," he says. Still, when I ask what he would be doing if he were back behind a desk at the BBC today, he replies, half joking: "Resigning, I think."

As it is, he has never stopped working. In 1997, he was filming a series in New Zealand when he received a phone call telling him that Jane had suffered a brain haemorrhage. He returned to Britain and was with her in hospital when she died. As he reflected later in his memoir Life on Air, he felt the focus of his life was gone. Jane had looked after the children and the animals, had met every flight he took home. She had made his career possible: "Now, I was lost." Work steered him through grief, and he has immersed himself in new projects ever since.

A hallmark of his career has been his desire to push back the boundaries (he introduced colour television to Britain on BBC2 in 1967). Now he is experimenting with 3D. On Christmas Day he appeared in Sky's Flying Monsters, squashed into a hang-glider as an animated pterosaur - a giant winged lizard, 65 million years extinct - whirled around him. "I was thinking, 'Gosh, a national treasure's going up in a glider,'" says Anthony Geffen, the show's producer. "And the helicopter with the rig was flying literally within feet of the glider to get him to speak to camera. It was hair-raising." But Attenborough, says Geffen, is always game. "He just goes in and likes to think the best will happen, and that nothing bad will happen." That adventurous spirit has never been quelled, nor has his work ethic. "David is rigorous," Geffen says. "He wants to get it right and he will get it right . . . He doesn't suffer fools gladly. If you get it wrong, you'll certainly know about it."

Yet he is also fun. The pair recently worked together again on First Life, a series on fossils. At the end of a day's shooting the crew would return to the hotel assuming that the presenter would be tired and retreat to his room. Far from it, Geffen says. "When we get back he's fired up and wants to go out and talk and drink red wine - and we're up till pretty late and off again very early." Attenborough will talk about anything, Geffen says; he reads extensively. "He's a very good partner in Trivial Pursuit, because he answers everything."

Not long after I met Attenborough, I went to hear him speak at the Institute of Education in London. The hall was packed, and even though it was a cold Monday evening in November, in the middle of term, the rows were full of children shuffling in excitement. At the end of the talk, their hands shot up to ask questions. He has always had this effect on children, has never lost what Dawkins describes as his "boyish enthusiasm". Perhaps that is why people can be overcome with affection for him. He is a voice and a face from our earliest years, when we sat too close to the television as a grey-haired man crouched behind a bush and explained something extraordinary about nature.

Stars in his eyes

Dawkins says that Attenborough's longevity and undimmed energy have made him arguably the most respected person in Britain. He quite seriously imagines what would happen if we had to elect a monarch: "David Attenborough would be the one person the whole country would unite behind. Attenborough for king!" Not that the man would relish the adulation. Geffen had to persuade him, when they made First Life, that the public would enjoy an additional film about Attenborough's life ("He didn't really want to make it because he likes to concentrate on what he's filming").

Eventually he agreed, and they took an extra crew on location for First Life to shoot Attenborough's Journey, tracing his lifelong passion for fossils. At the end, the presenter sits on a beach in Australia and muses on the circularity of choosing the very earliest creatures as the subject of what he says will be his final big series. "In that curious way, the end - of making my last series like this - is my beginning."

As Geffen says, this was unusually introspective. "He doesn't sit there, reflecting like that in a sentimental way, very often. He's not fundamentally sentimental about himself. He is very, very modest . . . but it's a rare chink we got out of him and quite a big moment: the final element of the whole strand of programming he's made for years and years." That this was as difficult to capture on film as a snow leopard in the wild reflects Attenborough's lack of self-interest. (Fothergill notes that, in Attenborough's films, he never says "I" - "he is clear that the stars are the animals".)

His boundless curiosity is instinctive. "That's what being alive is about," Attenborough says. "I mean, it's the fun of it all, making sense of it, understanding it. There's a great pleasure in knowing why trees shed their leaves in winter. Everybody knows they do, but why? If you lose that, then you've lost pleasure."

He seems uncharacteristically sombre for a moment. Then he says: "I feel regret that there are some people who've never even savoured it. It never occurs to people to wonder why a hummingbird and a hummingbird hawkmoth do the same things. It's a delight. So I suppose there are some people who don't do these things and are very happy and have perfectly happy lives. Who's to patronise them? But all I can say is that the pleasure of it all is not virtue, or high morality. It's just fun."

Sophie Elmhirst is an assistant editor of the NS.