The first natural history programme David Attenborough worked on adopted a very straightforward approach. "A keeper from London Zoo would put some creature in a sack," he recalls. "It would then be transported to Alexandra Palace, where we'd have put a doormat on a table. The keeper would put the animal on the doormat and then say to the camera, 'this is a lion cub'. In fact it could make for quite good television," he laughs. "The animal would sometimes escape and bite the keeper. But it really wasn't much to do with zoology."

Within a year or two Attenborough had broken free of the studio and begun to create a body of work that ranks among the most memorable and technically innovative television ever made. From his first trip to Sierra Leone in 1954, where he filmed keepers from London Zoo searching for the bald-headed rock crow, to this month, when the 83-year-old Attenborough will visit the north pole, he has hardly been off our screens. In that time he has become one of the most trusted people on television and his programmes some of the best loved. National treasure status was granted long ago, and was most recently evidenced by him not only occupying Alistair Cooke's slot on Radio 4 but also appearing at the last night of the Proms in a performance of Malcolm Arnold's uproarious A Grand, Grand Overture for orchestra, vacuum cleaners, floor polisher and rifles. Attenborough played the floor polisher with gusto.

In the half century he has been broadcasting, the perception of Attenborough's subject has changed. Study of the natural world used to be a prime example of the apolitical, the reassuring and the timeless. It has become contentious, alarming and urgent. And Attenborough has not entirely escaped the crossfire. George Monbiot crystallised the complaint of some environmentalists that Attenborough's vision of the world too often underplayed the extent of humanity's impact on the environment. "There are two planet Earths. One of them is the complex, morally challenging world in which we live, threatened by ecological collapse. The other is the one we see on the wildlife programmes." Monbiot saw Attenborough's "invocation of a fantastic, untainted world" as dangerous, claiming he had become, "in two respects, godlike. He can, in the eyes of all who worship him, do no wrong. And he has created a world which did not exist before. He's a fine man, but for 50 years he has perpetuated one of humanity's most dangerous myths."

The campaigner Jonathon Porritt recognises the charge. He remembers environmentalists having "unbelievable respect" for the awareness Attenborough engendered in millions of people around the world. "But that was tempered by a sense that he was less outspoken than he might have been in terms of the implications of human activity. Those views persisted well into the 90s. It wasn't until comparatively recently that he emphatically said that we needed to get our act together. That was an incredibly powerful moment, and his recognition of human impact on the natural world has become a more confident and up-front thread throughout his broadcasting."

Porritt and Attenborough are now fellow patrons of the Optimum Population Trust, which campaigns on issues of human population and its impact on environmental sustainability. Attenborough says that "the rather dated observation that you can travel to the heart of Africa and end up holding a Coca-Cola bottle was a kind of joke that has become an obvious reality. But the important point is that the planet has become overrun with humanity and we can't go on expanding. If we were another species, then predators or lack of food or lack of territory would deal with it. Somehow we have accommodated ourselves, albeit often uncomfortably. But none of these things is sustainable. And unless we take some action, we will run out of food and places to live."

He acknowledges that this approach "strikes at the fundamental rights of a human being to decide how many children they should have. The only answer I have is that in every society where there is literacy and where women are treated as equals and have control over their own bodies, the birthrate drops. So the way to stop population growth is to raise standards of living."

When he was a boy, the idea of environmentalism, as we now think of it, didn't really exist. "I was interested in the natural world, but it was nothing to do with saving the planet. There were people who thought the country had been desecrated in terms of putting up pylons and things like that. But the idea that you could actually destroy the Earth didn't really occur. So we poured raw sewage into the sea because the seas were infinite. If there were unpleasant smells from a factory then you would build a higher chimney. The atmosphere was so big it could just absorb it all. If you wanted to dig up a bit of forest, no one was going to stop you. It was assumed the world was big enough. Maybe it was. But when I was a kid there were only a third of the people on the planet that there are today. It doesn't seem big enough any more."

David Attenborough was born in 1926, the middle of three brothers, and was brought up in Leicester. His father, an Anglo-Saxon scholar, was head of a college at the university. He attended a local grammar school and "within a half-hour bike ride I could get into a hedgerow or a wood or a wild field, fishing for newts or looking for fossils."

In 1945 he won a scholarship to read natural sciences at Cambridge. "It was the most marvellous time of my life. It's a cliché, but mostly clichés are true. I encountered whole new areas of human experience: music and painting and talk with other students. And there were great men around the place who had proved that continents moved or had mapped the history of the North Sea through pollen analysis. But mostly I had an overwhelming feeling of good fortune. I was sitting alongside people who had lost arms or legs – fighter pilots who had been shot up. To say you were humbled would be putting it mildly."

After university he completed his national service in the Navy and says that by the time he was demobbed he had begun to doubt whether he had the necessary dedication to be a scientist. "I also didn't fancy going back to living on a grant. I was used to earning a wage. And I wanted to get married." He and Jane Oriel were married in 1950, by which time he was a junior copy editor in an education publishing house. They had two children and were married for 47 years until Jane's death in 1997. Attenborough still lives in the Richmond home where they moved in 1952, the year he joined the BBC. He had initially applied, unsuccessfully, for a job in radio, but his CV was passed to the fledgling television service and, despite being unable to offer a critique of their programmes because he didn't own a television set, he was appointed as a trainee producer.

Opportunities for working on screen quickly presented themselves, despite an internal report on an early appearance insisting that he should not be used again because "his teeth are too big". There was a strong tradition of natural history on radio, and a lot of nature films had been made by independent companies, but Attenborough and his colleagues were essentially inventing natural history television. Travel to the likes of Sierra Leone, Indonesia or New Guinea was unbelievably exotic. "People knew what elephants and giraffes looked like. But the komodo dragon we filmed wasn't in any zoo. We were the first to film lemurs in Madagascar, the first to film birds of paradise displaying in the wild. This was all entirely new."

Attenborough found himself back in front of the camera – despite the teeth – when a London Zoo keeper was taken ill on location. He says he did a bit of amateur dramatics at school – "although not as much as my brother did" – but never had any ambition to perform. "And it was all staff no fee in those days," he laughs. Michael Palin, perhaps the most travelled television presenter since Attenborough – and the first to perpetrate the many TV parodies of him with Monty Python – describes him as the consummate presenter: "There was no blather. I learned from him that you couldn't pretend to know what you are talking about, you really had to know. He's also a very good actor. He knows how to draw an audience in, how to pause, how to create tension and how to play a laugh. There's an awful lot of craft at work."

Despite burgeoning fame, in the mid-1960s, after 10 years as a presenter, Attenborough enrolled on a part-time anthropology course at the LSE. "It was wonderful, but after two terms I was given this chance to run BBC2, so I had to properly ask myself whether I was a broadcaster or an academic. And I realised I was a broadcaster."

He took over in 1965 and built the new channel as an alternative to BBC1 by encouraging new forms of programmes across the range of television output. "So we televised floodlit football and snooker, which hadn't been done before. We did 26-part classic drama serials. We did science fiction. In comedy we had Peter Cook and Dudley Moore and The Likely Lads. Of course it is very much more difficult to do anything new today," he says. "But even so, the number of genres you now see on television is lamentably small. All those cooking and property programmes are a bit depressing."

Another promotion followed, to the BBC's director of programmes, which, Attenborough says, "meant doing much more dogsbody work. I had to fire people and work on budgets. I was even responsible for the introduction of computers, which my children fall over laughing about as even now I don't use email." He plays down the suggestion that he turned down the job of director general. "My name was mentioned because I was a senior guy who'd been around for a while, but I wouldn't have been any good at it and I wouldn't have enjoyed it. And I'd got to the point where I thought, why on earth would I do something that I didn't enjoy. So it was never me saying take away your gilded chariots."

So he returned to programme making and began to build on his BBC2 legacy. His ambition to reinvent documentary television coincided with the arrival of colour, and the upshot was Kenneth Clark's mammoth art history series, Civilisation. It was followed by other "tombstone" projects, as they became known, such as a history of science presented by Jacob Bronowski, The Ascent of Man, and Alistair Cooke's America. "So why not natural history?"

Life on Earth took three years to complete and aired in 1979. Its combination of rare animals and state-of-the-art photography ensured it was an immediate success, and its follow-up series have built a globally successful franchise. They have even cracked America. "When we first went there, their nature programmes were all whiz-bang stuff such as catching elephants or lassoing rhinoceroses. They thought the idea that you could do a programme on plants or caterpillars was absurd. But people seemed to like it."

Its reception in America was also an early indicator that Attenborough's subject matter was becoming more contentious. The series became embroiled in rows with the evolution and creationist lobbies. Television, he says, does not deal easily with divisive opinions. "The population issue has barely been covered at all, partly because there are large groups of people who might find it offensive. And television, by and large, is not a crusading medium. The BBC doesn't like to take sides, and commercial television doesn't want to antagonise large chunks of its audience, because that doesn't encourage advertisers."

He says climate change provides an interesting example of this predicament. "When do you move from something being debated to being implacable fact? You first have to decide whether it is beyond the various extremes that have afflicted the world for the past thousand years. The next debate is what is responsible. It's a very fine judgment. And the BBC has a responsibility not to be too far ahead or too far behind general opinion."

So is he comfortable putting his head above the parapet on population? "No, but I think you do have an obligation to speak the truth as you see it. And while people say, with good cause, that it's all doom and gloom, I also think we've come a long way. It's not so long ago that the idea of having a minister for the environment would have been absurd. Now no party could possibly be elected without a policy on the environment. I think people are increasingly realising that if we get out of kilter with the natural world, we are heading for catastrophe. And the associated emotional, spiritual and physical loss is the road to madness. The natural world is still a source of solace and pleasure and delight and beauty and reassurance."

After his north pole trip, Attenborough will travel to the south pole next spring and is also writing the script for a new BBC television series called Life – "not an entirely original title" – which will be screened next month. "Yet again they have come up with some amazing material. I've just been looking at film of weedy sea dragons. They are like elongated seahorses, and their fins have turned into what looks like fantastically coloured seaweed, so every part is fringed with fronds and tassels. The film is of their courtship, which takes place off the Australian coast at dusk, so very few people have seen it before. It is just the most remarkable thing. You could cry at how beautiful it is. It certainly doesn't need words from me. You just sit and gape at this wonder.

"No matter how long I have been involved with such things, it is simply impossible to become blasé and not be moved by something like this indescribably beautiful and wonderfully surreal creature."