Obliging soul that he is, David Attenborough suggested that he come to the NS office for this interview. He arrives brandishing his pensioner's rail pass and extolling the virtues of Ken Livingstone, first architect of subsidised travel for the capital's elderly. "Does Ken get my vote for mayor? Oh yes, absolutely. These cards are the only thing it's worthwhile being over 65 for. They cost a lot of money, you know." The curious aspect of Attenborough's bargain odyssey from the Surrey fringes to London Victoria is its contrast with the itinerary for the making of his most recent television series, The Life of Birds.

His journey was 256,000 miles, the equivalent of a trip to the moon or ten laps of the world. Its starting point was the office of the then head of the BBC's Natural History Unit, where Attenborough initially, and modestly, tried to scotch the top man's enthusiasm for an ornithological marathon. "I told him: 'I don't know anything about birds. I do know the difference between a thrush and a blackbird, but I'm not one of your Bill Oddie fanatics.' " Such quibbles overruled, Attenborough embarked on his ten-part series. Naturally it was a great success. Inevitably the accompanying tome - winner of the 1998 Natural History Book Award - will stuff thousands of Christmas stockings.

"Award?" says Attenborough, vaguely. "Oh, that book." A great self-deprecator, he is loath to flaunt the merits of his own work, beyond acknowledging its value to a ravaged BBC. Several years ago, as president of the British Society for the Advancement of Science, he warned that the government's proposed broadcasting reforms would mean doom for the corporation. Does he now believe that John Birt has presided over a disaster? "It's easy and sort of fashionable for someone like me to say that. There is no question but that Birtism . . . has had some terrible results. On the other hand, the BBC had to change. Now it has to produce programmes no one else can do. Otherwise, forget the licence fee."

Attenborough, now 72, became controller of BBC2 (and later director of programming for the corporation) in the mid-1960s. Driven by Reithian high-mindedness laced with populist guile, he was offered a tabula rasa. "I asked what the policy should be, and they said: 'That's up to you, dear boy. You tell us.' " Attenborough introduced classic serials - Stendhal, Henry James - along with one-day cricket, Pot Black, floodlit rugby league and Match of the Day - a package designed as sport for the masses and now reinvented as fodder for Murdoch.

"Well, of course, that is misery really - the market economy. I think commercialisation has ruined rugby football, but who am I to tell Will Carling that he shouldn't be paid and that it was better in the old days, when the game was played by people with beer bellies and no teeth? It's just the way of the world."

Surely, in Murdoch's case, it is the way of Thatcher and now Blair, who have not been eager to rein him in? "That's right. And the changes at the BBC are all market forces and marketing. Gets up my nose a bit," he grumbles. "I do have a frisson of despair at the standards, except I know I was there at a golden time." Now, at the start of the digital era, he foresees a post-television age in which anything of interest will find another medium. "Maybe it'll be the Internet or CD-Rom. Technologically, we are in a state of flux that has corroded and dissolved the old things. But they will crystallise somewhere else."

Attenborough got out of management long ago, relinquishing the painful process of natural selection within broadcasting for a more conventional study of Darwinism. His first natural history series, Life on Earth - made more than two decades ago - is about to be reshown on a cable station. As he suggests, lions are not faddish. "They do what they do. If you film it well, it will last."

By contrast, Attenborough, once a director-general-in-waiting, is deemed to have changed enormously. Interviewers always ask him if he likes animals, as if he has mutated into a dotty old dear with a herd of pampered cats in the back kitchen. "Worse, they ask me if I love animals. I don't love earwigs. But earwigs are great. What are those pincers on the back for? How do they live and mate? Fascinating stuff, but I'm not in love with earwigs."

What Attenborough really loves is high standards; a cult of excellence instilled in him by his father, the self-educated son of a village shopkeeper in Nottinghamshire. Attenborough pere got a starred first in Anglo-Saxon at Cambridge, became a don at Emmanuel College, Cambridge, and then the head of University College, Leicester.

David, grammar-school educated and his middle son, was told that he did not deserve to take the natural science tripos at Cambridge unless he could prove his worth by winning an open scholarship. "It still brings tears to my eyes as I speak. I was digging on our allotment, planting potatoes. I can see father running down, limping on his bad leg, shouting 'You've got it.' "

Richard - the film director and the oldest son - was less promising material. "Father couldn't understand Dick. Dick was hopeless. He couldn't even pass his school certificate, because he was in the local theatre all the time. Father couldn't understand how a child of his could fail. It was a big family crisis, particularly since wanting to be a film actor was anathema to father. So he got on to his academic pals, found the most difficult drama scholarship in the country - the Leverhulme at Rada - and gave Dick £50 as a 17th birthday present to submit himself. The deal was that if he got it, fine; if not, then he could stop this nonsense and do some work. He got it, and father said: 'Now you will wish to learn about Hamlet. I shall teach you.' But there was no more controversy. From then on, Dick was going to be an actor."

The older Attenborough brothers have remained close friends and neighbours who see each other often and will meet for Christmas lunch. There is a third son, John, who has kept out of the public eye and became an executive with Alfa Romeo. "He was a very bright guy who took languages. Later on he ran a group of garages in the south of England. He is now retired." Attenborough sounds as if both occupations are wholly beyond his ken.

Although he now plans to rest for a while - "lie in the bath and look at the ceiling" - he has no plans to stop working. Despite his age, he retains the relatively youthful looks of a less dog-eared John Simpson and a constitution able to withstand, when he must, the rigours of the job. "My motto is that any bloody fool can be uncomfortable. If I'm filming birds of paradise, I know I'll be living in a tent with pouring rain and mud for three weeks. But I don't say: 'Whoopee, there's going to be mould on my boots and leeches in my underpants.' I'm never lonely. I love the people I work with."

Attenborough does command extraordinary loyalty. Jane, his wife of 47 years, who died recently after a brain haemorrhage, insisted - almost to the end of her life - on attending to all his business affairs. "She was seriously ill two years before she died and, when she went into hospital, the one thing she was worried about was the VAT. She kept asking me: 'How will people know where the books are? You're hopeless. You don't know anything.' " Now Susie, one of Attenborough's two children, has put her career on hold to look after her father's financial affairs.

Such devotion does not betoken any autocracy on his part. Attenborough, mild and pragmatic, abhors posturing. You will find him on no Gaia-style platform, bewailing the potential extermination of the rainforest. There is, in his view, a lot of it left. Nor is he moved by Jeremiahs pointing to the extinction of species. "Often they can't name one . . . To parrot cliches about conservation is dangerous - particularly for me, since I have a bogus authority. Because I appear on TV, people think I'm one of the great naturalists. That's rubbish. Really, I'm a journalist."

A television producer both to Anthony Eden during the Suez crisis and to Harold Wilson, Attenborough - though "a standard, boring left-wing liberal" - has eschewed any direct involvement with the Blair government, citing old BBC notions of impartiality. "But I think the government has made a good fist of it, really. Politics is the art of compromise, and they've compromised."

Certainly he has no special deference for politicians. He was recently spotted in a box at the Albert Hall, inspecting Peter Mandelson's nether regions and assuring him that he could find no demonic pointed tail. Mandelson, reportedly, was not amused. Nor does Attenborough extrapolate much from the animal world to the political. You would not hear him, for instance, advancing the Stephen Jay Gould line that natural selection is "essentially Adam Smith's economics read into nature".

One aspect of post-Darwinianism - as demonstrated by politicians and other public figures - does perplex him. "It's when you see people doing barmy things for what the popular press calls love - people ruining their careers for sex. It seems incomprehensible what people will do; all down to the selfish gene. The sex bit has got slightly detached from reproduction. Nonetheless, we are driven to reproduce ourselves. That is why the sexual urge is so dominant and powerful."

But most current political sex "scandals" involve gays, whose motivation is certainly not explicable by traditional selfish-gene theory. "No, the gay business is very curious. A facile generalisation may be that the size of population is coming to be a threat, and people don't want to reproduce. We've seen - in dense populations of the species - behaviour in which the sexual thrill or frisson has become one in which there is no possibility of reproduction. One obvious answer is that even where there is no need to increase the population, the need to have sexual congress or satisfaction of some kind is so powerful that it has to find an outlet somewhere."

He is, as his Life of Birds itinerary suggested, an intrepid traveller. We have moved from the physiology of the earwig to the link between post-Malthusian population theory and the News of the World splash, passing Clapham Common and the Ministry of Agriculture on the way. David Attenborough, a man of the broadest horizons, fishes out his pensioner's travel card and departs.