Desmond Morris is keen on the concept of playfulness, and thinks it tell us a lot about humanity's evolution. Our playfulness fuels our ingenuity and inventiveness, allowed us to defeat physically more powerful species to become global top dogs, and explains how we ended up wearing clothes, playing computer games, dropping "smart" bombs and watching Strictly Come Dancing. We play, therefore we are.

You can't help thinking about Morris's playfulness theory when you meet him: a large, beaming, soon-to-be-80-year-old surrounded by books (many of which were written by him) and surrealist paintings (all of which were painted by him) in a converted coach house at the back of his rambling home in Oxford - the very house, he tells me at least twice, in which 19th-century lexicographer James Murray laboured for more than 30 years on the Oxford English Dictionary (he died having got to T). We talk, meanderingly, for three hours; I'm knackered by the end, but Morris remains arm-wavingly irrepressible, and only stops because his wife is ill and rings from the house for a cuppa (we also seem to be ending at tea). Otherwise, I'd probably still be there.

Morris has just produced The Naked Man, a companion book to The Naked Woman, which appeared in 2004, and the latest in an evolutionary line stretching back to his career-defining, life-changing, collective-mentality-altering The Naked Ape, an instant bestseller when it was published in 1967 and now heading for sales of 20m. He has made a career out of telling us that 10,000 years of "civilisation" cannot offset several million years of hunter-gathering, and that we now live a life that is, in many ways, at odds with our genetic inheritance.

Morris's theories do not endear him to feminists. The phrase he uses most often to describe men is "risk-takers". He says that for a million years or more men had to go out and face down woolly mammoths and sabre-toothed tigers, develop strategies to overcome them (cooperation to fight common foes, he emphasises, is genetically more important to man than the competitive urge), and "bring home the bacon - literally", while women cleaned the cave, raised the serial litter that is unique to the human species, and organised every other aspect of life. His view, anathema to many fellow scientists as well as to feminists, is that this genetic separation can't be wished away by equality legislation.

"There was considerable specialisation for the male and the female," he says. "Men became more athletic and did the hunting; women did everything else. They were multitasking, to use a phrase that has become very popular, and were at the centre of society. It's not a question of superiority and inferiority. It's a question of differences, and these differences are very real." Men were expendable; women, because of their reproductive function, weren't. "You couldn't afford to lose a woman," he says, "They were too valuable, so the males became the specialist hunters."

For the whole of prehistory, this separation of roles worked well, Morris says - a perfect union designed to rear young and perpetuate the species. The problem came when we started to expand from our natural tribal groups of around 100 and develop big settlements. "For a million years, women were at the centre of society and men were on the periphery," says Morris. "I don't see why women get so upset about this: women ran society, men were in the hunting ground. The sad thing for women is that, over a period of time, the hunting grounds became the city centres, and so instead of being on the periphery, men were now in the centre of the cities running things. The cities were the hunting grounds, although now the hunting was metaphorical. Urbanisation favoured the male."

Feminists could take this two ways. The upside is that Morris thinks women should run more or less everything. "We'd be much better off if women ran most of the organisations," he says. "If women ran the political world rather than men, for instance. I don't think men are suited to politics. Women are much more suited because they are genetically more cautious and are not going to make stupid mistakes." The downside, feminism-wise, is that Morris thinks that men, because of their natural risk-taking, will always be better inventors and artists.

"For every one great woman artist, there are 100 men," says Morris. "There are more male geniuses than female geniuses, and there are more male idiots than female idiots. If you're a human female, you can't afford to be a risk-taker and you can't afford to be a dimwit. You have to be in between those two extremes." Man's artistic dominance, he argues, cannot be explained by opportunity or social conditioning. Over the whole of evolution, women have produced more art than men - in the form of decorated pottery and clothing - but they have tended to produce traditional art. It is men who have flouted the rules and produced most of the great, mould-breaking art.

Morris also has a theory about car crashes. "Women have more accidents but men have bigger accidents," he says. "If you're going to have a big car crash, it's always a man; if you're going to get a dented fender, it's a woman. You hardly ever hear of a high-speed crash involving a woman; it's always a man." He expresses his theory forcefully, vividly; you sense, anecdotally, that there may be something in it; but you would also like to see the insurance stats to prove it. Morris paints with a very broad brush.

The other area where Morris's new book has already proved controversial is his take on homosexuality. This is dealt with in the final chapter of the book and feels tacked on, as if the publisher insisted the subject be covered. Morris has borrowed a theory from his friend and fellow zoologist Clive Bromhall, who in his book The Eternal Child argued that a key feature of our evolutionary history has been the extension of childhood and the postponement of breeding. The social effect of that process has been that for 10 years, between the ages of about four and 14, boys and girls have tended to play together in separate groups. At puberty, the sexes - driven by the need to reproduce - come together, but for largely social reasons (these are left irritatingly vague) a small proportion of men and women continue to prefer their own sex.

Homosexuality is "genetically instigated but then environmentally influenced", he says. Gay men, he argues, are at the extreme end of the "playfulness" spectrum, and likely to be more creative than the average. One of the key features of humans, according to his overall theory, is that unlike other species we never lose the curiosity and inventiveness of youth. Homosexual people, he says, exhibit that "Peter Pan syndrome" to an extreme degree, giving them above-average intelligence, inventiveness and creativity. The theory, to my untutored ears, sounds tenuous, and for once Morris doesn't express it very fluently - he seems embarrassed to be relying on someone else's thinking.

Morris, a genetically useful iconoclast and provocateur, prides himself on simplifying science for the general reader without distorting, but I wonder about the way these large hypotheses on homosexuality are skipped over in just seven pages of what is effectively a coda to the book.

The Naked Man exemplifies the tensions - and the pleasures - in Morris's richly varied career. The book is a long way from The Reproductive Behaviour of the Ten-Spined Stickleback, for which he was awarded his doctorate at Oxford in 1954. He could have had a distinguished career as what might be called an institutionalised zoologist - and for much of his time he has had exactly that - but in the late 1950s he fell among that dangerous tribe of television people, joining Granada to make the programme Zoo Time, which taught him how to popularise without dumbing down too much, made him a name and led to him writing The Naked Ape, which treated humans as animals and sought to explain all human behaviour in zoological terms. Man was not a moral or a spiritual being; he was an animal, an ape with pretensions to be something greater.

"I spent the first half of my life studying animal behaviour," says Morris, "and when I eventually wrote a book about human beings, I wrote about them as if I was writing about another animal species. I had met Tom Maschler, then a young publisher at Cape, at a party and told him I was going to write a zoology of human beings one day, and not even use the term human beings. Instead I'd write it as if I was an alien who had come to this planet and seen this extraordinary ape which doesn't have any fur on its body. Tom loved the idea, but it took him three years to persuade me to write it, and when I eventually set to work I had to complete it in four weeks because I was so busy. I just sat down and wrote. It was written with such intensity that I can remember it to this day."

It was a publishing sensation that caught a moment when everything was being rethought: while attractive to radicals, who wanted to strip away layers of political and religious authority and liked the notion of unfettered man, it infuriated feminists, who accused it of promoting a male-centred view of human evolution. It became a cause celebre - Maschler's instinct had been spot on - and made zoology part of the late-60s zeitgeist.

The irony is that Morris was an accidental zoologist. His first love was art - he intended to be a surrealist painter - and he only applied to study zoology at Birmingham because he had a low opinion of art schools and felt a study of biomorphic shapes would be useful to his painting. He exhibited in London with Joan Miro in 1950, but that was surrealism's last throw in the UK and he realised he would never make a living. He had, in any case, been bitten by the zoology bug at Birmingham and couldn't resist the lure of the ten-spined stickleback.

Morris never stopped painting, though he didn't exhibit between 1952 and 1974, and his coach house is neatly divided into halves that sum up the two compartments of his life. "I have a library with all my scientific books and a room with my art books and my paintings," he says. "It's like two hemispheres of my brain." Who else, you wonder, could in the mid-1960s have gone from being head of mammals at London Zoo - the job he had been given after the success of Zoo Time - to director of the Institute of Contemporary Arts? "From rhinos to Picassos," as he sums up a wondrously odd move.

He didn't stay at the ICA long. The runaway success of The Naked Ape allowed him to run away - he says he knew it was a sensation when he heard Burt Lancaster had bought the film rights. He quit his job and, with his wife Ramona, went to live in Malta, where he started painting again in earnest, lived a life of sunny, self-conscious hedonism, and produced a son, Jason.

Morris was almost 40 when he left for Malta - burned out, he says, after 15 years of frenetic activity. "A few months before I wrote The Naked Ape my health collapsed. When I recovered I said, 'Right, that's it,' and I cut out a lot of things. I realised that I had to cut down. I was trying to live like a machine when in fact I was just an animal. I focused all my efforts on one thing, which was The Naked Ape. I sat down and wrote it in the autumn of 1966. I hadn't any hopes for it; I didn't expect it to do anything. People say I must have known what I was doing. They think I deliberately wrote it to make a shocking bestseller, but it wasn't like that."

Intentional or not, that was what it became, and he was determined to enjoy the money and freedom it brought him. "I didn't intend to stay in Malta for very long, but it stretched for five or six years. I had a studio and painted. I wrote in the winter, painted in the summer. We had all our friends out to stay. My childlike curiosity, which I've maintained all my life, demands new experiences, and this was something I'd never experienced before. My mother was horrified. She said, 'Put the money in the bank,' as mothers do, but I said, 'No, I'm going to spend it all, and when it's all spent I'll come back to work, because I like work.' And that's what I did. I spent most of it, and then came back and started research again here at Oxford."

In 1973 he returned to the UK with his family to take up a research fellowship at Wolfson College, seeking to combine hard science with populist books. The Human Zoo and Intimate Behaviour, both written in Malta, were followed by Manwatching (which developed the concept of "body language"), The Soccer Tribe, Catwatching, Dogwatching and Babywatching. He resumed his TV career and travelled widely - a map of the world in the corridor that connects his art-world with his science-world is covered with pins marking the places he has visited.

He has clearly had a ball, adores his wife of 55 years, remains fascinated by art and film, and has even come to quite like his fellow humans after early doubts. An intense only child, he had spent much of his adolescence watching his father die of wounds received in the first world war and, as a teenager confronted with the loss of his father and the start of a second global conflict, he took a dim view of his fellow naked apes, finding solace in art and the natural world.

Regrets? "I should have stuck at the art," he says. "I think I had the potential to be a good artist. I still consider myself a serious artist but a very minor one, and I'm a minor artist because I've been doing too many other things. My strength is that I do lots of different things; my weakness is that I don't stick at any of them long enough."

Does he envy David Attenborough, who 50 years ago used to present Zoo Quest on the BBC while Morris did Zoo Time on ITV? His TV rival - and good friend - carried on making programmes and remains a vital presence on television, lauded for his commitment to quality broadcasting.

"I'm not an envious person," says Morris. "It's one of my few good qualities. David has stuck to his guns, gone right the way through the whole animal kingdom, and made the best programmes about animals ever. They will be a lasting monument to him. David has his huge collection of natural history programmes which together make a brilliant study. I would have liked to have been somebody who had done a major study, but on the other hand I've had the pleasure of being like a child all my life and having new toys all the time." The perfect image of the playful, risk- taking male. Happiness may lie in having theories that match one's own personality.

· The Naked Man is published by Jonathan Cape (£17.99) on January 3. To order a copy for £16.99 with free UK p&p go to theguardian.com/bookshop or call 0870 836 0875.