Every child born on this earth starts by being interested in the natural world. You have only got to turn over a stone and see a worm or earwig underneath and the child is fascinated.

I grew up in Leicestershire, and the north-eastern part is full of rocks full of fossils. Every time you put a rock under a hammer there was a chance you'd see something new, and yours were the first human eyes ever to see that. The notion that there's a sea creature in the rocks beneath the ground on which you walk which was alive 100m years ago I find romantic and amazing.

One of the first series we did was birds, because everybody's interested in birds. That worked, so we did another one. Now we have come to the last, which is amphibians and reptiles. It wasn't until five years ago that I thought: 'It's possible we could do a series about every major group that lives on land.' And so it's turned out.

My children are 55 and 51. Of course there were episodes in their lives I missed which I'm sorry about, but you can't have your cake and eat it.

When you make an animal series, you conceal yourself to get the behaviour as if you were not there. After Big Brother, eavesdropping on people has become more acceptable; but if you tell them then they'll behave in a different way, and their behaviour loses a lot of its validity.

Life is not all high emotion. Some of the most interesting things are when it's not highly emotional: little details of relationships and body language.

Every society that's ever existed has felt it necessary to have creation myths. Why should I believe one? People write to me and say: 'You show us birds and orchids and wonderful, beautiful things - don't you feel you should give credit to He who created those things?' My reply says: what about a parasitic worm that's boring through the eye of a four-year-old child on the bank of an African river? It confuses me that I should believe in a god who cares individually for each and every one of us and could allow that to happen.

The cameraman has all those qualities people have generously ascribed to me: patience, endurance... I have very little of those.

If you're sitting next to an 18-stone gorilla, you don't shout - you're respectful. I'm caricatured as speaking in this breathy way, but most of the words I've spoken on TV aren't like that. I shouldn't object: to have a characteristic people know as yours is a compliment.

If it wasn't for eco-tourism, I'm quite sure there would be no mountain gorillas left. People in and around the forest have a better standard of living than if they were just growing maize.

At some stage we're going to have to control the size of the population. Human beings are demanding more and more of the natural world for their own particular purposes, and the world is finite. If we don't fix it, then the natural world will fix it for us in a devastating way. It's already doing it with famines. Human beings will cut their numbers without the need for draconian measures - the evidence, happily, is that if you provide female education and the technical facilities, that's the way people choose.

My next project is about Charles Darwin. He says in a letter to Emma, his wife, something like: 'I sat down on a bench and saw a bird singing in the trees and saw a wide mass of life going on around me, and I thought I didn't care what the process was that brought this into place because it's so wonderful.' If I lost that feeling, I'd go and do something else.

·Life in Cold Blood starts on BBC1 on 4 February at 9pm