I think the origin of sex is unique, I think it happened just once. Sex meaning meiosis, the production of haploid gametes and then the fusion of them, I think that happened just once. And there... it happened somewhat a thousand million years ago, it's always difficult to be sure that one's got the right explanation, I mean, how do you test your ideas? But that's one area where I think there are difficulties. The other area... you see, sex can mean so many different things to a biologist. I've been using sex to mean, as I say, meiosis and the fusion of two equal gametes.

[Q] That had to happen once only, because it's too similar in...

Yes, yes, it's incredibly similar in animals and plants and fungi, the actual mechanisms are almost identical. But the whole world of bacteria go in for sex - I think we should talk about this later on, perhaps - but they don't do it the same way at all, they're sort of just swapping individual genes in a curious way, and that also requires an explanation for its origin and its maintenance and so on, and we really have almost no ideas about it. Well, I've got ideas about it, but the formal mathematics hasn't been done, we're not even sure what experiments we ought to do, it's much more sort of open, because people have only just started thinking about it.