There are two theories on that, as it happens, two quite convincing theories; either one of which would be fine if only the other one didn't exist. The first one is actually due to Geoff Parker and myself, who did a game theory analysis of... we started with an isogamous population, a population of organisms producing equal sized gametes, and asked, 'Is this evolutionarily, and could it be invaded by individuals, either producing much bigger gametes or much smaller ones?' And we showed that there were circumstances in which it could so be invaded. But interestingly, the invasion would be, you'd have small motile gametes, and then the invader would be the one producing great big gamete, which shows that in spite of what it says, you know, that males are actually the first sex, which is nice, we were there first and women only came along afterwards. That's one view, and I think it's... there's even some supporting evidence. Looking at... well particularly there's a group of algae called volvox, in which the theory predicts exactly which one should be sexual and which one shouldn't and it works out very nicely. Not sexual but dimorphic.

[Q] How could the first one have been the male, because it doesn't make the economic contribution?

Well, perhaps I'm stretching a point by claiming they were males, but they were motile, they had little, you know, they had flagella to drive the cells along.

[Q] But also tiny.

And tiny, but not so tiny that they didn't have enough cytoplasm there to make themselves work. The essence of being a female is to produce a large non-motile gamete, and all gametes were small and motile initially, I think. But there is an alternative view, which I quite like, which is due to Lawrence Hurst and Bill Hamilton, I think, and others, which is that it really has to do with the inheritance of these organelles - chloroplasts and mitochondria. And it turns out that almost universally, organisms transmit their organelles, their... their mitochondria, let's say, only from one of the two parents. Of course, even if you're isogamous - it sounds crazy this - but an organism which is isogamous nevertheless has what's called plus and minus mating types. You can't tell them apart except chemically, by looking at them you can't tell, but plusses will only fuse with minuses. And this is, obviously, you'd say, well, that's because otherwise, no sooner has an organism produced gametes than they'd fuse with one another and you'd be back where you started. And I think that's right, it prevents selfing. But it then turns out that even in organisms with isogametes, the organelles are inherited from only one side. In chlamydomones, for example, which is a little alga, the mitochondria [are] inherited from one mating type and the chloroplasts from the other. And the argument is that the reason that's important is that it doesn't offer the opportunity for selfish behaviour on the part of mitochondria, the mitochondria don't fight one another, so to speak, and the chloroplasts don't fight one another. And I think that's essentially correct as an explanation. And some people have wanted to suggest that this is the origin of unequal gametes. I mean, the gametes are already unequal in a sense, they either contain chloroplasts or they don't. And the argument has been that you evolve large and small gametes as a way of preventing other kinds of selfish elements, parasites and so on, spreading. I'm not convinced that the argument works, but better men than I am think it does, so we'll see. I like the one that I invented myself, one always does.

[Q] Yes, I confused that with Parker, Baker and Smith, which is similar.

It's the same idea. No, I'm perhaps making unfair claims for myself here. Parker, Baker and Smith, came up with the idea, and it's essentially the same idea. They showed it to work in a particular case, by computer simulation. I, in my sex book, showed that the principle is quite general, and applied game theory to it to show that the conditions under which it would work, so I... in a sense, I did the analytical proof that it works, but they had... the basic idea was theirs.