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the progression of AIDS. This is partly because AIDS is a disease of the immune system and immune surveillance of these diseases is therefore lifted, but it also makes evolutionary sense. If your host is going to die, you had better multiply as fast as possible. So-called opportunist infections therefore hit you when you are down. Incidentally, one scientist has suggested that the cross-reactivity of the immune system (infection with one strain causes immune resistance to another strain of the same species of parasite) might be the parasite’s way of slamming the door on rival members of its species once it is inside.25

If it pays a parasite to go for broke when a rival appears, then it pays a host to prevent cross-infection with two strains of parasite. And nowhere is the risk of cross-infection greater than during sex. A sperm fusing with an egg risks bringing its cargo of bacteria and viruses as well; their arrival would awaken the egg’s own parasites and cause a battle for possession that would leave the egg sick or dead. To avoid this, therefore, the sperm tries to avoid bringing into the egg material that might harbour bacteria or viruses. It passes just the nucleus into the egg. Safe sex indeed.

Proof of this theory will be hard to come by, but suggestive support comes from Paramecium, a protozoan that mates by conjugation: passing spare nuclei through a narrow tube. The procedure is hygienic, in the sense that only the nuclei travel through the tube. Two paramecia stay linked for only two minutes or so; any longer and cytoplasm would also pass through the tube. The tube is too narrow even for the nucleus, which only just squeezes through. And it may be no accident that Paramecium and its relatives are the only creatures that possess such tiny nuclei, which are used as stores of genes (‘coding vaults’ they have been called) and from which larger, working copies are made for everyday use.26

Decision Time

Gender, then, was invented as a way to resolve the conflict between the cytoplasmic genes of the two parents. Rather than let such conflict destroy the offspring, a sensible agreement was reached: all the cytoplasmic genes would come from the mother, none from the father. Since this made the father’s gametes smaller, they could specialize in being more numerous and mobile, the better to find eggs. Gender is a bureaucratic solution to an anti-social habit.

This explains why there are two genders, one with small gametes, the other with large ones. But it does not explain why every creature cannot have both genders on board. Why are people not hermaphrodites? Were I a plant, the question might not arise: most plants are hermaphrodites. There is a general pattern for mobile creatures to be dioecious (with separate genders) and sessile creatures like plants and barnacles to be hermaphroditic. This makes a sort of ecological sense. Given that pollen is lighter than seed, a flower that produces only seed can have only local offspring. One that also produces pollen can father plants that spread far and wide. A law of diminishing returns applies to seed but not to pollen.

But it does not explain why animals took a different route. The answer lies in those muttering organelles left behind at the gate when the sperm entered the egg. In a male any gene in an organelle is in a cul-de-sac, because it will be left behind in the sperm. All of the organelles in your body and all of the genes in them came from your mother; none came from your father. This is bad news for the genes, whose life’s work, remember, is to pass into the next generation. Every man is a dead end for organelle genes. Not surprisingly, there is a ‘temptation’ for such genes to invent solutions to their difficulty (i.e., those that do solve the problem spread at the expense of those that do not). The most attractive solution for an organelle gene in a hermaphrodite is to divert all of the owner’s resources into female reproduction and away from male.

This is not pure fantasy. Hermaphrodites are in a state of constant battle against rebellious organelle