Free What a Wonderful World by Marcus Chown

selection, when it gets a good idea, runs with it. In the oceans, where life began, the sexes evolved coordinated behaviour, releasing eggs and sperms simultaneously into the water in order to maximise the chance of their union. Such a strategy was impossible, however, after animals moved onto the land. Instead, internal fertilisation became advantageous. Matched genitals evolved so that males could penetrate females and fertilise them. ‘Sexual intercourse began/In nineteen sixty-three/(which was rather late for me),’ wrote the poet Philip Larkin. 20 But, actually, it was rather earlier than that. Finally, to protect a developing embryo better, females evolved a womb, or uterus, in which an embryo could develop in relative safety.
    The road to modern sex has been a long one but at least the major milestones along that road appear clear. Nevertheless, sex very much remains that ‘riddle wrapped in a mystery inside an enigma’. And this is evident even when looking around at the human world today.
Other sex mysteries
    Take homosexuality, defined as sex between a same-sex couple. Since the only way for genes and the characteristics they encode to propagate down the generations is through sex between a male and female, genes that contribute to homosexuality should, by rights, become rapidly extinct. ‘We are machines built by DNA whose purpose is to make more copies of the same DNA,’ says Dawkins. ‘It is every living object’s sole reason for living.’ 21
    Yet the frequency of homosexuality is thought to be constant across cultures at about 3 per cent in men and 2 per cent in women. How can this be?
    One obvious possibility is that homosexuality has no genetic component – that there is no gene or genes that determine homosexuality. In fact, Dawkins’s basic ‘selfish gene’ idea has been increasingly tempered by the realisation that the environment plays a role in the expression of genes. According to the field of epigenetics, cells read DNA more like a script to be interpreted – depending on, for instance, environmental chemicals – than as a super-strict blueprint. ‘My mother made me a homosexual,’ goes the joke. To which someone replies, ‘If I give her the wool, will she make me one too?’
    Another possibility is that homosexuality has a genetic component that, though it is not beneficial in promoting the cause of selfish genes, comes along with a gene that is. This is not uncommon. For instance, there is a particular gene that gives people immunity to malaria. But if, instead of having one copy of the gene, a person has
two copies
– one from each parent – they get sickle cell anaemia, in which blood cells becomeflattened and block capillaries. Sickle cell anaemia – a cripplingly painful disease – persists because, in most people, the gene that causes it has a beneficial effect and boosts their chance of survival.
    Of course, homosexuality might persist because homosexuals
do
get their genes into the next generation. Although there is a tendency to pigeon-hole sexuality, in fact there is a whole spectrum, ranging from 100 per cent heterosexuality through bisexuality to 100 per cent homosexuality. ‘Sexuality is as wide as the sea,’ said English film-maker Derek Jarman. People may not be totally homosexual – or might be homosexual only at certain times in their lives. This would mean that homosexuals do sire enough children – at least to make sure their genes persist through the generations, and that homosexuality persists from generation to generation.
    But there is a possibly more plausible way that homosexuals could get their genes into the future. If they help in the rearing of children who are genetically related to them – perhaps the offspring of a brother or sister – they will actually be acting selfishly to ensure their genes propagate into the future. This is similar to the argument often employed to make sense of another great mystery of biology: altruism. Why do