Breaking the Spell: Religion as a Natural Phenomenon

I mentioned music in the previous chapter, and we will eventually turn to a more detailed examination of its possible evolutionary sources, but I want to warm up first on some easier things we love. What about alcohol? What about money? What about sex? Sex presents some of the most interesting and challenging problems in evolutionary theory, because, on the face of it, sexual reproduction is a bad bargain indeed. Forget—for the moment—about our human kind of sex (sexy sex), and consider the most basic varieties of sexual reproduction in the living world: the sexual reproduction of almost all multicellular life-forms, from insects and clams to apple trees, and even many single-celled organisms. The great evolutionary biologist François Jacob once quipped that the dream of every cell is to become two cells. Every time this fission happens, a complete copy of the cell’s genome is copied into the offspring. The parent clones itself, in other words; the resulting organism shares 100 percent of its genes. If you can make perfect genetic copies of yourself, why would you go to the expense of reproducing sexually, which involves not just finding a mate but, much more important, passing on only half of your genes to your offspring?⁴ This 50 percent reduction (from the gene’s point of view) is known as the cost of meiosis (the kind of fission that occurs in sex cells, to distinguish it from the cloning fission of mitosis). Something must pay for this cost, and it must pay on delivery, not at some future date, since evolution lacks foresight and cannot approve bargains on the speculative basis of eventual return at some distant time.

Sexual reproduction is thus a costly investment that has to pay for itself in the short run. The details of theory and experiment on this topic are fascinating (see, e.g., Maynard Smith, 1978; Ridley, 1993), but for our purposes a few highlights from the currently front-running theory are most instructive: sex (in vertebrates like us, at least) pays for itself by making our offspring relatively inscrutable to the parasites we endow them with from birth. Parasites have short lifespans compared with their hosts, and typically reproduce many times during their host’s lifetime. Mammals, for instance, are hosts to trillions of parasites. (Yes, right now, no matter how healthy and clean you are, there are trillions of parasites of thousands of different species inhabiting your gut, your blood, your skin, your hair, your mouth, and every other part of your body. They have been rapidly evolving to survive against the onslaught of your defenses since the day you were born.) Before a female can mature to reproductive age, her parasites evolve to fit her better than any glove. (Meanwhile, her immune system evolves to combat them, a standoff—if she is healthy—in an ongoing arms race.) If she gave birth to a clone, her parasites would leap to it and find themselves at home from the outset. They would be already optimized to their new surroundings. If instead she uses sexual reproduction to endow her offspring with a mixed set of genes (half from her mate), many of these genes—or, more directly, their products, in the offspring’s internal defenses—will be alien or cryptic to the ship-jumping parasites. Instead of home sweet home, the parasites will find themselves in terra incognita. This gives the offspring a big head start in the arms race.

Could such a bargain pay for itself? That is the question at the heart of current research in evolutionary biology, and if the positive answer holds up to further scrutiny, then we will have found the ancient but ongoing source, in evolution, of the huge system of activities and products that we normally think of when we think of sex: marriage rituals and taboos against adultery, clothing and hairstyles, breath fresheners and pornography and condoms and HIV and all the rest. To explain why each and every facet of this huge complex exists, we will have to resort to many different kinds and levels of theory, not all of it biological. But none of this would exist if we weren’t sexually reproducing creatures, and we need to understand the biological underpinnings first if we are to have a clear view of what is optional or mere historical accident, what is highly resistant to perturbation, what is exploitable. There are reasons why we love sex, and they are more complicated than you might think.