My father used to say that a chicken was merely an egg's way of making another egg. When I was young, this statement annoyed me. Now I realize that he was many years ahead of his time.

Of course, the modern version is dressed up in a little more scientific language. Richard Dawkins, who coined the phrase "the selfish gene" in a book with that title, gave a very clear statement of the main idea: any organism, everything from a walrus to a wisteria (and including human beings) is no more than a mechanism designed to make sure that its own genetic material will survive in the future.

Individuals die, but their DNA (for some viruses it's RNA) continues, virtually immortal and changing only very slowly over time. Since the DNA that defines even the largest plant or animal fits easily on the head of a pin, in this view of the world we are nothing but enormous, lumbering monsters designed to carry, safeguard, and assure the copying of something too small to be seen with the naked eye.

This is an attractive and powerful notion, able to explain many elements of nature. It runs into a problem, however, when we ask the question, "Why is there sex?" Your DNA is high-quality stuff, developed and fine-tuned over four billion years of life on Earth. It is you, the essence of you, the only way for you to continue an existence in the future. There is no way that you should be willing to risk its integrity.

So what do you do? You mate with a genetic stranger. At that point your unique and wonderful DNA becomes mixed, fifty-fifty, with other DNA about which you know very little. Even if you have known your partner all your life, it is still true to say that the two of you are strangers at the DNA level. Indeed, the best bet from the point of view of your genes would be to mate with a close relative, where you share a high proportion of common genetic material.

This is not, of course, what happens. Incest is taboo in human societies, and mating outside the immediate family seems generally preferred everywhere in the plant and animal kingdom.

What is going on? Why, taking a gene's-eye view of things, is sexual reproduction such a big hit? Why do all the most complex life forms on Earth employ, all or part of the time, sex as a tool for propagation?

Biologists today offer a variety of answers to those questions. The most popular one is that sexual reproduction, mixing and matching genetic material half-and-half from a mother and a father, permits numerous variations of form; the offspring thus can adapt more rapidly to a changing environment. A non-sexual organism, however - and there are many of them - is stuck with genetic material that can change only through accidents of DNA copying or DNA swapping, and these are rare and random events taking thousands or millions of years to produce significant variations. If the world changes quickly, the non-sexual reproducer will be in trouble.

There are other theories for the popularity of sex in life on Earth. One is that sex was developed as a way of protecting organisms against disease. We are invaded all the time by a variety of tiny critters, which see us as a plentiful food supply. We have built up protections against them, but they in turn have become cunning at penetrating our defenses. When an organism employs non-sexual reproduction, and some other parasitic organism finds a way around the defenses, the game is over. The same trick will penetrate the defenses of future generations with identical genetic make-up. Sexual reproduction, however, scrambles the genes, and makes the offspring less susceptible to invasion. Thus, sex provides a partial fresh start with each generation.

Which theory is right? I don't know. Maybe both are, since they are not inconsistent with each other. Sexual reproduction allows organisms to adapt more rapidly to new environmental niches, and also serves as a defense against disease.

In any case, sex seems like a good idea - at least to most of us, most of the time.

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