About eight months ago I wrote in this column about the puzzling question of sex, and why it is such a big hit in the plant and animal world. The mystery, in brief, begins with the observed fact that most organisms (including humans) go to great lengths to make sure that their own genetic material will survive into the future. Individuals die, but their DNA lives on, virtually immortal and changing only very slowly over time.

However, seen from this point of view sex is a very chancy business. DNA is high-quality material, developed and fine-tuned for survival over four billion years of life on Earth. It is the very essence of an individual, the only way to continue some kind of a presence in the future beyond the individual's own death. Why, then, does any organism choose to reproduce by mating, which dilutes that unique and wonderful DNA with the DNA of some other individual?

One plausible explanation, which we might call the "Quick-change" answer, argues that mixing and matching genetic material from a mother and a father permits almost infinite variations in the offspring. Some of those offspring will be positioned to take advantage of rapid changes of environment, for the better or for the worse. Those offspring will flourish. A non-sexual organism, by contrast, is stuck with genetic material that changes only through accidents of DNA copying. These are rare and random events that take thousands or millions of years to produce significant variations. If the world changes quickly, the non- sexual reproducer will be in trouble.

Another explanation, and the one which I want to explore here, argues that sex is forced on organisms by the existence of other organisms; specifically, by the existence of parasites.

Parasites are far more common in humans than most of us would prefer to think. They can be relatively large animals, such as tapeworms and hookworms. The biggest tapeworm found in a person was twenty meters long, while it's estimated that one-fifth of all human beings have hookworms inside them. (You don't? How can you be sure?) Or the parasite can be very tiny like the single-celled organism Plasmodium, small enough to live inside red blood cells and cause malaria. A person dies of malaria somewhere in the world every twelve seconds. Far more people than that are weakened by the disease and lead diminished lives.

The basic idea linking sex and parasites is simple: the mixing of genetic material makes individuals diverse from generation to generation, so it is harder for a parasite to home in on its continuously varying target. The problem, as in many situations in biology, is testing the idea. We need to be able to run a control experiment. How well does a parasite do in invading individuals who reproduce without sex, versus the parasite's performance in cases involving sexual reproduction?

This sounds like a difficulty, since we reproduce solely via sex. However, numerous species propagate both sexually and asexually. The most familiar examples are probably plants that can easily be grown from seeds (sexual), by bulb division (asexual), or in principle from cuttings (asexual, but mine always die). However, we would prefer some organism closer to humans. This is more difficult. The original work that explored the interaction of parasites and sexual reproduction was done with snails, which are far from humanoid.

The particular snail was a small one with the formidable name of Potamopyrgus antipodarum, which I will loosely translate as "The tower-shaped river thing from Down Under." It's not a bad name, since this snail lives in lakes and streams in New Zealand. There is a particular kind of worm (more accurately, a fluke) that lives and multiplies inside the snail. Those parasites, it was found, could more easily establish themselves in snails that lived in lakes than those that lived in streams.

So now for the crucial test, which was conducted by an American named Curtis Lively. Do snails in habitats where the parasites are more common reproduce more by sexual reproduction than snails in places where the parasites are found in smaller numbers? If so, then there would seem to be an evolutionary pressure for sexual reproduction of the snails, caused by the presence of their parasites.

To end the suspense you must be feeling, the answer is yes. Sexual reproduction was preferred in situations where there were more parasites. Sex, in some sense, confers a protection against parasitic invasion.

If you happen to be a snail, that is. The case for humans is less clear. On the other hand, we don't have a choice. If people want offspring, it's sex or nothing. Fortunately, we have other tools in our arsenal to rid us of or protect us from parasites. We have medications, which on the one hand may prevent infection (vaccines) or may cure us when we are already infected with diseases and parasites (antibiotics and worm-killers).

Before we become complacent, let's note that the parasites and diseases have not given up. They are fighting back. Fifty years ago, malaria (invasion by Plasmodium) could be treated very well using the drug chloroquine. Today, chloroquine-resistant variants of Plasmodium can be found almost everywhere. If you want to find the world's most-antibiotic resistant organisms, however, you should not go abroad. The place to find them is the hospitals of the United States. As for worms and head lice, don't even ask. Just visit your local elementary school.

Meanwhile, we still have more than one theory as to why sexual reproduction is so widespread. Is it the Quick-change, or is it Parasite-protection? Actually, it doesn't have to be either. It can be both. Earth's living organisms, with four billion years of experience to guide them, will automatically pick whatever combination works best.

In the future, if and when human cloning becomes an alternative to sexual reproduction, we will face similar choices. The difference is we will be obliged to make them consciously.

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