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In Praise of Parasites

In 1933 primatologists discovered a red-and-black colored monkey living in the canopy of West Africa's forests, which they named Miss Waldron's Red Colobus. Recently it was pronounced extinct - the first documented extinction of a primate since the 1700s. After searching for six years, a team of scientists have not been able to find any sign of the monkey, leading them to conclude that heavy logging in the region killed off the last member of the species. But when Miss Waldron's Red Colobus vanished, it did not go alone. It also took with it into oblivion an unknown number of parasite species that made the monkey their home - parasites that might have included viruses, bacteria, protozoans, fungi, tapeworms, and other gruesome creatures.

We may be in the process of causing mass extinctions the likes of which the planet has not seen since an asteroid crashed into earth 65 million years ago. Surprisingly, it is parasites that will disappear in the greatest numbers, because parasites represent the vast majority of Earth's biodiversity. Although researchers are divided over how many species exist - estimates vary from 5 to 30 million - it appears that four out of every five species is a parasite of some sort.

It can be difficult to grasp the fact that we live in an overwhelmingly parasitic world. After all, we typically think of parasites with fear and loathing. They are things to be eradicated or at the very least not discussed in polite company. But parasites are nature's great success story. They have been around for billions of years, and have evolved into a bizarre panoply of forms - nematodes that can curl up in a single muscle cell, crustaceans that clamp onto the eyes of Greenland sharks, flatworms that live in the bladders of desert toads buried in the sand for 11 months out of every year.

Parasites have evolved into remarkably sophisticated creatures along the way. They can biochemically castrate their hosts, so that they don't waste energy making eggs or finding mates when they could be feeding the parasites. Parasites can even control the behavior of their hosts to ensure their own reproductive success. Many species of parasites, for example, need to live inside two or more species to complete their life cycle. Often the first host is prey for the second, and so the parasites helps the predators catch their prey. Toxoplasma, a protozoan that starts out in rats and other mammals, makes cats its final host. A rat infected with Toxoplasma is perfectly healthy, but it loses its instinctive fear when it smells a cat. By altering the rat's neurochemistry, Toxoplasma may make its hosts easier targets.

Parasites may not win anyone's love, but they should win our respect. So when we take strides to preserve biodiversity, we should not forget the parasites that live inside endangered hosts, like the ones that lived in Miss Waldron's Red Colobus. For there are practical reasons to preserve biodiversity, and they apply to parasites just as much as to their hosts. Many of the most effective drugs, for example, were originally discovered in plants or animals.

Parasites are masters of biotechnology. Consider hookworms. These nasty creatures dig their fangs into the linings of the intestines in order to drink blood and lacerated flesh. Normally, clots would form in the wound, making it impossible for the hookworm to feed. But the hookworm has evolved the ability to produce a molecule that elegantly jams up the cascade of chemical reactions that create clots. Biotech researchers are so impressed by the hookworm that they've synthesized the molecule and are testing it as a blood-thinning medicine that can be used during surgery.

The hookworm is but one species out of millions of parasites; other parasites may produce chemicals that can be used to stop organ rejection or to alter the brain. If they disappear into extinction, they will take their secrets with them.

The contempt that many people have for parasites hides a profound uneasiness about our own role in nature. We pride ourselves as the masters of nature, with the beasts of land and sea at our disposal. Yet parasites can sneak past our defenses and turn our bodies into their own playgrounds. The fact that we will always be part of the natural world is, in some ways horrifying. That horror is what makes parasites such a great device for science fiction movies such as Alien.

But parasites make us uneasy in another way. When you look at an aerial photograph of housing developments spreading into prairies or logging operations obliterating rain forests, it's hard to avoid entertaining a frightening possibility: we are parasites. Our host is the biosphere, which we exploit and consume for our own advantage, and to our host's detriment.

There may be something to this metaphor, but the lesson I take from it is not what others might. To be called a parasite is not necessarily a bad thing. Parasites have been incredibly successful over the past four billion years of life's history. If we are in fact parasites, we are not very good ones. Parasites make very careful use of their hosts, because killing them off too quickly will leave them without a home. Unlike other parasites, we have only one host, which means that we must be especially careful. Judging from the state of the world's rain forests, wetlands, and coral reefs, we're not. We should learn from the masters.