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.

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