Wow, what a book!
This is a non-fic that can change your world picture. When I started it, I expected more or less average run of the mill collection of stories about parasites. However, this is so much more!
The book starts with initial theories of parasites: people observed them thru the centuries but they posed a problem: never found (in their final form) outside the body, are they an integral part of an organism or invaders? Why there is nothing in Genesis on them? After Darwin’s evolution theory took hold, parasites fared not much better: To Darwin, life was not a ladder rising up to the angels but a tree, bursting upward with all the diversity of the species on Earth alive today and long past, all rooted in a common ancestry. And parasites, instead of develop, actually lose developed functions. One influential voice for these ideas belonged to the British zoologist Ray Lankester, who wrote “Degeneration: A Chapter in Darwinism.”, where virulently attacked species that not only stopped rising but actually surrendered some of their accomplishments. They degenerated, their bodies simplifying as they accommodated themselves to an easier life. They were the sine qua non of degenerates, whether they were animals or single-celled protozoa that had given up a free life. This critic quickly spilled over to social activism, were everyone, from communists to Nazis started accusing their opponents of parasitism, a practice present even today.
However, parasites aren’t that simple. They manage to fool immune systems better than the best of our drugs. Blood-drinkers often keep their point of contact clean and inject anti-coagulants, but don’t let the whole body bleed. For each species there are estimated four species that parasitize on it, all extremely narrow specialists.
Many books on parasites stress the “Ew” factor, but not this one. It shows that there is a whole world often disregarded by ‘classical’ biologist. Say, usually wolf-moose relation is usually viewed as predators keeping a herd of prey healthy by weeding out the slowest ones. In truth, wolves are the final hosts for one of the smallest tapeworms in the world, Echinococcus granulosus. It doesn’t cause its final host much harm, but its eggs can be vastly vicious. They are eaten by herbivores such as moose, where they slowly transform themselves into cysts. One of the tapeworm’s favorite sites for forming its cyst is the lungs. As a result, when wolves sweep down on a herd of moose, they’re more likely to pick out the slow, wheezing one and kill it. The result is that the tapeworm brings the wolf to the moose so that it can get into the wolf. The thinning of the herd is an illusion, not the service of the predator but the side effect of a tapeworm traveling through its life.
Parasites are paramount part of the environment. To disregard them, assuming healthy elements in modelling the eco-system brings wrong results.
Also, there is a common sense that given time parasites might evolve into a new species better adapted to a new host. If they did, they would be amply rewarded by natural selection for any mutation that caused less harm to their new host. After all, if their host died off, the parasites would die with it. In truth, familarity could breed contempt. Natural selection favors genes that can get themselves replicated more often than others. Obviously, a gene that makes a parasite instantly fatal to its host won’t go very far in this world. Yet, a parasite that is too well mannered won’t have any more success. Because it takes almost nothing from its host, it won’t have enough energy to reproduce itself and will come to the same evolutionary dead end. The harshness with which a parasite treats its host—what biologists call virulence—contains a trade-off.
A great eye opener!