Once considered a "degenerate" form of life, parasites are being seen as important indicators of how evolution has progressed over 4 billion years. Zimmer credits them with being the driving force for biological diversity. He substantiates this claim with a sweeping, evocative survey of what is known today about parasites. That, he regretfully concedes, is little enough. What is known is that many early conceptions about parasites needed to be thrown aside as more information about this highly adaptable and widely variable range of organisms emerges.
While we may recoil at the term "parasite", Zimmer identifies but one villain in this book. Ray Lankester, a devoted Edwardian-era evolutionist, postulated that parasites were a "regressive" form of organism. He thought they shed evolutionary advantages as they simplified their bodies through their life cycles. Lankester thus set the tone for generations - biologists avoided studying parasites as offering no additional information revealing evolution's processes. Zimmer explains that since parasites are predators, it was thought they ought to follow the patterns of other predators - stalking prey like lions, or following scent gradients like sharks.
Instead, as more about them came to light, it was revealed how adaptive parasites are. Some, in fact, have developed the talent of making "prey" come to them. One fluke invades a snail early in its career. In an intermediate, but distinctive form, it then moves to an ant. Residing in the ant's brain, at some point it directs the ant to climb a grass stalk. There it waits for the grass, along with the ant and itself, to be eaten by a cow. The fluke cruises through the cow's stomach before taking up residence in the liver as adults, yet another body form. When the eggs are produced, they return to the intestinal tract to be later deposited on the ground, awaited by the snails. Looking at each phase, residing in a different host, you would be inclined to see it as a separate species.
This note is but one of the endless chorus of parasite adaptations Zimmer relates in this excellent book. He joins the refrain of older scientists lamenting the lack of upcoming researchers needed in parasite studies. Unlike the animals we see around us, most parasites have astonishingly varied body forms as they go through the phases of their life cycles. For years, this catalog of body plans was thought to display different species. Only recently has it been demonstrated that these creatures changed shape and function dramatically as they changed living environments. Identifying each stage, the invader's function there, the impact on the host and other elements requires long, patient and dedicated work.
Those of us in the urban world think we can keep parasites at a distance, flooding our farms and wetlands with chemicals to fend them off. This is false confidence, Zimmer reminds us. Parasites are the most adaptable forms of life on the planet. They are as likely to promote change as respond to it. Zimmer cites Robin Dunbar's thesis that grooming for parasites ultimately allowed humans to develop speech and language. He explains how our immune systems and parasites enter a modus vivendi that allows the parasite and host alike to survive. Recognising how that process evolved could lead to better coexistence through "taming" the invaders.
Coexistence with these minute creatures turns out to have many implications. It's now clear that the development of agriculture made human society vulnerable to invaders unknown on the savannah. Human bodies became less robust and mortality rates rose. How far back in time have they had influences on us and what are those? Zimmer suggests that some monkeys have developed "manners" in resource or mate competition. They scream and cavort, but don't scratch or bite rivals for fear of bloodworm infection. Others use particular leaves to clear digestive tracts of infestations. We hear of researchers seeking "genes for" schizophrenia, homosexuality, even "gods". Zimmer thinks we're looking in the wrong place. Instead, he urges, we should identify the "flukes for" these and other aspects of human behaviour and form. [stephen a. haines - Ottawa, Canada]