will raise the grue in most readers. There are passages that could make some people sick. But eventually, we begin to agree with the parasitologists: these creatures, with their exquisite adaptations to their habitats, and their beautifully sequenced metamorphoses of their life cycles, are fascinating; and the title does not exaggerate their importance. Zimmer’s final conclusion, that humans are parasites on Gaia, is sobering, especially when we consider that unlike the other, successful parasites, we have apparently not learned how to tame our voraciousness to just the right level to guarantee that our host, and therefore we, will survive in its present form. If we don’t learn how to do this, we may change our host so drastically that it can no longer support us.
A book that should be read, but I suspect many, perhaps most, readers will come away with a conviction that parasites should be eradicated. This would be a dangerously wrong inference, as Zimmer shows very clearly that without parasites ecosystems would be very different, that there is a fine line between symbiosis and parasitism, and that very likely we would not have evolved: we appear to be a collaboration of symbionts. Evolution of ever more complex creatures may in fact be a response to parasites: every trick that a parasite develops is countered by a trick developed by its host. This attack and counter-attack system will almost inevitably result in increasing complexity.
But not only is our complexity merely the effect of our ancestors’ attempts to evade parasites, there is some evidence that much of the “junk DNA” may be parasitic, too. It’s pretty well established that cells are symbiotic systems: the mitochondria look too much like bacteria to be anything else. Now it looks like much of our DNA may be viruses that have permanently joined us, their hosts.
From the beginning of life, in other words, some life forms survived by attaching themselves to others and using their food, their bodily substance, and their reproductive machinery for their own interests. What’s more, this has now become the dominant mode: all animals and fungi live by eating other life forms. Even plants depend on other living things: the remains of dead animals supply essential nutrients to almost every plant. Very few plants can subsist on nothing but water and minerals. Most bacteria and all viruses need living hosts for at least part of their life cycle. Life lives by devouring life. Gaia exists by cycling matter through complex webs of interdependence, the whole system driven by energy derived from the sun and released by the breakdown of molecules deep in the oceans and the crust. Tennyson, with his nature red in tooth and claw didn’t know the half of it. *** (2003)
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