Saturday, November 26, 2011

The most primitive life is still with us

kw: book reviews, nonfiction, viruses

I have read a number of books on viruses and virology, including a few that I have reviewed in this blog. The latest covers no new ground, but is a very informative introduction to the modern view of viruses: A Planet of Viruses by Carl Zimmer.

Using a baker's dozen case accounts to cover the breadth of the subject, Zimmer introduces us to viruses large and small, from ancient foes to recent eruptions. Though it may have been with us the longest, smallpox was the first to be cured, partly because it is the most obvious. In contrast to HIV, which is very recent, smallpox makes a person sick immediately, with unmistakable symptoms, and runs its course, deadly or not, in a few weeks. This has led to it being the first virus to be eradicated in the wild.

Viruses are fearsome in part because there are no known "beneficial" varieties. Just as snakes are universally predatory, viruses thrive only by parasitism of cellular organisms. As it happens, though, just as there are billions of bacteria for every "higher" organism, there are huge numbers of virus varieties that parasitize only bacteria and are thus beneficial to us. Before the discovery of antibiotics, viruses called bacteriophages (for "eaters of bacteria") were cultivated and used to cure bacterial diseases. Their only drawback is that viruses are very specific, so it takes quite a cocktail of phages is needed to combat bacteria that exist in multiple strains.

It is now known that the sea is a "virus ocean", with many millions of virus particles per liter of sea water. It is likely that, without viruses, the seas would become a cesspool of bacterial goop! The air is filled with suspended viruses as well, though to a lower density. As numerous as they are, viruses are so small that it takes a few million to outweigh the average bacterium, so they are (probably) not the heaviest component of the biosphere.

A recent discovery shows they are not all that small. Mimiviruses are called that because they mimic small bacteria. They are visible in an optical microscope, being about a micron in size. The smallest viruses known are one-hundredth the size, and most are about one-fortieth to one-twentieth that size.

The most interesting viruses to me are the retroviruses, those that insert their genomes within the genome of their host. So many of these have become "endogenous", meaning incorporated permanently, that about 8% of any animal's genome, including ours, consists of viruses that can be reactivated (according to other accounts I have read, about another quarter of our genome consists of fragmentary virus genomes).

If we consider the ways that life may have originated, it is likely that viruses may have either preceded the earliest cells, or that they arose along with them. That means that living things have never existed in isolation, but have always partaken of a grand kind of cross-species interbreeding facilitated by viruses. They are sometimes called the third sex, although before binary sex arose, they'd have been called the opposite sex! (were there anybody there with sufficient brains to do the calling).

The book is an easy read, and an enjoyable one. For many, it will introduce many subjects that one can then pursue in other works, and the bibliography contains plenty of excellent resources for that.

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