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The Visitors That Came to Stay
(A review of Virolution by Frank Ryan)
The Wall Street Journal, June 18, 2011

We are part virus. This bizarre yet inescapable fact has been revealed over the past 30 years, as scientists have spelunked their way through the human genome and encountered stretches of DNA with the telltale chemical signatures of viruses. All told, they've found 100,000 such segments so far. As Frank Ryan explains in "Virolution," these pieces of virus DNA ended up in our genome through a peculiar kind of infection. From time to time, viruses slipped their DNA into the eggs and sperm of our ancestors. Parents then passed down the virus DNA to their offspring. These viruses could no longer escape their hosts, but they could still make new copies of their DNA, which were then inserted back into our ancestors' genomes. And so it is that, after millions of years of infection, viruses now make up at least 8% of the human genome. Our "own" genes--the genes that encode the proteins that constitute our bodies--make up a measly 1.2%.

Dr. Ryan, a British physician and science writer, argues that this discovery demands a new vision of how evolution works--hence the name of his book and a splashy boast on its cover: "The most important evolutionary book since Dawkins's 'Selfish Gene.' " In that landmark 1976 work, Richard Dawkins presented a gene-centered view of evolution. Evolution did not work for the good of the species, he argued. Altruism and other selfless traits were actually strategies that evolved so that genes could make more copies of themselves. In "Virolution," by contrast, Mr. Ryan presents a virus-centered view of life, in which viruses help steer the course of evolution. "Natural selection alone could not have given rise to the evolution of life," Mr. Ryan declares.

When Mr. Dawkins wrote about natural selection 35 years ago, he was describing a process in which genes mutate as they are passed down from one generation to the next. Viruses, Mr. Ryan notes, can deliver entire sets of new genes all at once. Sometimes this viral DNA can become domesticated: It evolves from a recipe for a parasite to a vital service for its host. Placentas, for example, stick to the wall of uteruses thanks to proteins that originated in viruses. Without our viral DNA, in other words, none of us would have ever been born.

These days the evidence that viruses have had at least some role in our evolution is very strong. But this doesn't demand a "virolution." When Dr. Ryan claims that natural selection alone cannot be responsible for evolution, he is arguing against a straw man. If you open up a scientific journal on evolutionary biology these days, you'll find papers on natural selection, and you'll find papers on other processes, too. Sexual selection--the competition for mates--drives extraordinary changes in animals and plants. Mutations can also spread through chance alone, a process called "neutral evolution."

Evolutionary biology as practiced these days explains a great deal about how viruses shape our genome. A newly arrived virus may cause a devastating disruption, or it may be harmless, depending in part on where it gets pasted into our DNA. Natural selection will weed out the harmful mutations and tolerate the ones that don't have much effect for good or ill.

The battle between viruses and their hosts is so intense that our ancestors evolved defenses against them. The human genome contains proteins that have evolved to grab onto certain kinds of virus DNA and prevent them from invading the genome. We also have defenses that fight against parasitic DNA as it copies itself and reinserts itself back into our genes.

A lot of our viral DNA has been so degraded by later mutations that it is just baggage that gets carried along from one generation to the next. But if a mutation happens to salvage an old virus gene and give it a new, useful function, it will spread through a species via natural selection.

"Virolution" offers a number of illuminating passages on these matters, but Dr. Ryan mixes them with dreary transcripts of technical conversations that he has had over the years. One chapter is little more than a series of summaries of talks at a conference he organized in 2008.

Dr. Ryan also meanders beyond viruses, to other examples of intermingled genomes. He tells the now-familiar story of mitochondria, the sausage-shaped structures in our cells that produce fuel for us to burn. Mitochondria started out as free-living, oxygen-consuming bacteria that somehow became residents in our single-celled ancestors. It's all interesting. It's all accurate. But it doesn't contribute to any coherent argument about viruses.

It's too bad that "Virolution" suffers from these shortcomings, because it has an important message. Scientists have identified a few compelling examples where our inner viruses do useful things for us. But in many cases we don't know what, if anything, they're doing. These viruses can have a dark side: In cancer cells, for example, viral genes seem to help speed up their growth. Discovering the true nature of our inner viruses isn't just a matter of natural history. It's potentially a matter of life and death.

Copyright 2011 Carl Zimmer
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