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Zimmer2006_Did_DNA_Come_from_Viruses_ScienceNews -...

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12 MAY 2006 VOL 312 SCIENCE www.sciencemag.org 870 CREDITS: COURTESY OF PATRICK FORTERRE Scientists who deal in the history of life have never been quite sure what to do with viruses. One measure of their uncertainty is the Tree of Life Web Project, a collective effort to record everything known about the relationships of liv- ing and extinct species. The first page of its Web site—entitled “Life on Earth”—shows the broadest view: From a single root come three branches representing the domains of life (www.tolweb.org). One limb, Eubacteria, includes bacteria such as Escherichia coli . Another, Archaea, includes microbes of a differ- ent lineage that are less familiar but no less com- mon. The third, Eukaryotes, includes protozoans as well as multicellular organisms such as our- selves. And just below the tree there’s a fourth branch floating off on its own, joined only to a question mark. It is labeled “Viruses.” A growing number of scientists hope to get rid of that question mark. They recognize that a full account of the evolution of life must include viruses. Not only are they unimaginably abun- dant—most of the biomass in the ocean is made up of viruses—but they are also extraordinarily diverse genetically, in part because they can acquire genes from their hosts. They can later paste these genes into new hosts, potentially steering their hosts onto new evolutionary paths. Patrick Forterre, an evolutionary biologist at the University of Paris-Sud in Orsay, France, believes that viruses are at the very heart of evolution. Viruses, Forterre argues, bequeathed DNA to all living things. Trace the ancestry of your genes back far enough, in other words, and you bump into a virus. Other experts on the early evolution of life see Forterre’s theory as bold and signifi- cant. But although they find much to agree with—particularly the importance of viruses to evolution—many also regard Forterre’s ideas as controversial. “I really applaud the bravery and intellectual power to come up with this picture,” says Eugene Koonin of the National Center for Biotechnol- ogy Information (NCBI) in Bethesda, Maryland. “But it would be strange if we agreed on all the parts of the picture.” A new domain Forterre has been developing and elaborating his theory over many years. He began his scien- tific career in the early 1970s studying the repli- cation of DNA. He investigated how E. coli , the common gut bacteria, use special enzymes to make new copies of their genes without letting the double helix of DNA become tangled. As Forterre was studying bacteria, another group was developing a complex new view of simple organisms. Carl Woese of the University of Illinois, Urbana-Champaign, demonstrated that some bacteria are not bacteria at all. They belong to a separate branch on the tree of life, which came to be known as Archaea. Archaeans turned out to have a distinct biology. Forterre and his colleagues discovered that they use peculiar enzymes for DNA replication that work
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