I, virus.

"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 living 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.

"One limb, Eubacteria, includes bacteria such as Escherichia coli. Another, Archaea, includes microbes of a different lineage that are less familiar but no less common. The third, Eukaryotes, includes protozoans as well as multicellular organisms such as ourselves. 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 abundant -- 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."