Don't forget about viruses.

THERE IS A GLARING OMISSION IN THE RECENT Tree of Life Special Issue (13 June, pp. 1691–1709). For too long, viruses have been overlooked by biologists attempting to frame the big picture of life’s history. Earlier views maintained that viruses are not alive because of their lifestyle as obligate parasites, or that viruses are too changeable and inscrutable for inclusion in the tree of life. We argue that these views are no longer tenable. Viruses exhibit the same primary features common to all life forms (e.g., internal homeostatic controls promoting survival, structural organiza t ion based on heritable nucleic acids, reproduction, and so on). Furthermore, there are a legion of other obligate parasites that we do include in the tree (E. coli in our guts and mycorrhizal fungi, for example). Viral inscrutability is diminishing, and although the study of viral origins and evolution is in its infancy, the number of wellsupported virus phylogenies is increasing, and, most importantly, there is a growing set of putative homologous genes linking various virus clades to nonvirus life forms, with more homologs likely to be discovered as genome sequencing efforts for diverse taxa continue. Integration of viruses into the tree of life is not just a viral birthright. It also promotes understanding of viral origins and their roles in emergent disease, in genome evolution through horizontal transfer of genes among hosts, and in the ecology of organismal evolution, as the diversification of viruses and their colonization of virtually all habitats and taxa on Earth have been significant in the creation, maintenance, and loss of biodiversity. Life is a clade, and, to be comprehensive, the tree of life must encompass all of the clade’s diversity. DAVID P. MINDELL1 AND LUIS P. VILLARREAL2 1Department of Ecology and Evolutionary Biology and Museum of Zoology, University of Michigan, Ann Arbor, MI 48109–1079, USA. 2Department of Molecular Biology and Biochemistry, University of California at Irvine, Irvine, CA 92697, USA.