Viruses are essential agents within the roots and stem of the tree of life.

In contrast with former definitions of life limited to membrane-bound cellular life forms which feed, grow, metabolise and replicate (i) a role of viruses as genetic symbionts, (ii) along with peripheral phenomena such as cryptobiosis and (iii) the horizontal nature of genetic information acquisition and processing broaden our view of the tree of life. Some researchers insist on the traditional textbook conviction of what is part of the community of life. In a recent review [Moreira, D., Lopez-Garcia, P., 2009. Ten reasons to exclude viruses from the tree of life. Nat. Rev. Microbiol. 7, 306-311.] they assemble four main arguments which should exclude viruses from the tree of life because of their inability to self-sustain and self-replicate, their polyphyly, the cellular origin of their cell-like genes and the volatility of their genomes. In this article we will show that these features are not coherent with current knowledge about viruses but that viral agents play key roles within the roots and stem of the tree of life. (c) 2009 Elsevier Ltd. All rights reserved.