Eukaryotes, prokaryotes, archaea, … and viruses?
Recently, there’s been a lot of discussion about giant viruses (as in “Biggest virus ever: We just don’t understand anything anymore!” The same issue of The Scientist from which we learned that the last universal common ancestor was “a sophisticated cellular organism,” not Darwin’s “primordial form” also asks, editorially, whether giant viruses should be the fourth domain of life.
More-recent discoveries of giant viruses with massive genomes have the potential to shake up evolutionary biology once again. Truly giant viruses, with bigger genomes than many bacteria and archaea, escaped detection for decades because microbiologists defined viruses, in part, by their ability to pass through filters with pores 0.2 μm in diameter. Didier Raoult (“Viruses Reconsidered”) describes the discovery of these megaviruses (0.4 μm) that, among other surprises, contain genes encoding the translational machinery for their replication. Woese added archaea to the tree of life based on rRNA sequencing. Viruses have no rRNA, and most biologists do not consider them to be living organisms. But Raoult argues that phylogenetic trees based on transfer RNA and RNA polymerase genes “show that viruses are at least as old as the three traditional domains proposed by Woese” and may warrant classification as a fourth branch of life.
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One of the giant viruses is Pandoravirus:
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See also: The Science Fictions series at your fingertips (origin of life)