Viruses are supposed to be small and simple—not even alive, just mobile genetic material after all. So what do we make of giant double-stranded DNA (dsDNA) viruses, one of which—the newly discovered Pandoravirus salinus—has an even larger genome than a hunky parasitic eukaryote called Encephalitozoon? The recent identification of P. salinus adds evidence to growing speculation that it and other mammoth viruses evolved from cellular ancestors and represent domains of life that likely existed on Earth before the last universal common ancestor (LUCA). Increasing numbers of scientists are coming around to this point of view; some of whom like Gustavo Caetano-Anollés at the University of Illinois at Urbana-Champaign go even further, asserting in Microbe magazine that “giant viruses not only existed at the same time as the LUCA of cellular life, they’re direct descendants of the lineage that gave rise to it.” Caetano-Anollés does not say this lightly—he has protein data to substantiate the claim.
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