If you were to guess right now what kinds of organisms have driven some scientists to believe that another branch of life exists, what would they be? Hideous, green aliens or maybe man-eating flies? Actually, the organisms in question are neither alive nor visible to the naked human eye. Recently identified giant viruses – literally just uncommonly large viruses – are the organisms responsible for this new line of thinking.
The number of genes Pandoraviruses have is especially noteworthy. Their genomes range from 1,900 to 2,500 genes. In comparison, the influenza virus has a mere 13 genes while humans have roughly 24,000. What’s more, only 7% of their genes have been seen before in other organisms, meaning that the other 93% is foreign. According to Jean-Michel Claverie and Chantal Abergel, who are researchers on Pandoraviruses, “the lack of similarity of most of their genes with other life forms might be an indication that they originated from a totally different primitive cellular lineage.” As of now, three domains of life are recognized: Bacteria, single-celled Archaea and Eukaryotes, the domain plants, animals, and humans belong to. Pandoraviruses’ foreignness alludes to the possibility of a fourth branch of life distinct from the other three and greater biological diversity.
Hand in hand with the theory of a fourth domain is the idea that Pandoraviruses and other giant viruses descended from primitive, living cells separate from bacteria, archaea, and eukaryotic cells. If so, then ancient cells must have embarked on two different evolutionary paths: some gave rise to “modern life” while others evolved into viruses.