We've talked about viruses as being an outlier that strains the definition of life. Viruses do not have the ability to reproduce on their own, but must harness a host cell of another species to replicate. But now there's a new type of life form that is even more primitive than viruses called obelisks.
Obelisks consist of a viroid-like disc of RNA, but they do not have the protein shell that viruses have. Their genetic information is completely distinct from any known species, putting them into a unique phylogenic group. They reproduce by infecting another cell, like viruses do. They haven't yet been classified into the Tree of Life because scientists aren't really sure where they belong. But obelisks have been found in the microbiome of humans, in the digestive tract. One type of obelisks studied infects the bacteria Streptococcus sanguinis, found in human mouths. The impact of obelisks in the human biome has not been determined, but they've been there all this time and just haven't been found until recently. -via Metafilter
(Image credit: Ivan N. Zheludev et al. via Cell magazine)
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