The origin and evolution of viruses may be hard to determine, but new research discovered evidence that they are alive and may have emerged ahead of the first modern cells.
The analysis, published in the journal Science Advances on Sept. 25, backs the hypothesis that viruses are living organisms that share a long evolutionary history with cells. The study traces viral evolution back to when both viruses and cells exist in their currently recognized forms.
Gustavo Caetano-Anollés, professor at the University of Illinois and the Carl R. Woese Institute for Genomic Biology who led the analysis, said viruses have just been placed on the tree of life, on the senior-most position at the bottom of the tree and existing in their own unique group.
Right now it is called the “viral supergroup,” just short of “superkingdom” or “domain,” according to Caetano-Anollés.
Graduate student Arshan Nasir, who partnered with Caetano-Anollés in the study, asserted that viruses are living, but “simply have an atypical mode of living that is slightly different” from that of humans.
“They are not fully independent. Instead, they move in and out of our bodies, stealing the resources and producing their offspring,” Nasir explained, calling for a broader definition of how life and its associated activities are defined.
Viruses are a challenge to study due to the rapidly changing sequences encoding their genomes. The researchers then ventured to look at “folds,” which are the structural building blocks of proteins giving them a complex and three-dimensional appearance.
The researchers analyzed all of the folds in 5,080 organisms – including 3,460 viruses – that represent every branch of the tree of life, and identified 442 protein folds shared between cells and viruses, and 66 unique to the latter.
“You’ve found a multitude of features in viruses that have all the properties that cells have,” Caetano-Anollés said, asserting that viruses also have components unique from those they shared with cells.
Caetano-Anollés and Nasir employed computational methods on protein-fold data to build trees of life, with data suggesting that viruses hailed from multiple ancient cells – likely containing segmented RNA genomes – and co-existed “with the ancestors of modern cells.”
Data from this analysis suggests, too, that not long after modern cells existed, most viruses acquired the ability to encapsulate themselves in capsids or protective protein coats. Capsids eventually became sophisticated and allowed viruses to become infectious to cells that resisted them previously – the “hallmark of parasitism,” added Nasir.
According to the analysis, viruses can be deemed as “cells that have lost and lost genetic material in exchange for reaping the benefits of their interactions with other cells.”
Photo: NIAID | Flickr
