Viruses have been difficult to classify, with some scientists arguing that they are just nonliving bits of DNA and RNA, yet new research not only finds that they are very much alive, but that they also emerged before the first modern cells.
Viruses have therefore just been placed on the tree of life, occupying the senior-most spot right at the bottom of the tree. They are not "animal, vegetable or mineral," as the saying goes, but exist within their own unique group, according to a new study.
"For now, we call it the 'viral supergroup,' just short of 'superkingdom' or 'domain,' which are words that are quite charged with meanings," Gustavo Caetano-Anoll's of the University of Illinois and the Carl R. Woese Institute for Genomic Biology told Discovery News. He co-authored the study, published this week in the journal Science Advances.
Photos: The Art of Microbiology
Arshan Nasir, who is Caetano-Anollés' graduate student and also worked on the study, added that "viruses are living. They simply have an atypical mode of living that is slightly different from ours. They are not fully independent. Instead, they move in and out of our bodies, stealing the resources and producing their offspring. In short, we need to broaden how we define life and its associated activities."
Viruses are challenging to study because the sequences that encode their genomes are subject to rapid change. As a result, the scientists elected to study what are known as "folds": the structural building blocks of proteins that give proteins their complex, three-dimensional shapes.
The scientists compared fold structures across different branches on the tree of life, reconstructing the evolutionary history of the folds and of the organisms whose genomes code for them. The researchers did this for 5,080 organisms representing every branch of the tree of life, including 3,460 viruses.
They identified 442 protein folds shared between cells and viruses, and 66 that are unique to viruses.
Viruses Pass Major Test to Enter Ranks of Living
Nasir said that "a large number of viral genes are nothing like we have seen so far in the cellular world. They are most likely new genes created by viruses."
The researchers theorize that viruses evolved at a time when primordial cells were extruding genetic material, which viruses could then acquire. Most viruses then gained the ability to encapsulate themselves in protective protein coats, called capsids, which became more sophisticated over time. Capsids allowed viruses to become infectious to cells that had previously resisted them.
Viruses, Caetano-Anollés said, "can be visualized as cells that have lost and lost genetic material in exchange for reaping the benefits of their interactions with other cells."