Viruses are alive!

Viruses are largely made of proteins and nucleic acids. Some like the giant viruses have molecular complexities that resemble the complexities of cells. They can also be as big as small bacterial microbes. While they are unable to replicate they do so inside their cellular hosts and their molecular makeup evolves very much as that of typical living organisms, with the caveat that evolution is faster. While they appear to highjack the host cellular machinery, one can also view the infected cell as part of a complex lifecycle in which 'viro-cells' engage or not in panspermic dispersion of genetic material.

An analysis of a vast repertoire of 3-dimensional protein structures, called ‘folds’, that are encoded in the genomes of all cells and viruses revealed that the origin of viruses is cellular and very ancient. The identification of all known protein folds in 5,080 organisms representing every branch of the tree of life and 3,460 viruses revealed 442 folds that are shared between cells and viruses and 66 that are unique to viruses. Advanced phylogenomic analysis of this data confirmed the existence of an ancient stem line of descent that is common to cells and viruses. These findings justify placing viruses in a phylogeny of life, a 'tree of life' that now describes the evolution of 4 supergroups, Viruses, Archaea, Bacteria and Eukarya. This changes our current paradigm.

While the traditional view that viruses are non-living entities has been pervasive, the new massive genomic dataset that supports these studies suggests viruses are part of complex obligatory lifecycles. A study of gene dispersion also reveals viruses spread genetic wealth through our planet using horizontal mechanisms of genetic exchange. We should regard viruses as agents of change extruded by cells, since they promote and spread the generation of useful molecular innovations that are needed to sustain life.

1. Nasir A, Caetano-Anolles G (2015) A phylogenomic data-driven exploration of viral origins and evolution. Science Advances 1: e1500527.
2. Malik SS et al. (2017) Do viruses exchange genes across superkingdoms of life? Frontiers Microbiology 8: 2110.

Artwork illustration by Anson Call, Iowa State University