Reassessing viral origins and evolutionary placement in the tree of life

The quest to fit all cellular beings in one picture frame as the universal Tree of Life (ToL) has always been a daunting task for evolutionary biologists. Over the decades, ToL has emerged from a dichotomous topology to its present form with three domains; bacteria, archaea and eukarya. But this phylogenetic placement is also questionable due to the miscellaneous nature of certain housekeeping genes, horizontal gene transfers (HGT), and also due to incomplete pathways of pathogenic organisms. Furthermore, the ambiguous nature of viruses has always puzzled researchers about their placement in ToL. Despite the multiple attempts, lack of common genes, and their coevolution with host systems, placement of viruses has always been controversial and has often yielded scattered phylogeny among themselves. Recent discoveries-especially of giant viruses sharing genes with cellular domains-offer fresh insights that support the inclusion of viruses in the ToL framework. By focusing on the RNA polymerase subunit β (RpoB) gene, a conserved marker across bacteria, archaea, eukarya, and giant viruses, this study reconstructs phylogenies that reveal giant viruses clustering closely with eukaryotes, suggesting viruses may occupy a distinct yet integral position in the evolutionary landscape. Though perfect declaration of viruses as fourth domain is still dubious, their placement in ToL is as important as any other cellular organism.