Why some viruses are enveloped while others lack an outer lipid bilayer is a major question in viral evolution but one that has received relatively little attention. The viral envelope serves several functions, including protecting the RNA or DNA molecule(s), evading recognition by the immune system, and facilitating virus entry. Despite these commonalities, viral envelopes come in a wide variety of shapes and configurations. The evolution of the viral envelope is made more puzzling by the fact that nonenveloped viruses are able to infect a diverse range of hosts across the tree of life. We reviewed the entry, transmission, and exit pathways of all (101) viral families on the 2013 International Committee on Taxonomy of Viruses (ICTV) list.
By doing this, we revealed a strong association between the lack of a viral envelope and the presence of a cell wall in the hosts these virusesinfect. We were able to propose a new hypothesis for the existence of envelopedand nonenveloped viruses, in which the latter represent an adaptation to cellssurrounded by a cell wall, while the former are an adaptation to animal cellswhere cell walls are absent. In particular, cell walls inhibit viral entry andexit, as well as viral transport within an organism, all of which are critical waypoints for successful infection and spread. Finally, the impact of our newmodel for the origin of the viral envelope and its impact on the overallunderstanding of virus evolution is discussed.