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In sum, our extensive review has revealed a close association between cell walls and nonenveloped viruses that was not bound to particular types of host organism. The cell wall provides a physical barrier that hinders the interaction of receptors on the viral envelope with receptors in the cell membrane, an interaction that is central to the infection of animal cells. Although there are exceptions to this important evolutionary generality, we show that they can be considered to be individual adaptations. We also propose that early viruses were nonenveloped and that the viral envelope has evolved several times independently, reflecting the diversity of hosts encountered; this provides a new perspective on our understanding of virus origins and evolution.