These "molecular fossils" also revealed that viruses and proteins of immune system share the same structure. One plausible explanation may be that viral building blocks served as precursors for the evolutionary more recent immune system of animals.
These results are an outcome of recent advances in structural biology, a new discipline integrating applied physics, biochemistry and biology, which have permitted detailed comparison of viral structures in atomic detail. Such comparison led to the discovery that viruses without any sequence homology may be related.
Earlier approaches to delineate evolutionary history of viruses have failed. Since Charles Darwin evolutionary history of multicellular organisms has been delineated using fossils, e.g. physical remains of animals and plants. Later genome sequencing provided support for this framework at the molecular level. However, as virus sequences diverge rapidly and they do not leave behind physical fossils due to their small size, these methods could not be used in studies of the evolution of viruses.