Introduction

Viruses are a class of genetic element, dependent on suitable host cells for their propagation. Thus, the evolutionary development of viruses must have been contingent on the prior evolution of potential host cells, and the genetic material of viruses must primordially have derived from cellular nucleic acids. Many virus coded proteins, particularly enzymes, have characteristic sequence similarities to non-viral proteins, and this is taken to indicate that many viral genes have cellular genes as progenitors. Having made these almost axiomatic assertions, it is by no means easy to fill in further strategic detail on virus origins. We would like to know what kind of pre-viral elements gave rise to early viruses; what forces acted in the genesis and evolution of viruses; at what epochs in the evolution of cellular organisms viruses appeared; and to what degree contemporary viruses have radiated from common progenitors. This chapter aims to examine some aspects of these questions, but it will quickly become apparent just how limited is our capability in obtaining firm or detailed answers.

Two general classes of data on viruses are applicable to considering details of viral origins, evolution and relatedness. The first includes what we know of basic phenotypes in terms of virion structure (including gross structure of the genome) and replicative cycle; ‘higher’ aspects of phenotype, namely properties relating to pathogenesis and disease, are less useful for these purposes. The second class consists of direct information on genotypes, that is, determined DNA sequences and their interpretations in terms of gene organization and encoded protein sequences.