2Chapparvoviruses are a highly divergent group of parvoviruses (family 3 Parvoviridae) first identified in 2013. Interest in these poorly characterized 4 viruses has been raised by recent studies indicating that they are the cause of 5 chronic kidney disease that arises spontaneously in laboratory mice. In this 6 study, we investigate the biological and evolutionary characteristics of 7 chapparvoviruses via comparative analysis of genome sequence data. Our 8 analysis, which incorporates sequences derived from endogenous viral 9 elements (EVEs) as well as exogenous viruses, reveals that 10 chapparvoviruses are an ancient lineage within the family Parvoviridae, 11 clustering separately from members of both currently established parvoviral 12 subfamilies. Consistent with this, they exhibit a number of characteristic 13 genomic and structural features, i.e. a large number of putative auxiliary 14 protein-encoding genes, capsid protein genes non-homologous to any hitherto 15 parvoviral cap, as well as a putative capsid structure lacking the canonical fifth 16 strand of the ABIDG sheet comprising the luminal side of the jelly roll. Our 17 findings demonstrate that the chapparvovirus lineage infects an exceptionally 18 broad range of host species, including both vertebrates and invertebrates.19Furthermore, we observe that chapparvoviruses found in fish are more closely 20 related to those from invertebrates than they are to those that infect amniote 21 vertebrates. This suggests that transmission between distantly related host 22 species may have occurred in the past. Our study provides the first integrated 23 overview of the chapparvovirus group, and revises current views of parvovirus 24 evolution 25 26 AUTHOR SUMMARY 27 Chapparvoviruses are a recently identified group of viruses about 28 which relatively little is known. However, recent studies have shown that these 29 viruses cause disease in laboratory mice and are prevalent in the fecal virome 30 of pigs and poultry, raising interest in their potential impact as pathogens, and 31 utility as experimental tools. We examined the genomes of chapparvoviruses 32 and endogenous viral elements ('fossilized' virus sequences derived from 33 ancestral viruses) using a variety of bioinformatics-based approaches. We 3 1 show that the chapparvoviruses have an ancient origin and are evolutionarily 2 distinct from all other related viruses. Accordingly, their genomes and virions 3 exhibit a range of distinct characteristic features. We examine the distribution 4 of these features in the light of chapparvovirus evolutionary history (which we 5 can also infer from genomic data), revealing new insights into chapparvovirus 6 biology. 7 8