Birds were likely the ancestral hosts of hepatitis B virus and the human disease emerged after a bird-mammal host switch reports a paper published in Nature Communications this week. Up to now the origin of this virus had remained elusive due to the small number of integrated sequences present in hosts’ genomes.

The field of paleovirology allows for the identification of genomic relics that arise as a consequence of insertion of virus derived DNA in the host’s genome after infection. Alexander Suh and colleagues take advantage of presence or absence patterns of these viruses’ relics among closely and less closely related species of birds to construct a time map of hepatitis infiltrations in the lineage. They find a complete gene sequence of one of these viruses - a Mesozoic paleovirus - and estimate that hepatitis is about 63 million years older than previously known. This provides direct evidence for the existence of Hepadnaviridae during the Upper Cretaceous. They also hypothesise that a predecessor of Hepadnaviridae probably infected an ancestor of birds that lived around 324 million years ago and show that the compact genomic organisation of Hepadnaviridae has remained largely unchanged for the last million years of evolution.

Hepatitis B is a major global health problem with a third of the human population infected and this study brings one step closer to understand its evolution.