Recently scientific research was published that reported that it had found – among others – endogenous viral elements (EVEs) from a Hepatitis B Virus in Crocodilians. Analysis reveals genome fragments from the virus family were inserted into the genome of this snake over the past 50 million years.
. These elements can be seen as the genetic ‘fossils’ of which can be detected in whole genome sequence data millions of years later. Endogenization of viruses is not rare; in fact, it appears to be a recurrent and on-going process.
The scientists searched the recent saltwater crocodile, gharial, and American alligator draft genome assemblies using whole viral genomes of the duck Hepatitis B Virus and the Mesozoic endogenic Zebra finch Hepatitis B Virus (eZHBV) and identified two endogenous crocodilian Hepatitis B viruses (eCRHBVs): eCRHBV1 and eCRHBV2.
eCRHBV1 is present in all crocodilians except alligators and is 63.8–102.6 Millon Years (MY) old, which means that it is of Cretaceous origin. The second, eCRHBV2 is exclusively shared between saltwater and dwarf crocodiles: its endogenization thus occurred during the Paleogene in the ancestor of Crocodylidae (30.7–63.8 MYA).
So, the Hepatitis B Virus has been on the planet for millions of years. No wonder, then, that it has become so versatile. It changes, mutates and jumps species without any problem. As the Greek philosopher Heraclitus already said in the 4th century BC: There is nothing permanent except change.
 Suh et al: Early Mesozoic Coexistence of Amniotes and Hepadnaviridae in PloS Genetics - 2014
 Katzourakis et al: Endogenous viral elements in animal genomes in PLoS Genetics – 2010
 Holmes: The Evolution of Endogenous Viral Elements in Cell Host & Microbe – 2011