After a hepadnavirus inserts itself into a chromosome of an animal, fragments of the virus are passed on to all of that animal' s descendants  A hepadnavirus fragment is present in a chromosome of the zebra finch and in precisely the same location in a corresponding chromosome of the dark-eyed junco
The fact that these two bird species diverged from each other about 25 million years ago ttherefore means that the hepadnavirus is at least 25 million years old
When a virus inserts itself into an animal's chromosome, the insertion occurs at a random spot.