The second article in Nature, does a convincing job in showing through evolutionary bioinformatics (something I again admit to practicing as a hobby, in the spirit of full disclosure) that the hepatitis B virus entered eukaryotes ~83 million years ago. The oldest viral relic in a genome is the 99% conserved sequence for avian Hep-B in Zebra Finches. Chickens don’t have a relic of Hep-B. Since their oldest common ancestor lived ~82 million years ago, the virus must have entered the genome before that time. Bob’s your uncle!
Read both papers. They are entertaining as well as quite possibly true.