AA << phylogenetic analyses suggest that this major retroviral lineage, and therefore retroviruses as a whole, have an ancient marine origin and originated together with, if not before, their jawed vertebrate hosts >450 million years ago in the Ordovician period, early Palaeozoic Era. >>
Pakorn Aiewsakun, Aris Katzourakis. Marine origin of retroviruses in the early Palaeozoic Era. Nature Communications 8 , Article number: 13954 (2017) doi :10.1038/ncomms13954. 10 Jan. 2017.