Classes of ERVs Wikimedia Commons
Endogenous retroviruses (ERVs) are genes of retroviral origin that have been incorporated into the genome. They include the syncytins expressed in the placentas of various mammals. Six are known so far and each represents a separate capture of a retroviral gene (see previous post).
Now an ERV has been described that occurs in representatives of all four superordinal clades of placental mammal (here). In this case the gene is orthologous and thus represents a single integration event that must have predated the divergence of placental mammals in the Cretaceous (for possible dates see my recent post).
This work was published in a special issue of Phil Trans R Soc B on the theme "Paleovirology: insights from the genomic fossil record." Although sequence substitutions occur over time, an ERV is essentially a "fossil" record of a virus as it existed at the time of incorporation into the host genome. Thus the study of ERVs in various organisms (not just mammals) can provide insight into the long term history of viruses and virus-host interactions (see overview here).
In a historical overview of the field (here) Robin A. Weiss notes, "The advantage to the host of ERV protein expression is most dramatic in the evolution of the mammalian placenta." Incidentally, if you find the whole concept hard to swallow you are in good company as the early history of the field so clearly shows.