>>These are all never captured by historians.
They're all much deeper in time than what historians write about. History has things like one of the first waves of what eventually was called the Black Death putting paid to Justinian's dream of reuniting the fallen western Roman Empire with the then-ongoing Eastern one.
That's not to say that the interplay between retroviruses and eukaryotes isn't interesting. I know of nothing more recent than the introduction of syncitin -and live (vivaparous) birth, although you have me wondering how exactly a retrovirus got into germ-line cells. Thanks for this - although I take your point to have been the importance of fungi that linked article has sent me chasing down a rabbit hole to see if there's any evidence that mamaliaforms (ancestors to mamals) might not have separated germ from soma cells - which does not seem to be the case.
BTW/FWIW it's not uncommon for viruses to be syncitial (cause cells to merge together resulting in cells with multiple nuclei) - to the point that it seems a bit generic to name just one 'respiratory syncytial virus'. Stepping carefully around the rabbit hole of how muscle cells (which normally have multiple nuclei) got to be that way.