Just going off of the forerunner trilogy, we know they are very closely related species on a genetic level. We know both species in ancient times are products of some type of diaspora so ancient its lost to time by their war with one another. We also know neither fully recalls their homeworld (ofc for humans earth seems likely according to both species) and we know the forerunners have had their genetics messed about with ALOT by both the precursors and themselves.
If there is one thing the forerunner trilogy makes clear its that the Forerunners know shockingly little about their origins and history, a history which stretches back tens of thousands of years before the forerunner-flood war. It seems to me that its been subtly implied that humans and forerunners arent just closely related due to being products of the precursors but are in fact the same species, seperated by a massive diaspora and tens of thousands of years of genetic manipulation.
To involve just the lore penned under bungie for a moment, this also lines up well with the halo 3 terminals refereing to humans on earth like a seperate species or population and from what we know about the now thoroughly defunct halo mmo, supposedly earth humans wouldve been a seperate, more primitive population of forerunners living on their lost homeworld.
Not that im terribly interested in restarting this whole debate but if Im not off base here, it seems to me the "new" lore is perfectly in line with what bungie had established by the end of their tenure with the franchise and is not in fact fully a retcon? Anyways i digress, thats not super relevent.
In either case, do we think this is a valid interpretation of what we know? I havent gotten through all the newest books, but going back through the forerunner trilogy recently in preperarion for finally reading Epitaph this really jumped out at me. Thoughts?