A bit of crackpot theory I got here. Detailing that Ancient Humans could be descended from Forerunners.
So, we know Forerunners are seeded on Ghibalb. And that planet has been heavily irradiated ever since. From there, Forerunners settled on numerous planets. Even Mars, our neighboring planet isn’t exempt from that. From this, it could be assumed that some Forerunners could actually reside in Erde-Tyrene. Even Ancient Humans had to move their civilization to flee from Foreruners’ control, so Human ‘Homeworld’, Erde-Tyrene must be edge too close to Ecumene.
Ok, I already said that Forerunners are seeded on Ghibalb, while Humans arose from Erde-Tyrene (even Mythos confirmed that). The keyword here is ‘arose’, its never stated that Humans are actually seeded on Erde-Tyrene, only they ‘arose’ from there and its their homeworld. So its entirely possible that some Forerunners, who want nothing to do with Ecumene (like Soma the Painter) set up colony there.
While theory above might be plausible, there’s no motive for the Forerunners to do that. This is second part of my speculation.
We know Precursors announced to the Forerunners that they won’t be worthy of the Mantle, but Humans will be tested next. So, we know Forerunners absolutely ‘worship’ the Mantle to the point of mania. After knowing Forerunners won’t be trusted with the Mantle, why would they remain ‘Forerunners’? Why don’t they just become ‘Humans’? Because obviously Humans have legitimate claim to the Mantle.
Needless to say few groups of fanatical Forerunners who are hellbent in Inheriting the Mantle proposed this idea to the Council (or whatever their government is at the time). By becoming ‘Humans’, they won’t be straying from Precursors’ ‘wish’. The Council, of course, isn’t amused by the idea (becoming a Xeno (in a way) just to claim the Mantle? Pffft! Heresy!), but they decided to humour these Forerunners.
The problem is they don’t know what it means to become Humans. They only know that Procursors would test Humans next, that’s it. So they decided to just wing it. The plan is devolve them and set them up on Erde-Tyrene, and let evolution do the rest.
The experiment in becoming ‘Humans’ isn’t really successful. Well, because Council don’t really want it to suceeded. As Council always blasted Humans to Dark Age (they can’t kill them because it’d violate the Mantle). Unfortunately, Humans always rose again. As such, Builders devised Guardians as a tool to permanently keep sending back those Humans to stone age (thus Ancient Humans’ description in having went through several technological Dark Ages). Finally, the latest incarnation of Humanity was able to break away from Ecumene territory. And succeeded in setting up their nice little hut outside Orion Complex.
Humans originate from Forerunners explain their fast technological advancement (despite being blasted to stone dark age numerous times, their latest incarnation still considered to be technologically advanced by Forerunners). Their fast technological advancement came from reverse-engineering Forerunner tech, which could be explained that Humans still have memory of Forerunner tech somehow which helps their reverse-engineering efforts. Its impossible to have genetic memory? Well, it must be Lifeworker thing.
This may also explain Ur-Didact’s reluctance to exterminate them at first. Of course, maybe there are limited number of Forerunners who knew that. Builders and Warrior-Servants went great lengths in canon to hide true relation between Humans and Forerunners after all.
Forerunners’ and Humans’ (Modern) design aesthetic are somewhat same too. Blocky, angular, not bulbous. Both are also love practicality. Forerunners always wear armor and helmet, I assume they always want to be able to protect themselves just in case.
Primordium also implied that Humans are Forerunners are originally one before ‘splitted’.
So, there. Feel free to comment.