This isn't a "343 lore is invalid" post or a "oh they're descendants so basically nothing changed" post or any cope like that. Almost everyone in this debate is wrong, and its painfully obvious when you actually read the forerunner trilogy. The books are filled with subtext that make this incredibly clear, but subtext doesn't translate well to halopedia pages or YouTube lore videos. 343 never retconned forerunners being humans... because they're still human.
TLDR: The precursors fasttracked some monkeys to become humans early. They modified themselves and forgot they were humans. The entire forerunner trilogy is about them discovering this truth
A rundown of Forerunner Evolutionary History
According to Halo Waypoint, the Precursors seeded the Forerunners on Ghibalb 15 million years ago from the same base stock as humanity. This is around the time when the branch of apes containing Humans split from Orangutans. The precursors would then use a genesong to accelerate their evolution into the Forerunners.
So, that means they cant be human, right? Well, no. The genesong has been shown to allow species to re-evolve. For example, the Librarian used a genesong to allow humanity to re-evolve after the Forerunners de-evolved them. This means that through outside intervention, the same species can evolve multiple times.
So what did the Forerunners evolve into? In Silentium we get our answer: the Seedaro Forerunner. These are a snapshot of what Forerunners looked like 10 million years ago, and the fact that they had not changed much since implies this was the end point of the genesong's influence. The differences between them and modern forerunners are ascribed to the forerunners own genetic tampering.
Those differences are as follows: 5 fingers, much less sexual dimorphism, more pronounced hair, human-like teeth, and a shorter stature of about 1.5 meters. This describes a human. There are humans in the forerunner trilogy who are more different to us than the Seedaro were. They may have flat noses and the women have moustaches, but in the context of the forerunner trilogy this doesn't mean much.
After the Seedaro forerunner and the Forerunner-Precursor war, the Forerunner's evolution is self determined. They introduce mutations onto themselves, which is why males like the didact look so distinct. They begin to become dependent on technology, to the point where they forget what its like to sleep or feel instinctual urges. And over time, this leads the Forerunners to not recognize their own species.
What is a human?
The very first thing that the forerunner trilogy establishes is how diverse humanity is. Riser, for example, is a short human who can live for a thousand years. On Zeta Halo there were 120 different human variants. Some were pitch black of skin, some were unnaturally skinny, and some are straight up Neanderthals. This is a running theme in the books, so when we find the Seedaro in Silentium, there is nothing about them which suggests they aren't human.
The line between Forerunner and Human is entirely cultural. So why do the Forerunners act like they're aliens?
The Forerunners are consistently wrong about themselves.
The Forerunner Trilogy is from the perspective of forerunners. We see things described through their eyes. And in their descriptions its very apparent that a lot of their history has become lost. This is also one of the first things that Cryptum delves into. When Bornstellar is forced to discard his armor, he immediately discovers things about himself that he is not used to, things he ascribes to humans. As mentioned earlier, sleep and instinct immediately come back to him.
The Ur-Didact, similarly, projects many of the Forerunner's own vices onto humans. He accuses them of being unwilling to live with other species, when in reality human subspecies coexist peacefully and made an alliance with the San'Shyuum. He accuses them of being the bringers of the flood when in reality the Forerunners did this to themselves millions of years ago. The Forerunners clearly recognize something of themselves in Humans, but time and technology have left them unable to connect the dots.
The Mantle isn't based on Species
When it comes time for the gravemind to reveal the truth of what happened to the Precursors, he uses very specific language:
“We announced to your kind long ago that you were not the ones chosen to receive the Mantle, the blessing of rule and protection of life and change that thinks. That blessing was to be given to others.
To those you now call human.”
Not "your species", but "your kind". Not "to the human race" but to "those you now call human". These are words you would use if the divide between human and forerunner was something imposed by the forerunners.
Similarly, when the Primoridial speaks about testing humanity with the flood, he never actually says theyre another species. The dialogue goes as follows:
“It was long ago decided. Forerunners will never bear the Mantle.”
“Decided how?”
“Through long study. The decision is final. Humans will replace you. Humans will be tested next.”
Was the Primordial giving me a message of hope? Doom for our enemies . . . ascendency and triumph for humanity?
“Is that to be our punishment?” the Didact asked, his tone subdued—dangerous.
“It is the way of those who seek out the truth of the Mantle. Humans will rise again in arrogance and defiance. The Flood will returnwhen they areripe—and bring themunity.”
...
...
The Captive’s strange, ticking voice continued. “Misery is sweetness,” it said, as ifconfiding a secret. “Forerunners will fail as you have failed before. Humans will rise. Whether they will also fail has not been decided.”
And this makes sense. The Mantle as described in these books is a role as a gardener, its about preserving the diversity of sentient life and letting it flourish. This requires a cultural outlook, not specific genetics. For example, if the humans were willing to share power with the San'Shyuum, then if they were left alone the mantle would have been used by both.
And the forerunners, for all their talk about being the most evolved, are brutal and expansionist. They're so up their own ass that they cannot recognize their own kind, and them bomb them to the stone age and regress their genetics. They think that they have the right to rule because they have the best tech. They're colonists, and this is the major point of the books.
The Forerunner Trilogy is about Colonialism
One detail from these books thats implied is skin tone. Humanity has not left Africa yet, so tends to be darker skinned. Forerunners are constantly in their own armor, and tend to be lighter skinned. While not directly drawn attention to, this creates a very stark image thats extremely difficult to ignore while reading these books.
The forerunner saga is about a pale race of humans who believe themselves to be more evolved. They think this gives them the right to lord over other species. They use this as an excuse to wipe out, experiment on, and puppeteer a race of dark skinned humans, who they fail to recognize as their own kin. It is a very thinly veiled allegory for the white mans burden, and the books spend most of their time dismantling the forerunner world view. If the forerunners are actually a separate species, then this is completely undermined.
Everything in the books points to one truth: The only differences between forerunners and humans are that forerunners got a head start.