So humans and Forerunners appear to have been one, a long time ago. Then, they were 'sundered', presumably by the Precursors. The Precursors then tried to wipe out the Forerunners, but the Forerunners drove them off instead. But the Precursors sent the Flood to the galaxy as a test for the Forerunners... And that test turned Forerunner against Forerunner as they all fell to the Flood, which I presume is why the Forerunners failed. From what I gather from the end of the book, the Flood deliberately did not infect humans, because they were not the ones being tested. Only when the Master Builder started fucking around with the Flood on Installation 07 did they start infecting humans, but even then only a limited number actually got infected. Heck, Riser made his way across a Flood-formed wasteland filled with spores, and nothing happened to him. The captive states at the end that "humans will be tested next", I presume this is the point where
Chakas/Guilty Spark refers to humanity as the TRUE Reclaimers. When he says "reclaimer", I'm going to assume he means "to reclaim the Mantle", and not "to reclaim the Forerunners' empire" or some shit. The Precursors believed in the Mantle. If the Mantle is, truly, what the Forerunners believe it is, then it makes sense that they were deemed "unworthy" when they started fighting among themselves instead of protecting the galaxy against the Flood. It seems that humanity, if we consider the events of Halo 1/2/3 as 'their' test, may have passed because they actually banded together against the Flood. Or, what we saw in the first trilogy was only the start, because let's face it, the scale of Flood infection we saw during those games was peanuts compared to the galactic war the Forerunners are waging... The whole point of the 'Reclaimer'-trilogy may be humanity proving their worth as the 'reclaimers' of the Mantle, the heritage of the Precursors, for which the Forerunners were apparently deemed unworthy.
Now, about the Precursors. We know that the Captive was a Gravemind, but he was different than the mish-mash of bodyparts that Chakas came across during his journey. We also know that the Captive, while big, was a lot smaller than the only Gravemind we've actually seen 'in the flesh': the Halo 2/3 Gravemind. That one was a mass that spanned the entire bottom of the Library, and later on the entirety of High Charity. Yet he was able to move himself, so perhaps what we saw of the Gravemind was actually just a facade, a 'meat shield' if you will. Perhaps at the center of our Audrey-like friend, there was another being like the Captive. The real 'compound mind'. Probably not, but I'm brainstorming here.
Or perhaps, the Captive was as he is because of the way he was created. From what we know about the Precursors, it seems (and this is just pure conjecture here) like their technology was based on some kind of organic life. After all, their structures on Charum Hakkor were destroyed when the Master Builder set off a Halo near it. Now, because we havent found any trace of Precursor architecture in the timeframe of the games, Im going to assume that when the Halos wiped the galaxy clean, they wiped away all Precursor structures as well.