Well...the Forerunners created the Flood...it's incredibly complicated to explain unless you already know a lot about Halo 3...that, and Keyes wasn't turning into another "Gravemind", he was turning into a "Brain form", which is used, in this case, to interface with the Truth and Reconciliation's controls. Once they complete their objectives, the "Brain forms" can then later be absorbed by Gravemind, who uses their knowledge to increase his own.
I'm glad everyone here knows that the Ark is the "master" halo ring buried inside the Earth...I'm tired of explaining that one to my friends.
The humans, so far as I know, are not the decendants of the Forerunners...Although their homeworld just so happens to be the same planet where the Forerunners built the Ark (which is weird, I know), the reason that 343 calls Chief "Reclaimer" in Halo 1 is because he was wearing green armor, which is what powerful Forerunner warriors, such as Cephaeus (Halo 3 character) wore 100,000 years before Halo CE (again, it's complicated to explain), and 343 just so happens to be about 101,000 years old...most human AIs either die or go rampant after 5-7 years, so 343 can't tell the difference between incredibly tall, green-armored spartans from incredibly tall, green armored Forerunners. He then associates humans with the word "Reclaimer" because they have similar proportions to the Chief.
In his hand is single plasma grenade.
He ignites the grenade and turns to the Master Chief with
resolve in his eyes.
ARBITER:
You’ve done your part. Now flee,
Demon. My life for yours.