Please note this stuff is not mine, however I find it rather interesting.
I'd like to make a final statement about this, one last thread, professional, none of that... disrespectful stuff, or cussing. But I'll start off by giving you all of the evidence I have towards Forerunner and their existence.
In Halo, the last level, Called, "The Maw," You get to the bridge, 343 Guilty Spark, is pulling information from Human, databases. He is looking at the "Pillar of Autumn's" computers. And he says this. "You can't imagine how exciting this is! To have a record of all of our lost time! Human history, is it? Fascinating." 343 Guilty Spark pretty much just said: "he just got a record of all of his lost time..." "Human History is it?" Human, History.
1. 343 Guilty Spark said hat he was built by the Forerunners (In the books). Now, when 343 Guilty Spark referred to his lost time, how could it mean anything other than the forerunners?
2. Then, since 343 Guilty spark referred to that lost time, as human history, how could that not mean humans are forerunners...
3. Thirdly, if a long time ago, the forerunners (Now known as humans) were alive, and they built the Halo's. And when the Halo's were activated, and wiped out most of the forerunners, wouldn't that make for millions of years of, "Lost Time?" The humans, could just be a patch filling in for the forerunners, The Humans of NOW may not have built the Halo's, but they may have built them a long time ago.
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I don't want to get too in depth and start contradicting my insight that this is only a game and the plots don't go that far in, but Zuka Zammamee, when talking to the prophets on one occasion, said that the master Chiefs shields may in fact be better than his… The prophet went on to say that was classified as treason. Now, if you were a soldier, and you knew a weakness in your army, wouldn’t it be a good idea to speak up about it? Yes, it would the prophets don’t even want the main Covenant population THINKING about such things, that’s because it would make the prophets look like they got caught with their pants down if they found out the Humans were forerunner.
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And As for your other idea, if the Halo were to be fired, all sentient life would be destroyed. How many years ago was it destroyed? 100,000 years right? Hate to use other people's arguments, but who's system? Earths? Ours? The Covie system? Or the previous more advanced form of humans (i.e. Forerunners.) Which would be very important to recognize, as 343 would only assume his (More his Creators) solar system, he wouldn't automatically assume somebody else’s. And even so 343 acts like he knows the Chief. Do you remember anything from the book? How about The library level. How about that marine. Staff Sergeant Mobuto. Marvin, Mobuto.
"Ah," 343 Guilty Spark said, peering down over the Spartan's shoulder. "The other Reclaimer. His combat skin proved even less suitable than yours."
The soldier looked up over his shoulder. "What do you mean?"
"Is this a test, Reclaimer?" the Monitor seemed genuinely puzzled. "I found him wandering through a structure on the other side of the ring, and brought him to the same point where you started." Page 244 (Bottom paragraphs, word for word.)
That guy. You forget about him? I hope not, I hope nobody would forget about him, because that guy, is a real man. But more to the point. He called him the other Reclaimer. And note, "Reclaimer" is capitalized... That's right, meaning, he isn't just ANY reclaimer, he's the Reclaimer. Human. Not just the Spartans. It isn't just Master Chief that he thinks is the Reclaimer. He is calling human's Reclaimer. Humans. (In order to be a Reclaimer by the way, you have to own the thing being reclaimed.)
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Now, The Forerunner didn't give the armor and weapons and stuff to the covenant (Further proving my point) because in the game and books, 343 says "The other species." While referring to the covenant. And if 343 Doesn’t know who the covenant are, then the Forerunner obviously never really made any transactions such as giving them technology.
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Okay, now Check this out.
"Why would you hesitate to do what you've already done?" 343 Guilty Spark Demanded.
What YOU have already done. Humans. Possibly. And That proves that there was AT LEAST, a human looking figure on Halo before. Now then take into consideration he calls ALL humans Reclaimer. So he was very possibly saying, (Rewritten in my own words what he could in fact have been saying.) "Why would you hesitate to do what your people have done before?"
That's not it.
"If you are unwilling to help--I will simply find another," Spark said conversationally.
Doesn't a "Reclaimer" need to activate the installation? "...I will simply find another." He is referring to the human race as the Reclaimer.
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That's still not all.
What type of gravitation system do the covenant have? They have mysterious forms that just automatically attract you to the floor, now UNSC and Human artificial gravity is created how? (Remember in the boxing ring where Chief Really screwed up those four O.D.S.T.'s. It's call Centrifugal force. What type of artificial gravity does the halo have? You don't even need the book to know this one, it's a "ring" world. Obviously sense it is spinning there's it's gravitational poll. But if you're going to get cranky I'll give you the fricking book example.
"Captain," Cortana said, "the object is clearly artificial. There's a gravity field that controls the ring's spin and keeps the atmosphere inside."
That type of Artificial Gravity is Human based. Oh and would you like more about its gravity?
"But it appears that the ring has an oxygen-nitrogen based atmosphere and earth normal gravity."
Now I don't know if you caught all of that but its got earth normal gravity, and the atmosphere is perfect human living conditions, complete with perfect air, nice gravity, and weather conditions such as sun, snow, rain, etc... Wouldn't you think, that whoever built the Halo's, would build them to their own liking? Yes, I would think that and so would you. This is exactly as a human planet should be. The G-Forces not too extreme, and the air is perfect and breathable.
Now, lets go over everything, that I've said.
1. He is referring to the human race as Reclaimer. Not just Master Chief, and the forerunner activated it before, 343 says this, he says, that we've done this before. He was referring to the Forerunner, but we are human, he didn't know we call ourselves human. But never-the-less, he says we're Forerunner. (Inadvertently, in fact, does he ever do anything advertently?) Now to number two.
2. Earth Normal Gravity, and Oxygen-Nitrogen based air. Why would this be? Maybe because the builders of the halo wanted to survive on their own creation while they activated the Halo's, so they would obviously build something to their liking. Even further proof that Humans built the ring.
3. The gravity system is human based. It is a human model of artificial gravity.
4. He said he had considerable time to ponder the question, does "Considerable time" sound like 100,000 years? I think it does.
5. He said "Last time you asked me..." "...And I still say yes." 343 has talked to human life forms before. On Halo. And you know what? There is way more than just sufficient proof here.
Lastly, the (edited)Engineer(End edit), in The fall of Reach, it comes to a Human car, dismantles it, and then reassembles it... I'm willing to bet that the covenant did the same to the Forerunner tech a long time ago, and then just said that they were given the tech directly, making them their gods, and setting up the whole chain of command to have the grunts answer to everyone, the jackal’s to Elites, Elites to Prophets, and Prophets to nobody.
I'll finally to end with this. (You don't have to believe it but I will,

) Raiden2006 has a friend on Xbox Live, that works at Bungie, he said something about the Forerunner being human and he sharply replied back, "How did you know that?"
Flames are not welcome here, but if you have something to add to this, by all means, do so, in a non flaming way, don’t just say “You’re wrong.” Give me the evidence that I’m wrong. Have fun, and keep it professional.
I think both of these ideas are rather believable and sound.