It is my belief that humans are the forerunners. I know Im probalbly just another, same-old storytelling guy, but I would appreciate your time on this.
My first clue is, that there is no possible way an entire race, billions, perhaps trillions, of people just vanishing from the galaxy. There would have to be some form of righting or video communication. Yet all that is left is their technology. Nothing more, nothing less.
My second clue is Guilty spark, and one of the sentinels from Ghosts of Onyx would call us reclaimers. Despite our language difference, and technological difference. Guilty Spark even already knew of our history and race name, and even home planet upon arrival onto Installation 04. We are also the only ones who can operate their machines. Why would the forerunners leave their technology to us?
My third clue is Halo: Contact Harvest. We were declared war upon because the prophets finally found out that humans and our technology were of forerunner descent. It says it right there in the book. We would contradict their entire religion. Taking the San-Shyuum (prophets) out of power. We being a Sacralige to their gods is a bunch of bull.
If you have any contradicting evidence other than Evolutions and the Forerunner Saga, then please leave a post.