Sure, that's a popular theory, but it would entirely undermine human evolution. The Forerunners existed as an advanced race when, according to history, humans were still evolving. And saying it's just science fiction isn't a valid argument. Halo canon is modeled to fit actual history.
The Forerunners are meant to have chosen humankind to be their successors, much as the Forerunners were entrusted to be the successors of the Precursors (might be wrong on that name, bit short on time). I think it's a fair assumption to say they shared a similar anatomy to humans but I think they could have been a fair amount larger given the size of their facilities.
I haven't heard about any race by which the Forerunners were chosen; but I concur with the rest of what you said, because it follows what I've read in Halo Enyclopedia: The Definitive Guide to the Halo Universe.
I have no idea what game you guys are talking about. Is Forerunner the name of a game? Descriptive thread titles FTW!
Yeah, the Forerunners are the fictional race who created the Halo rings. I think they look like the protoss in starcraft, don't know why.
